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Matter

Matter

Matter is commonly referred to as the substance of which physical objects are composed. In physics, it is everything that is constituted of elementary fermions. Philosophically, matter constitutes the formless substratum of all things, which exists only potentially and from which reality is produced. In the sense of content, matter is also used in contrast to form.

Matter in physics

Matter occupies space and has mass. It is composed predominantly of atoms, which consist of protons, neutrons, and electrons. All gauge bosons (of which the photon is one), which mediate the four fundamental forces, are not considered matter, even though they certainly have energy and some also mass. Matter thus consists of quarks and leptons. There are six types of quarks (strange, charm, top, bottom, up, and down) which combine to form hadrons, primarily baryons and mesons, through the strong interaction and are actually thought to always be confined. Among the baryons are the proton and the neutron, which further combine to form the nuclei of all elements of the periodic table. Usually these nuclei are surrounded by a cloud of electrons. A nucleus with as many electrons as protons, which is thus electrically neutral, is called an atom, otherwise it is an ion. Chemistry is the science that studies how nuclei and electrons combine to form compounds. In bulk, matter can exist in several different phases, according to particle density and energy density or alternatively pressure and temperature. These phases include gases, plasmas, liquids, fluids, superfluids, solids, and Bose-Einstein condensate. As circumstances change, matter may change from one phase into another. These phenomena are called phase transitions, and their energetics are studied in the field of thermodynamics. In small quantities, matter can exhibit properties that are entirely different from those of bulk material. Homogeneous matter has a definite composition and properties and any amount of the matter has the same composition and properties. Homogenous matter may or may not be a mixture. Iron and brass would examples of each. Heterogeneous matter does not have a definite composition, for example, granite. Matter constitutes the observable Universe. It can be converted to energy (see annihilation), and vice versa - can be created out of energy (see matter creation) and undergo other formations and alterations.

Fermion

Fermions, named after Enrico Fermi, are particles which form totally-antisymmetric composite quantum states. As a result, they are subject to the Pauli exclusion principle and obey Fermi-Dirac statistics. The spin-statistics theorem states that fermions have half-integer spin. One possible way of visualizing spin is that particles with a 1/2 spin, i.e. fermions, have to be rotated by two full rotations to return them to their initial state. All elementary particles are either fermions or bosons. Composite particles composed of fermions may be either bosons (such as mesons) or fermions (such as baryons) depending on their total spin. The elementary particles which make up matter are fermions, belonging to either the quarks (which form protons and neutrons) or the leptons (such as electrons). The Pauli exclusion of fermions is responsible for the stability of the electron shells of atoms, making complex chemistry possible. It also allows the stability of degenerate matter under extreme pressures. Examples of fermions:
- electrons
- quarks
- protons
- neutrons
- neutrinos

Content

Content can mean
- Comfort and a feeling of satisfaction
- Creations, as in open content or free content.
- In narrative art such as many novels and movies, content is often the subject of the plot or the events and characters contained within. In this and in more abstract art such as some painting and music content is also the details or stuff that make up the form or structure. "Form is supposed to cover the shape or structure of the work; content its substance, meaning, ideas, or expressive effects." (Middleton 1999) See: musical form.
- Online information as distinct from its mode or channel of presentation.
- In mathematics, content is a generalization of the idea of volume to arbitrary dimensions
- In publishing and media content is information and experiences created by individuals, institutions and technology to benefit audiences in venues that they value.
- On a webpage, content is material of interest, put there and changed frequently in order to encourage visits to that page, possibly producing income.

Trolling

The "content" phenomenon has recently been discovered. It is a trolling technique in which a page entitled "content" is added to wikis and merely filled up with ASCII art. The intent is most likely humor, but content pages often become so large as to slow down the entire wiki.

Source

- Richard Middleton (1999). Form. Key Terms in Popular Music and Culture, p. 141. Malden, Massachusetts. ISBN 0631212639. ja:%E3%82%B3%E3%83%B3%E3%83%86%E3%83%B3%E3%83%84

Space

:This article is about space — the scientific and philosophical concepts. For other uses of space, see space (disambiguation). Attempting to understand the nature of space has always been a prime occupation for philosophers and scientists. Perhaps as a result of this considerable discussion, it is difficult to provide an uncontroversial and clear definition of the nature of space, except its physical definition (see below). This article looks at the way space is dealt with variously by physicists, mathematicians and philosophers, and at the relation between space and the mind.

Physics and Space

Space is one of the few fundamental quantities in physics meaning it can't be defined via other quantities because there is nothing more fundamental known at present. Thus, similar to the definition of other fundamental quantities (like time and mass), space is defined via measurement. Currently, the standard space interval, called a standard meter or simply meter, is defined as the distance traveled by light in a vacuum during a time interval of 1/299 792 458 of a second (exact). In classical physics, space is a three-dimensional Euclidean space where any position can be described using three coordinates. Relativistic physics examines spacetime rather than space; spacetime is modeled as a four-dimensional manifold, and currently, there are theories that can support even eleven-dimensional spaces. Before Einstein's work on relativistic physics, time and space were seen as independent dimensions. Einstein's work unified the two into spacetime. In spacetime, measurements of space and time are held to be relative to velocity.

Measurement

The measurement of physical space has long been important. Geometry, the name given to the branch of mathematics which measures spatial relations, was popularised by the ancient Greeks, although earlier societies had developed measuring systems. The International System of Units, (SI), is now the most common system of units used in the measuring of space, and is almost universally used within science. Geography is the branch of science concerned with identifying and describing the Earth, utilising spatial awareness to try and understand why things exist in specific locations. Cartography is the mapping of spaces to allow better navigation, for visualisation purposes and to act as a locational device. Astronomy is the science involved with the observation, explanation and measuring of objects in outer space.

Astronomy and space

In astronomy, space refers collectively to the relatively empty parts of the universe. Any area outside the atmospheres of any celestial body can be considered 'space'. Although space is certainly spacious, it is now known to be far from empty, and filled with a tenuous plasma. In particular, the boundary between space and Earth's atmosphere is conventionally set at the Karman line.

Mathematics and space

In mathematics, a space is a set, with some particular properties and usually some additional structure. It is not a formally defined concept as such, but a generic name for a number of similar concepts, most of which generalize some abstract properties of the physical concept of space. In particular, a vector space and specifically a Euclidean space can be seen as generalizations of the concept of a Euclidean coordinate system. Important varieties of vector spaces with more imposed structure include Banach space and Hilbert space. Distance measurement is abstracted as the concept of metric space and volume measurement leads to the concept of measure space. As far as the concept of dimension is defined, this need not be 3: it can also be 0 (a point), 1 (a line), 2 (a plane), more than 3, and with some definitions, a non-integer value. Mathematicians often study general structures that hold regardless of the number of dimensions. Kinds of mathematical spaces include:
- Banach space
- Euclidean space
- Hilbert space
- Metric space
- Probability space
- Projective space
- Topological space
- Vector space

The philosophy of space

Space has a range of definitions.
- One view of space is that it is part of the fundamental structure of the universe, a set of dimensions in which objects are separated and located, have size and shape, and through which they can move.
- A contrasting view is that space is part of a fundamental abstract mathematical conceptual framework (together with time and number) within which we compare and quantify the distance between objects, their sizes, their shapes, and their speeds. In this view space does not refer to any kind of entity that is a "container" that objects "move through". These opposing views are relevant also to definitions of time. Space is typically described as having three dimensions, and that three numbers are needed to specify the size of any object and/or its location with respect to another location. Modern physics does not treat space and time as independent dimensions, but treats both as features of spacetime – a conception that challenges intuitive notions of distance and time. An issue of philosophical debate is whether space is an ontological entity itself, or simply a conceptual framework we need to think (and talk) about the world. Another way to frame this is to ask, "Can space itself be measured, or is space part of the measurement system?" The same debate applies also to time, and an important formulation in both areas was given by Immanuel Kant. In his Critique of Pure Reason, Kant described space as an a priori notion that (together with other a priori notions such as time) allows us to comprehend sense experience. With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic framework we use to structure our experience. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur. Similar philosophical questions concerning space include: Is space absolute or purely relational? Does space have one correct geometry, or is the geometry of space just a convention? Historical positions in these debates have been taken by Isaac Newton (space is absolute), Gottfried Leibniz (space is relational), and Henri Poincaré (spatial geometry is a convention). Two important thought-experiments connected with these questions are: Newton's bucket argument and Poincaré's sphere-world.

The psychology of space

The way in which space is perceived is an area which psychologists first began to study in the middle of the 19th century, and it is now thought by those concerned with such studies to be a distinct branch within psychology. Psychologists analysing the perception of space are concerned with how recognition of an object's physical appearance or its interactions are perceived. Other, more specialised topics studied include amodal perception and object permanence. The perception of surroundings is important due to its necessary relevance to survival, especially with regards to hunting and self preservation. "Veridical perception" is the term used to describe the processing of the information provided by the sensory organs to an extent whereby it allows interaction with the actuality of that perceived. It is worth noting that the way we perceive space may not necessarily be representative of the actuality of space.

Anxiety and space

Space can also cause anxiety in people, with agoraphobia manifesting itself in some people as a fear of open spaces, and claustrophobia being the fear of enclosed spaces. Astrophobia is the fear of celestial space, Kenophobia is the fear of empty spaces and spacephobia is the fear of outer space.

Personal space

The term personal space refers to the amount of space a person likes to maintain between their own person and that of other people.

Use of space

The definition of physical space in relation to ownership, in which space is seen as property, has long been an important issue. Whilst some cultures assert the rights of the individual in terms of ownership, other cultures will identify with a communal approach to land ownership. Spatial planning is a method of regulating the use of space at land-level, with decisions made at regional, national and international levels. Space can also impact on human and cultural behaviour, being an important factor in architecture, where it will impact on the design of buildings and structures, and on farming. Ownership of space is not restricted to land. Ownership of Airspace and of waters is decided internationally. Public space is a term used to define areas of land which are open to all, whilst private property is that area of land owned by an individual or company, for their own use and pleasure.

Reference

[http://search.eb.com/eb/article?tocId=46639 Space perception]. Encyclopædia Britannica from Encyclopædia Britannica Online. Accessed June 12, 2005. Category: Topology Category:Environments ko:공간 ja:空間 simple:Space

Mass

:For other senses of this word, see mass (disambiguation). Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. Unlike weight, the mass of something stays the same regardless of location. It is a central concept of classical mechanics and related subjects. Strictly speaking, there are three different quantities called mass:
- Inertial mass is a measure of an object's inertia: its resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily.
- Passive gravitational mass is a measure of the strength of an object's interaction with the gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller force than an object with a larger passive gravitational mass. (This force is called the weight of the object. In informal usage, the word "weight" is often used synonymously with "mass", because the strength of the gravitational field is roughly constant everywhere on the surface of the Earth. In physics, the two terms are distinct: an object will have a larger weight if it is placed in a stronger gravitational field, but its passive gravitational mass remains unchanged.)
- Active gravitational mass is a measure of the strength of the gravitational field due to a particular object. For example, the gravitational field that one experiences on the Moon is weaker than that of the Earth because the Moon has less active gravitational mass.

Introduction

Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. One of the consequences of the equivalence of inertial mass and passive gravitational mass is the fact, famously demonstrated by Galileo Galilei, that objects with different masses fall at the same rate, assuming factors like air resistance are negligible. The theory of general relativity, the most accurate theory of gravitation known to physicists to date, rests on the assumption that inertial and passive gravitational mass are completely equivalent. This is known as the weak equivalence principle. Classically, active and passive gravitational mass were equivalent as a consequence of Newton's third law, but a new axiom is required in the context of relativity's reformulation of gravity and mechanics. Thus, standard general relativity also assumes the equivalence of inertial mass and active gravitational mass; this equivalence is sometimes called the strong equivalence principle. If one were to treat inertial mass m_i, passive gravitational mass m_p, and active gravitational mass m_a distinctly, Newton's law of universal gravitation would give as force on the second mass due to the first mass :

m_a_2=\frac.

Newton's third law, of reciprocal actions, shows that active and passive mass are proportional. As a result they can be defined to be equal.

Units of mass

In the SI system of units, mass is measured in kilograms (kg). Many other units of mass are also employed, such as: grams (g), metric tons, pounds, ounces, long and short tons, quintals, slugs, atomic mass units, Planck masses, solar masses, and eV/c². The eV/c² unit is based on the electron volt (eV), which is normally used as a unit of energy. However, because of the relativistic connection between (rest) mass and energy, E = mc² (see below), it is possible to use any unit of energy as a unit of mass instead. Thus, in particle physics where mass and energy are often interchanged, it is common to use not only eV/c² but even simply eV as a unit of mass (roughly 1.783 × 10^-36 kg). Because the gravitational acceleration is approximately constant on the surface of the Earth, a unit like the pound is often used to measure either mass or force (e.g. weight), although the pound is officially defined as a unit of mass. For more information on the different units of mass, see Orders of magnitude (mass).

Inertial mass

Inertial mass is the mass of an object measured by its resistance to acceleration. To understand what the inertial mass of a body is, one begins with classical mechanics and Newton's Laws of Motion. Later on, we will see how our classical definition of mass must be altered if we take into consideration the theory of special relativity, which is more accurate than classical mechanics. However, the implications of special relativity will not change the meaning of "mass" in any essential way. According to Newton's second law, we say that a body has a mass m if, at any instant of time, it obeys the equation of motion :

F = \frac (mv)

where F is the force acting on the body and v is its velocity. For the moment, we will put aside the question of what "force acting on the body" actually means. Now, suppose that the mass of the body in question is a constant. This assumption, known as the conservation of mass, rests on the ideas that (i) mass is a measure of the amount of matter contained in a body, and (ii) matter can never be created or destroyed, only split up or recombined. These are very reasonable assumptions for everyday objects, though, as we will see, the situation gets more complicated when we take special relativity into account. Another point to note is that, even in classical mechanics, it is sometimes useful to treat the mass of an object as changing with time. For example, the mass of a rocket decreases as the rocket fires. However, this is an approximation, based on ignoring pieces of matter which enter or leave the system. In the case of the rocket, these pieces correspond to the ejected propellent; if we were to measure the total mass of the rocket and its propellent, we would find that it is conserved. When the mass of a body is constant, Newton's second law becomes :

F = m \frac = m a

where a denotes the acceleration of the body. This equation illustrates how mass relates to the inertia of a body. Consider two objects with different masses. If we apply an identical force to each, the object with a bigger mass will experience a smaller acceleration, and the object with a smaller mass will experience a bigger acceleration. We might say that the larger mass exerts a greater "resistance" to changing its state of motion in response to the force. However, this notion of applying "identical" forces to different objects brings us back to the fact that we have not really defined what a force is. We can sidestep this difficulty with the help of Newton's third law, which states that if one object exerts a force on a second object, it will experience an equal and opposite force. To be precise, suppose we have two objects A and B, with constant inertial masses m_A and m_B. We isolate the two objects from all other physical influences, so that the only forces present are the force exerted on A by B, which we denote F_AB, and the force exerted on B by A, which we denote F_BA. As we have seen, Newton's second law states that :

F_ = m_A a_A \,

and

F_ = m_B a_B \,

where a_A and a_B are the accelerations of A and B respectively. Suppose that these accelerations are non-zero, so that the forces between the two objects are non-zero. This occurs, for example, if the two objects are in the process of colliding with one another. Newton's third law then states that :

F_ = - F_ \,

Substituting this into the previous equations, we obtain :

m_A = - \frac \, m_B

Note that our requirement that a_A be non-zero ensures that the fraction is well-defined. This is, in principle, how we would measure the inertial mass of an object. We choose a "reference" object and define its mass m_B as (say) 1 kilogram. Then we can measure the mass of every other object in the universe by colliding it with the reference object and measuring the accelerations.

Gravitational mass

Gravitational mass is the mass of an object measured using the effect of a gravitational field on the object. The concept of gravitational mass rests on Newton's law of gravitation. Let us suppose we have two objects A and B, separated by a distance |r_AB|. The law of gravitation states that if A and B have gravitational masses M_A and M_B respectively, then each object exerts a gravitational force on the other, of magnitude :

|F| =

where G is the universal gravitational constant. The above statement may be reformulated in the following way: if g is the acceleration of a reference mass at a given location in a gravitational field, then the gravitational force on an object with gravitational mass M is :

F = Mg \,

This is the basis by which masses are determined by weighing. In simple bathroom scales, for example, the force F is proportionate to the displacement of the spring beneath the weighing pan (see Hooke's law), and the scales are calibrated to take g into account, allowing the mass M to be read off.

Equivalence of inertial and gravitational masses

The equivalence of inertial and gravitational masses is sometimes referred to as the Galilean equivalence principle or weak equivalence principle. The most important consequence of this equivalence principle applies to freely falling objects. Suppose we have an object with inertial and gravitational masses m and M respectively. If the only force acting on the object comes from a gravitational field g, combining Newton's second law and the gravitational law yields the acceleration :

K = \frac g

This says that the ratio of gravitational to inertial mass of any object is equal to some constant K if and only if all objects fall at the same rate in a given gravitational field. This phenomenon is referred to as the universality of free-fall. (In addition, the constant K can be taken to be 1 by defining our units appropriately.) The first experiments demonstrating the universality of free-fall were conducted by Galileo. It is commonly stated that Galileo obtained his results by dropping objects from the Leaning Tower of Pisa, but this is unlikely to be true; actually, he performed his experiments with balls rolling down inclined planes. Increasingly precise experiments have been performed, such as those performed by Roland Eötvös, using the torsion balance pendulum, in 1889. To date, no deviation from universality, and thus from Galilean equivalence, has ever been found. More precise experimental efforts are still being carried out. It should be noted that the universality of free fall only applies to systems in which gravity is the only force acting. All other forces, especially friction and air resistance, must be absent or at least negligible. For example, if a hammer and a feather are dropped from the same height, we all know that the feather will take much longer to reach the ground. This happens because the feather is not really in free fall: the force of air resistance on it is about as strong as the force of gravity. On the other hand, if the experiment is performed in a vacuum, where there is no air resistance, the hammer and the feather should fall at the same rate and reach the ground together. This demonstration was, in fact, carried out in 1971 during the Apollo 15 Moon walk, by Commander David Scott. A stronger version of the equivalence principle, known as the Einstein equivalence principle or the strong equivalence principle, lies at the heart of the general theory of relativity. Einstein's equivalence principle states that it is impossible to distinguish between a uniform acceleration and a uniform gravitational field. Thus, the theory postulates that inertial and gravitational masses are fundamentally the same thing. All of the predictions of general relativity, such as the curvature of spacetime, are ultimately derived from this principle.

Relativistic relation among mass, energy and momentum

Special relativity is a necessary extension of classical physics. In particular, special relativity succeeds where classical mechanics fails badly in describing objects moving at speeds close to the speed of light. In relativistic mechanics, the mass (m) of a free particle is related to its energy (E) and momentum (p) by the equation :

\frac = m^2 c^2 + p^2

. where c is the speed of light. This is sometimes referred to as the mass-energy-momentum relation. The first thing to notice about this equation is that it can cope with massless objects (m = 0), for which it reduces to :

E = pc \,

In classical mechanics, massless objects are an ill-defined concept, since applying any force to one would produce, via Newton's second law, an infinite acceleration - a nonsensical result. In relativistic mechanics, they are objects that are always traveling at the speed of light; an example being light itself, in the form of photons. The above equation says that the energy carried by a massless object is directly proportional to its momentum. Let us now consider objects with non-zero mass. For these, the quantity m has a simple physical meaning: it is the inertial mass of the object as measured in its rest frame, the frame of reference in which its velocity is zero. (Note: massless objects do not possess a rest frame; they are moving at the speed of light in any frame of reference.) The way we would measure m is exactly the same as in classical mechanics, which we described above: bouncing it off a reference object and measuring the accelerations. As long as the velocity of each object remains much smaller than the speed of light during this procedure, relativistic corrections to classical mechanics will be utterly negligible. In the rest frame, the velocity is zero, and thus so is the momentum p. The mass-energy-momentum relation thus reduces to :

E = mc^2 \,

which states that the energy of an object as measured in its rest frame - its "rest energy" - is equal to its mass times the square of the speed of light. Some books follow this up by stating that "mass and energy are equivalent", but this is somewhat misleading. The mass of an object, as we have defined it, is a quantity intrinsic to the object, and independent of our current frame of reference. The energy E, on the other hand, varies with the frame of reference; if the frame is moving at a high velocity relative to the object, E will be very large, simply because the object has a lot of kinetic energy in that frame. Thus, E = mc² is not a "good" relativistic statement; it is true only in the rest frame of the object. Some authors define a quantity known as the relativistic mass, which is basically the quantity E/c². This makes the "equivalence" of "mass" and energy true by definition, though neither quantity is frame-independent! "Relativistic mass" was used in many early writings on relativity, and it is still used in books for laymen as well as introductory physics classes. However, the concept is downplayed or discouraged by many physicists nowadays, for reasons explained in the article on relativistic mass. Following the modern usage, whenever we refer to "mass" in this article we always mean the rest mass, unless otherwise identified. Having defined the mass of an object, let us look at how it behaves when not at rest. We can arrange the mass-energy-momentum relation in the following way: :

E = mc^2 \sqrt

When the momentum p is much smaller than mc, we can Taylor expand the square root, with the result :

E = mc^2 +  + \cdots

The leading term, which is the largest, is of course the rest energy. The object always has this minimum amount of energy, regardless of its momentum. The second term is the classical expression for the kinetic energy of the particle, and the higher-order terms are basically relativistic corrections for the kinetic energy. Under normal circumstances, the rest energy of an object is inaccessible, in the sense that it cannot be used to do mechanical work. When the object hits something, it can do work by transferring its momentum, and thus its kinetic energy, to whatever it hit. However, the rest energy depends only on the mass of the object, which does not change during collisions, so it cannot be transferred along with the kinetic energy. On the other hand, it is possible to access the rest energy using processes that split or combine particles. The reason is that mass, as we have defined it, is not conserved during such processes. The simplest example is the process of electron-positron annihilation, in which an electron and a positron annihilate each other to produce a pair of photons: the electron and positron both have non-zero mass, but the photons are massless. Other examples include nuclear fusion and nuclear fission. Metabolism, fire and other exothermic chemical processes also convert mass to energy, however the mass change from these is negligible. Energy, unlike mass, is always conserved in special relativity, so, roughly speaking, what is happening in these reactions is that the rest energy of the reactants is being transformed into the kinetic energy of the reaction products. The fact that rest energy can be liberated in this way is one of the most important predictions of special relativity.

References

- R.V. Eötvös et al, Ann. Phys. (Leipzig) 68 11 (1922)

External links

- [http://math.ucr.edu/home/baez/physics Usenet Physics FAQ]
- [http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html Does mass change with velocity?]
- [http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html Does light have mass?]
- [http://www.teleles.nl/pdf/total_artikel.pdf Mass & energy]
- [http://www.geocities.com/physic1525/inertiaenergy.html The law of the inertia of the energy and the speed of the gravity. See chapter 3 The energy has mass ]
- [http://www.geocities.com/physics_world/stp/title.htm Dialog: Use and abuse of the concept of mass (from Spacetime physics by Edwin F. Taylor and John A. Wheeler)]
- [http://arxiv.org/PS_cache/physics/pdf/0111/0111134.pdf Photons, Clocks, Gravity and the Concept of Mass by L.B.Okun]
- [http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html The Apollo 15 Hammer-Feather Drop]
- [http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units] Category:Physical chemistry Category:Classical mechanics
- ko:질량 ms:Jisim ja:質量 simple:Mass th:มวล

Atom

:For alternative meanings see atom (disambiguation). An atom (Greek άτομον from ά: non and τομον: divisible) is a submicroscopic structure found in all ordinary matter. It is the smallest unit of an element to retain all the chemical properties of that element. The word atom originally meant a smallest possible particle of matter, not further divisible. Later, the objects that had been called atoms were found to be further divisible into smaller subatomic particles, but the word atom nonetheless continues to refer to them. Most atoms are composed of three types of massive subatomic particles which govern their external properties:
- electrons, which have a negative charge and are the least massive of the three;
- protons, which have a positive charge and are about 1836 times more massive than electrons; and
- neutrons, which have no charge and are about 1838 times more massive than electrons. Together, protons and neutrons form the nucleus of an atom, which is surrounded by the electrons. Atoms can differ in the number of each of the subatomic particles they contain. Atoms of the same element have the same number of protons, although the same element can differ in the number of neutrons which are then called isotopes of that element. Atoms are electrostatically neutral if they have an equal number of protons and electrons. Atoms which have either gained or lost electrons are called ions. Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Atoms are able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen and one oxygen atom.

Properties of the atom

Subatomic particles

:see main article subatomic particles Up until 1961, the subatomic particles were thought to consist of only protons, neutrons and electrons. However, protons and neutrons themselves are now known to consist of varieties of a still smaller particle called the quark, and the electron is considered a type of lepton. Therefore in modern atomic theory, the two basic constituents of matter are the lepton and the quark of which the above three particles of the atom are composed. All particles exhibit a wave-particle duality so that the electron is better understood as a wave when drawn about a nucleus. Unlike planets revolving around the sun, the electron is not held around the nucleus of the atom by gravity, but rather by electromagnetism.
Atom sizes
The atom is many times smaller than the wavelength that human vision can detect in any kind of microscope. However, there are ways of projecting the atom so as to obtain amplified images of it. These include: scanning tunneling microscopy (STM), atomic force microscopy (ATM), and nuclear magnetic resonance (NMR). In measuring an atom, the size of the area that an electron can travel in must be determined. Electrons travel in areas called atomic orbitals. This area forms a cloud where the electron may be situated. In the helium atom above (shown in its ground state), the atomic orbital where the electron may be situated describes a sphere. However, the cloud or atomic orbital in which an electron can travel changes shapes depending on the energy of the electron. So some electrons travel in the shape of a dumbbell with the nucleus in the smallest space in-between. There are other more complicated shapes as well. And the heavier the element, the more electrons there are and the more shapes there are for the orbitals in the atom. It therefore not only becomes more complicated to measure the size of the atom, but it becomes complicated to create models of the atoms of heavier elements. Since the electron orbitals are considered clouds, then the size of an atom is not easily defined since the places where the electron can be just gradually go to zero as the distance from the nucleus increases. For atoms that can form solid crystals, the distance between adjacent nuclei can give an estimate of the atom size. For atoms that do not form solid crystals other techniques are used, including theoretical calculations. As an example, the size of a hydrogen atom is estimated to be approximately 1.0586×10 m. Compare this to the size of the proton which is the only particle in the nucleus of the hydrogen atom which is approximately 10 m. Thus the ratio of the sizes of the hydrogen atom to its nucleus is about 100,000:1. Atoms of different elements do vary in size, but the sizes are roughly the same to within a factor of 2 or so. The reason for this is that elements with a large positive charge on the nucleus attract the electrons to the center of the atom more strongly. To illustrate the size of an atom, one million atoms can fit within the breadth of a strand of hair. An atom is mostly space. A basic analogy for the ratio of space inside an atom is this: if an atom were the size of a baseball stadium, the nucleus would be the size of a marble on second base and the electrons would orbit the perimeter.
Elements, isotopes and ions
Atoms are generally classified by their atomic number, which corresponds to the number of protons in the atom. The atomic number defines which element the atom is. For example, carbon atoms are those atoms containing six protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical behavior. The various kinds of atoms are listed in the periodic table in order of increasing atomic number. The mass number, atomic mass number, or nucleon number of an element is the total number of protons and neutrons in an atom of that element, because each proton or neutron essentially has a mass of 1 amu. The number of neutrons in an atom has no effect on which element it is. Each element can have numerous different atoms with the same number of protons and electrons, but varying numbers of neutrons. Each has the same atomic number but a different mass number. These are called the isotopes of an element. When writing the name of an isotope, the element name is followed by the mass number. For example, carbon-14 contains 6 protons and 8 neutrons in each atom, for a total mass number of 14. The simplest atom is the hydrogen atom, which has atomic number 1 and consists of one proton and one electron. The hydrogen isotope which also contains one neutron so is called deuterium or hydrogen-2; the hydrogen isotope with two neutrons is called tritium or hydrogen-3. Tritium is an unstable isotope which causes the atom to lose mass in a process called radioactivity. The elements in the periodic table beginning with number 86, radon, and those that follow have no stable isotopes and are all radioactive. The atomic mass listed for each element in the periodic table is an average of the isotope masses found in nature, weighted by their abundance. Although most sources state that there are 92 elements that occur naturally on earth from hydrogen up to uranium in the periodic table, it has been recently discovered that plutonium, the 94th element, also occurs naturally. Most of these elements were created through stellar nucleosynthesis and supernova nucleosynthesis. Several elements that do not occur on earth have been found to be present in stars. Elements not normally found in nature have been artificially created by nuclear bombardment, but they are usually unstable and spontaneously change into stable natural chemical elements by the processes of radioactive decay. Atoms that have either lost or gained electrons are called atomic ions (with either positive(+) or negative charge(−), respectively). Atoms are canonically distinguished from ions by their balanced electrical charge.
Atomic spectrum
:see main article Atomic spectroscopy Each element in the periodic table therefore consists of an atom in a unique configuration i.e. with different amounts of protons in the nucleus. Each atom of each element can also be uniquely described by the shapes of its atomic orbitals and the number of electrons within them. There is also another way in which each element with its own configuration is distinctive, that is, by its atomic spectrum. A spectrum is created when light is passed through a prism and the light breaks up into its component colors. Spectroscopy studies the spectrum of each element. Each atom of each element creates its own light pattern unique to itself, its own spectral signature. Scientists can use a spectrometer to study the atoms in stars and other distant objects, and due to the distinctive spectral lines that each element produces, are able to tell the chemical composition of distant planets, stars and galaxies.
Electron configuration
:see main article electron configuration The chemical behavior of atoms is largely due to interactions between electrons. Electrons of an atom remain within certain, predictable electron configurations. Electrons fall into shells based on their relative energy level. Generally, the higher the energy level of a shell, the further away it is from the nucleus. The electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus. valence electrons of a hydrogen atom. The principal quantum number is at the right of each row and the azimuthal quantum number is denoted by letter at top of each column.]] An electron shell can hold up to 2n² electrons, where n is the number of the shell. Whichever occupied shell is currently most outward is the valence shell, even if it only has one electron. In the most stable state, an atom's electrons will fill up its shells in order of increasing energy. Under some circumstances an electron may be excited to a higher energy level (that is, it absorbs energy from an external source and leaps to a higher shell), leaving a space in a lower shell, but at some point it will fall back to its previous level, emitting its excess energy as a photon. Electron shells also have distinctive shapes denoted by letters. In the illustration, the letters s, p, and d describe the shape of the atomic orbital. Electrons also have another property that describes their configuration due to the fact that they rotate in space. Thus electrons are said to have spin (physics).
Valence and bonding
:see main article valence electrons and chemical bond The number of electrons in an atom's outermost shell (ie the valence shell) governs its bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the periodic table of the elements. Group (i.e. column) 1 elements contain one electron on their outer shell; Group 2, two electrons; Group 3, three electrons; etc. As a general rule, the fewer electrons in an atom's valence shell, the more reactive it is. Group 1 metals are therefore very reactive, with caesium, rubidium, and francium being the most reactive of all metals. Every atom is much more stable (i.e. less energetic) with a full valence shell. This can be achieved one of two ways: an atom can either share electrons with neighboring atoms (a covalent bond), or it can remove electrons from other atoms (an ionic bond). Another form of ionic bonding involves an atom giving some of its electrons to another atom; this also works because it can end up with a full valence by giving up its entire outer shell. By moving electrons, the two atoms become linked. This is known as chemical bonding and serves to build atoms into molecules or ionic compounds. Five major types of bonds exist:
- ionic bonds;
- covalent bonds;
- coordinate covalent bonds;
- hydrogen bonds; and
- metallic bonds.
Atoms and antimatter
:see main article antimatter Antimatter can also form atoms, composed of antielectrons (positrons), antiprotons, and antineutrons.

Atoms and the Big Bang

In models of the Big Bang, Big Bang nucleosynthesis predicts that within one to three minutes of the Big Bang all the current atomic material in the universe was created producing no heavier element than lithium, but mostly hydrogen and helium. However, although the basic atomic particles of matter were created, atoms themselves could not form in the intense heat. Big Bang chronology of the atom continues to approximately 379,000 years after the Big Bang when the cosmic temperature had dropped to just 3,000 K which allowed the first atoms to form. It was then cool enough to allow protons to capture one electron each and form neutral atoms of hydrogen. Hydrogen makes up approximately 75% of the atoms in the universe. Helium makes up 24% and all other elements make up 1%. Since the size of the universe is unknown, the total numbers of atoms in the universe is unknown, but the number is not thought to be infinite because current theory suggests we live in a finite universe. One thing we can say about the mass of the baryons in the universe, meaning the mass of the protons and neutrons, is that we can tell what the ratio of their density ought to be from the Big Bang model. Einstein's theory of General Relativity suggests that the universe is the same in all directions and from all viewpoints. Therefore, examining one region of the universe and the density of atoms in that region should tell us how densely atoms are scattered throughout the entire universe, but as said previously, does not tell us how far the universe extends and how many atoms exist in total. Big Bang Nucleosynthesis predicts that 1/20 of the total mass of the Universe is baryonic matter. (The baryon is the category used to describe neutrons and protons which are similar in mass but different in electric charge.) So theoretically we should be able to study a region of space and calculate the amount of matter we see through our telescopes and one-twentieth of the matter should be baryons. However, from the density we can see through telescopes of matter in regions of the visible universe, 99% of the baryons are missing. This has given rise to theories of dark matter (which should also be made of baryons--or if you prefer atoms, since baryons make up the nucleus of atoms) in order to make up the difference in missing matter. What that means is that there are probably more atoms out there than we can see through our usual means of detection. In other words, we cannot see visible light from these atoms nor have we detected electromagnetic radiation, but they exist. In fact, in some cases we have detected, through radio-wave detectors, entire galaxies such as Virgo H121 that do not appear in normal telescopes.

Atomic theory

The atomic theory is a theory of the nature of matter. It states that all matter is composed of atoms.

Historical theories

Democritus and Leucippus, Greek philosophers in the 5th century BC, presented the first theory of atoms (see article atomism for more details). They held that each atom had a different shape, like a pebble, that governed the atom's properties. Dalton and Avogadro rediscovered the works of Democritus and Leucippus and suggested in the 19th century that matter was made up of atoms, but they knew nothing of their structure. This theory was conflicting with the theory of infinite divisibility, which states that matter can always be divided into smaller parts. The controversy ended in 1911 when Jean Perrin demonstrated the existence of atoms through experimental validation of Einstein's theory of Brownian motion (which relied on atomic theory). For much of this time, atoms were thought to be the smallest possible piece of matter. However, in 1897, J.J. Thomson published his work proving that cathode rays are made of negatively charged particles (electrons). Since cathode rays are essentially emitted from matter, this proved that atoms are made up of subatomic particles and are therefore divisible, and not the indivisible "atomos" Democritus talked about. Physicists later invented a new term for indivisible units, namely elementary particles since the word atom had already been taken and come into common use. At first, it was believed that the electrons were distributed more or less uniformly in a sea of positive charge (the plum pudding model). However, an experiment conducted a few years later by Rutherford demonstrated that atoms are mostly empty space, with a lot of mass concentrated in a nucleus. In the gold foil experiment, he shot alpha particles (emitted by polonium) through a sheet of gold. He observed that most of the particles passed straight through the sheet without deflection (striking a fluorescent screen on the other side), but that, surprisingly, a small number were bounced right back (having come close to a nucleus). This led to the planetary model of the atom, in which the electrons orbited the nucleus like the planets orbiting the sun. The nucleus was later discovered to contain protons, and further experimentation by Rutherford found that the nuclear mass of most atoms surpassed the number of protons it possessed; this led him to postulate the existence of neutrons, whose existence would be proven in 1932 by James Chadwick. The planetary model of the atom still had shortcomings. Firstly, a moving electrical charge emits electromagnetic waves; according to classical physics, an orbiting charge would steadily lose energy and spiral towards the nucleus, eventually colliding with it. Secondly, the model didn't explain why hydrogen gas, when submitted to an electrical discharge, emitted light only in certain discrete spectra. Experiments by Max Planck and Albert Einstein demonstrated that energy is transferred in tiny fixed amounts known as quanta. In 1913, Niels Bohr used this idea in his Bohr model of the atom, in which the electrons could only orbit the nucleus in fixed circles. They couldn't spiral downwards because they couldn't lose energy in a continuous manner; they could only make quantum leaps between fixed energy levels. The Bohr model would eventually be replaced by a full quantum mechanics model in 1925.

Study of atoms

Because of their ubiquitous nature, atoms have been an important field of study for many centuries. Current research focuses on quantum effects, such as in Bose-Einstein condensate. The study of atoms was done largely by indirect means through the 19th century and early 20th century. In recent years, however, new techniques have made the identification and study of atoms easier and more accurate. The electron microscope, invented in 1931, can image large molecules, however, not the atom itself. Atomic force microscopy is another technique by which individual atoms can be visualized and even arranged into patterns. Methods also exist to identify atoms and compounds. Elemental analysis allows the exact identification of the types and amounts of atoms in a substance.

Practical uses of the atom

Atoms have given us the key to understanding our universe, understanding our earth and life upon it, improving technology, and creating life-saving pharmaceuticals. There does not exist a scientific field that is not affected by the understanding of the atom. Atoms are the basis for chemistry, physics, geology, astronomy and biology. Within the tiny atom are the powers to both create and destroy. Through fusion and fission man has learned to unleash the power of the atom. Our sun and other stars use fusion of the atom to create the heavier elements in the universe that were not created in the Big Bang. Fission of the atom is used to create power in nuclear power plants. Fusion of the atom may one day be used to create safer forms of power than current fuels that are destroying the delicate balance of earth's ecosystem.

External links

- [http://www.howstuffworks.com/atom.htm How Atoms Work] ko:원자 ms:Atom ja:原子 simple:Atom th:อะตอม

Proton

:For alternative meanings see proton (disambiguation). In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1.602 × 10⁻¹⁹ coulomb) and a mass of 938.3 MeV/c² (1.6726 × 10⁻²⁷ kg), or about 1836 times the mass of an electron. The proton is observed to be stable, with a lower limit on its half-life of about 10³⁵ years, although some theories predict that the proton may decay. The proton and neutron are both nucleons. The nucleus of the most common isotope (called protium) of the hydrogen atom is a single proton. The nuclei of other atoms are composed of protons and neutrons held together by the strong nuclear force. The number of protons in the nucleus determines the chemical properties of the atom and which chemical element it is. Protons are classified as baryons and are composed of two up quarks and one down quark, which are also held together by the strong nuclear force, mediated by gluons. The proton's antimatter equivalent is the antiproton, which has the same magnitude charge as the proton but the opposite sign. In chemistry and biochemistry, the term proton may refer to the hydrogen ion, H⁺. In this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).

History

Ernest Rutherford is generally credited with the discovery of the proton. In 1918 Rutherford noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle. Prior to Rutherford, Eugene Goldstein had observed canal rays, which were composed of positively charged ions.

Technological applications

Protons can exist in spin states. This property is exploited by nuclear magnetic resonance spectroscopy. In NMR spectroscopy, a magnetic field is applied to a substance in order to detect the shielding around the protons in the nuclei of that substance, which is provided by the surrounding electron clouds. Scientists can use this information to then construct the molecular structure of the molecule under study.

Antiproton

The antiproton is the antiparticle of the proton. It was discovered in the year 1955 by Emilio Segre and Owen Chamberlain, for which they were awarded a 1959 Nobel Prize in Physics. CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. For example, the charges of the proton and antiproton must sum to exactly zero. This equality has been tested to one part in 10^-8. The equality of their masses is also tested to better than one part in 10^-8. By holding antiprotons in a Penning trap, the equality of the charge to mass ratio of the proton and the antiproton has been tested to 1 part in 9×10^-11. The magnetic moment of the antiproton has been found with error of 8×10^-3 nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton.

External links

- [http://pdg.lbl.gov/ Particle Data Group] Category:Nucleon ko:양성자 ms:Proton ja:陽子 simple:Proton th:โปรตอน

Electron

The electron is a fundamental subatomic particle which carries a negative electric charge.

Overview

Within an atom the electrons surround the nucleus of protons and neutrons in an electron configuration. The word electron was coined in 1894 and is derived from the term electric, whose ultimate origin is the Greek word 'ηλεκτρον, meaning amber. Electrons in motion constitute electric current which may be used by scientists and engineers to measure many physical properties. Electric current existing for a finite time gives rise to a movement of charge (electricity) that may be harnessed as a practical means to perform work. The variations in electric field generated by differing numbers of electrons and their configurations in atoms determine the chemical properties of the elements. These fields play a fundamental role in chemical bonds and chemistry.

Electrons in practice

Classification of electrons

The electron is one of a class of subatomic particles called leptons which are believed to be fundamental particles (that is, they cannot be broken down into smaller constituent parts). The word "particle" is somewhat misleading however, because quantum mechanics shows that electrons also behave like a wave, e.g. in the double-slit experiment; this is called wave-particle duality. The antiparticle of an electron is the positron, which has the same mass but positive rather than negative charge. The term negatron is sometimes used to refer to standard electrons so that the term electron may be used to describe both positrons and negatrons, as proposed by Carl D. Anderson. Under ordinary circumstances, however, electron refers to the negatively charged particle alone.

Properties and behavior of electrons

Electrons have a negative electric charge of −1.6 × 10⁻¹⁹ coulombs, and a mass of about 9.11 × 10⁻³¹ kg (0.51 MeV/c²), which is approximately ¹⁄₁₈₃₆ of the mass of the proton. These are commonly represented as e⁻. According to quantum mechanics, electrons can be represented by wavefunctions, from which the electron density can be determined. The exact momentum and position of an electron cannot be simultaneously determined. This is a limitation described by the Heisenberg uncertainty principle, which, in this instance, simply states that the more accurately we know a particle's position, the less accurately we can know its momentum and vice versa. The electron has spin ½, which implies it is a fermion, i.e., it follows the Fermi-Dirac statistics. While most electrons are found in atoms, others move independently in matter, or together as an electron beam in a vacuum. In some superconductors, electrons move in Cooper pairs, in which their motion is coupled to nearby matter via lattice vibrations called phonons. When electrons move, free of the nuclei of atoms, and there is a net flow, this flow is called electricity, or an electric current. A body has a static charge when the body has more or fewer electrons than are required to balance the positive charge of the nuclei. When there is an excess of electrons, the object is said to be negatively charged. When there are fewer electrons than protons, the object is said to be positively charged. When the number of electrons and the number of protons are equal, their charges cancel out and the object is said to be electrically neutral. A macroscopic body can acquire charge through rubbing, i.e. the phenomena of triboelectricity. Electrons and positrons can annihilate each other and produce a pair of photons. Conversely, high-energy photons may transform into an electron and a positron by a process called pair production. The electron is an elementary particle — that means that it has no substructure (at least, experiments have not found any so far, and there is good reason to believe that there is not any). Hence, it is usually described as point-like, i.e. with no spatial extension. However, if one gets very near an electron, one notices that its properties (charge and mass) seem to change. This is an effect common to all elementary particles: the particle influences the vacuum fluctuations in its vicinity, so that the properties one observes from far away are the sum of the bare properties and the vacuum effects (see renormalization). There is a physical constant called the classical electron radius, with a value of 2.8179 × 10⁻¹⁵ m. Note that this is the radius that one could infer from its charge if the physics were only described by the classical theory of electrodynamics and there were no quantum mechanics (hence, it is an outdated concept that nevertheless sometimes still proves useful in calculations). The speed of an electron in a vacuum can approach, but never reach c, the speed of light in a vacuum. This is due to an effect of special relativity. The effects of special relativity are based on a quantity known as gamma or the Lorentz factor. Gamma is a function of v, the velocity of the particle, and c. The following is the formula for gamma: :

\gamma = 1 / \sqrt

The energy necessary to accelerate a particle is gamma minus one times the rest mass. For example, the linear accelerator at Stanford can [http://www2.slac.stanford.edu/vvc/theory/relativity.html accelerate] an electron to roughly 51 GeV. This gives you a gamma of 100,000 given that the rest mass of an electron is 0.51 MeV/c² (the relativistic mass of this fast electron is 100 000 times its rest mass). Solving the equation above for the speed of the electron gives a speed of: :

(1-\frac   \gamma ^)c

= 0.999 999 999 95 c. (The formula applies for large γ.)

Electrons in the universe

It is believed that the number of electrons existing in the known universe is at least 10⁷⁹. This number amounts to a density of about one electron per cubic metre of space. Based on the classical electron radius and assuming a dense sphere packing, it can be calculated that the number of electrons that would fit in the observable universe is on the order of 10¹³⁰. Of course, this number is even less meaningful than the classical electron radius itself.

Electrons in industry

Electron beams are used in welding as well as lithography.

Electrons in the laboratory

Early experiments

The quantum or discrete nature of electron's charge was observed by Robert Millikan in the Oil-drop experiment of 1909.

Use of electrons in the laboratory

Electron microscopes are used to magnify details up to 500,000 times. Quantum effects of electrons are used in Scanning tunneling microscope to study features at the atomic scale.

Electrons in theory

In relativistic quantum mechanics, the electron is described by the Dirac Equation. Quantum electrodynamics (QED) models an electron as a charged particle surrounded a sea of interacting virtual particles, modifying the sea of virtual particles which makes up a vacuum. Although this theory involves difficult theoretical problems where calculations produce infinite terms, a practical (although mathematically dubious) method called renormalization was discovered whereby infinite terms can be cancelled to produce finite predictions about the electron. The correction of just over 0.1% to the predicted value of the electron's gyromagnetic ratio from exactly 2 (as predicted by Dirac's single particle model), and its extraordinarily precise agreement with the experimentally determined value, is viewed as one of the pinnacles of modern physics. There are now indications that string theory and its descendants may provide a model of the electron and other fundamental particles where the infinities in calculations do not appear, because the electron is no longer seen as a dimensionless point. At present, string theory is very much a 'work in progress' and lacks predictions analogous to those made by QED that can be experimentally verified. In the Standard Model of particle physics, it forms a doublet in SU(2) with the electron neutrino, as they interact through the weak interaction. The electron has two more massive partners, with the same charge but different masses: the muon and the tau lepton. The antimatter counterpart of the electron is its antiparticle, the positron. The positron has the same amount of electrical charge as the electron, except that the charge is positive. It has the same mass and spin as the electron. When an electron and a positron meet, they may annihilate each other, giving rise to two gamma-ray photons, each having an energy of 0.511 MeV (511 keV). See also Electron-positron annihilation. Electrons are also a key element in electromagnetism, an approximate theory that is adequate for macroscopic systems, and for classical modelling of microscopic systems.

History

The electron as a unit of charge in electrochemistry had been posited by G. Johnstone Stoney in 1874. In 1894, he also invented the word itself. The discovery that the electron was a subatomic particle was made in 1897 by J.J. Thomson at the Cavendish Laboratory at Cambridge University, while he was studying "cathode rays". Influenced by the work of James Clerk Maxwell, and the discovery of the X-ray, he deduced that cathode rays existed and were negatively charged "particles", which he called "corpuscles". He published his discovery in 1897. The periodic law states that the chemical properties of elements largely repeat themselves periodically and is the foundation of the periodic table of elements. The law itself was initially explained by the atomic mass of the elements. However, as there were anomalies in the periodic table, efforts were made to find a better explanation for it. In 1913, Henry Moseley introduced the concept of the atomic number and explained the periodic law with the number of protons each element has. In the same year, Niels Bohr showed that electrons are the actual foundation of the table. In 1916, Gilbert Newton Lewis and Irving Langmuir explained the chemical bonding of elements by electronic interactions.

External links

- [http://www.aip.org/history/electron/ The Discovery of the Electron] from the American Institute of Physics History Center
- [http://pdg.lbl.gov/ Particle Data Group]
- Stoney, G. Johnstone, "[http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Stoney-1894.html Of the 'Electron,' or Atom of Electricity]". Philosophical Magazine. Series 5, Volume 38, p. 418-420 October 1894.
- Eric Weisstein's World of Physics: [http://scienceworld.wolfram.com/physics/Electron.html Electron]

References

-
-
- Brumfiel, G. (6 January 2005). Can electrons do the splits? In Nature, 433, 11. ko:전자 ja:電子 simple:Electron th:อิเล็กตรอน

Photon

In physics, the photon (from Greek φως "phos", meaning light) is a quantum of the electromagnetic field, for instance light. The term photon was coined by Gilbert Lewis in 1926. Gilbert Lewis The photon is one of the elementary particles. Its interactions with electrons and atomic nuclei account for a great many of the features of matter, such as the existence and stability of atoms, molecules, and solids. These interactions are studied in quantum electrodynamics (QED), which is the oldest part of the Standard Model of particle physics. In some respects a photon acts as a particle, for instance when registered by the light sensitive device in a camera. In other respects, a photon acts like a wave, as when passing through the optics in a camera. According to the so-called wave-particle duality in quantum physics, it is natural for the photon to display either aspect of its nature, according to the circumstances. Normally, light is formed from a large number of photons, with the intensity related to the number of them. At low intensity, it requires very sensitive instruments, used in astronomy or spectroscopy, for instance, to detect the individual photons.

Symbol

A photon is usually given the symbol

\gamma

(gamma), although in nuclear physics this symbol refers to a very high-energy photon (a gamma ray).

Properties

Photons are commonly associated with visible light, but this is actually only a very limited part of the electromagnetic spectrum. All electromagnetic radiation is quantized as photons: that is, the smallest amount of electromagnetic radiation that can exist is one photon, whatever its wavelength, frequency, energy, or momentum. Photons are fundamental particles. They can be created and destroyed when interacting with other particles, but are not known to decay on their own. Unlike most particles, photons have no detectable intrinsic mass, or "rest mass" (as opposed to relativistic mass). Photons are always moving at the speed of light with respect to all observers. Although they lack mass, photons have both energy and momentum proportional to their frequency (or inversely proportional to their wavelength). This momentum can be transferred when a photon collides with matter. The force due to a large number of photons falling on a surface is known as radiation pressure, which may be used for propulsion with a solar sail. Photons are deflected by a gravitational field twice as much as Newtonian mechanics predicts for a mass traveling at the speed of light with the same momentum as the photon. This observation is commonly cited as evidence supporting Einstein's theory of gravitation, general relativity. In general relativity, photons always travel in a "straight" line, after taking into account the curvature of spacetime. (In curved space, such lines are called geodesics).

Creation

Photons are produced by atoms when a bound electron moves from one orbital to another orbital with less (more negative) energy. Photons can also be emitted by an unstable nucleus when it undergoes some types of nuclear decay. Furthermore, photons are produced whenever charged particles are accelerated. Atoms continuously emit photons due to their collisions with each other. The wavelength distribution of these photons thus are related to their absolute temperature. The Maxwell-Boltzmann distribution provides the probability of a photon being a certain wavelength when emitted by a collection of atoms at a given temperature. The spectrum of such photons is normally peaked in the range between microwave and infrared, but hot objects (such as the surface of the Sun) will emit visible light as well. As temperature is further increased, some photons will reach even higher frequencies, such as ultraviolet and X-ray. Radio, television, radar and other types of transmitters used for telecommunication and remote sensing routinely create a wide variety of low-energy photons by the oscillation of electric fields in conductors. Magnetrons emit coherent photons used in household microwave ovens. Klystron tubes are used when microwave emissions must be more finely controlled. Masers and lasers create monochromatic photons by stimulated emission. More energetic photons can be created by nuclear transitions, particle-antiparticle annihilation, and in high-energy particle collisions.

Spin

Photons have spin 1, and they are therefore classified as bosons. Photons mediate the electromagnetic interaction; they are the gauge bosons of quantum electrodynamics (QED), which is a U(1) gauge theory. In general, a boson with spin 1 should have three distinct spin projections (−1, 0 and +1). However, the zero projection would require a frame where the photon is at rest. Since the photon's mass is zero, it always travels at the speed of light, and such a frame does not exist. Thus photons in empty space show only two spin projections, corresponding to the right- and left-handed circular polarizations of classical electromagnetic waves. The more familiar linear polarization is formed by a mixture of right- and left-circularly polarized photons.

Quantum state

Visible light from ordinary sources (like the Sun or a lamp) is a mixture of many photons of different wavelengths. One sees this in the frequency spectrum, for instance by passing the light through a prism. In so-called "mixed states", which these sources tend to produce, light can consist of photons in thermal equilibrium (so-called black-body radiation). Here they in many ways resemble a gas of particles. For example, they exert pressure, known as radiation pressure. On the other hand, an assembly of photons can also exist in much more well-organized coherent states, such as in the light emitted by an ideal laser. The high degree of precision obtained with laser instruments is due to this organization. The quantum state of a photon assembly, like that of other quantum particles, is the so-called Fock state denoted

|n\rangle

, meaning

n

photons in one of the distinct "modes" of the electromagnetic field. If the field is multimode (involves several different wavelength photons), its quantum state is a tensor product of photon states, for example: :

|n_\rangle\otimes|n_\rangle\otimes\dots\otimes|n_\rangle\dots

Here

k_i

denote the possible modes, and

n_

the number of photons in each mode

Molecular absorption

A typical molecule,

M

, has many different energy levels. When a molecule absorbs a photon, its energy is increased by an amount equal to the energy of the photon. The molecule then enters an excited state,

M^ -  \,

. :

M + \gamma \to M^ -  \,

Photons in vacuo

In empty space (vacuum) all photons move at the speed of light, c, defined as 299,792,458 meters per second, or approximately 3×10⁸ m s⁻¹. The meter is defined as the distance traveled by light in vacuum in 1/299,792,458 of a second, so the speed of light does not suffer any experimental uncertainty, unlike the meter or the second, which rely on the second being defined by means of a very accurate clock. According to one principle of Einstein's special relativity, all observations of the speed of light in vacuo are same in all directions to any observer in an inertial frame of reference. This principle is generally accepted in physics since many practical consequences for high-energy particles in theoretical and experimental physics have been observed.

Photons in matter

When photons pass through matter, such as a prism, different frequencies will be transmitted at different speeds. This is called dispersion of colors, where photons of different frequencies exit at different angles. A similar phenomenon occurs in reflection where surfaces can reflect photons of various frequencies at different angles. The associated dispersion relation for photons is a relation between frequency, f, and wavelength, λ. Or, equivalently, between their energy, E, and momentum, p. It is simple in vacuo, since the speed of the wave, v, is given by :

v=\lambda f = c\,

The photon quantum relations are: :

E=hf \,

and

p=h/\lambda \,

Here h is Planck's constant. So one can also write the dispersion relation as :

E=pc \,

which is characteristic of a zero-mass particle. One sees how remarkably Planck's constant relates the wave and particle aspects. In a material, photons couple to the excitations of the medium and behave differently. These excitations can often be described as quasi-particles (such as phonons and excitons); that is, as quantized wave- or particle-like entities propagating though the matter. "Coupling" means here that photons can transform into these excitations (that is, the photon gets absorbed and medium excited, involving the creation of a quasi-particle) and vice versa (the quasi-particle transforms back into a photon, or the medium relaxes by re-emitting the energy as a photon). However, as these transformations are only possibilities, they are not bound to happen and what actually propagates through the medium is a polariton; that is, a quantum-mechanical superposition of the energy quantum being a photon and of it being one of the quasi-particle matter excitations. According to the rules of quantum mechanics, a measurement (here: just observing what happens to the polariton) breaks this superposition; that is, the quantum either gets absorbed in the medium and stays there (likely to happen in opaque media) or it re-emerges as photon from the surface into space (likely to happen in transparent media). Matter excitations have a non-linear dispersion relation; that is, their momentum is not proportional to their energy. Hence, these particles propagate slower than the vacuum speed of light. (The propagation speed is the derivative of the dispersion relation with respect to momentum.) This is the formal reason why light is slower in media (such as glass) than in vacuum. (The reason for diffraction can be deduced from this by Huygens' principle.) Another way of phrasing it is to say that the photon, by being blended with the matter excitation to form a polariton, acquires an effective mass, which means that it cannot travel at c, the speed of light in a vacuum.