Atom
Helium atom model (not to scale) Showing nucleus with two protons (red) and two neutrons (green) and with a probability cloud (gray) of two electrons (yellow).
Classification
Smallest recognised division of a chemical element
Properties
Mass: atomic mass Electric Charge: 0 C Diameter: 10pm to 100pm

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 around us. It is the smallest unit of an element to retain all the properties of that element. The word atom originally meant a smallest possible particle of matter, not further divisible. Later, those objects to which the name atom had been assigned were found to be further divisible into smaller subatomic particles, but the word atom nonetheless continues to refer to them. Atoms are canonically distinguished from ions by their balanced electrical charge. When this charge is disrupted, the particle is then considered to be an atomic ion rather than an atom. Helium atom (not to scale) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... A chemical element, often called simply element, is the class of atoms which contain the same number of protons. ... The atomic mass of an element (also known as the relative atomic mass or average atomic mass or atomic weight) is the average atomic mass of all the chemical elements isotopes as found in a particular environment, weighted by isotopic abundance. ... The coulomb (symbol: C) is the SI unit of electric charge. ... To help compare different orders of magnitude this page lists lengths between 10 pm and 100 pm (10-11 m and 10-12 m). ... To help compare different orders of magnitude this page lists lengths between 100 pm and 1 nm (10-10 m and 10-9 m). ... An atom is the following: Generally, an atom is the smallest irreducible constituent of a chemical system. ... Matter is commonly referred to as the substance of which physical objects are composed. ... The term element can refer to: Chemical element — material that consists of atoms with the same number of protons in the nucleus (see also Periodic table). ... Helium atom (not to scale) Showing two protons (red), two neutrons (green) and a probability cloud (gray) of two electrons (yellow). ... An ion is an atom or group of atoms with a net electric charge. ...

Most atoms are composed of 3 types of massive subatomic particles which govern their external properties: A subatomic particle is a particle smaller than an atom: it may be elementary or composite. ...

• electrons, which have a negative charge;
• protons, which have a positive charge; and
• neutrons, which have no charge.

Antimatter can form atoms, usually composed of antielectrons, antiprotons, and antineutrons. Properties The electron is a fundamental subatomic particle which carries a negative electric charge. ... Negative has meaning in several contexts: Look up negative in Wiktionary, the free dictionary Negative and non-negative numbers Negative (photography) In optics, diverging lenses are also called negative lenses. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. ... Properties In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... In common usage positive is sometimes used in affirmation, as a synonym for yes or to express certainty. Look up Positive on Wiktionary, the free dictionary In mathematics, a number is called positive if it is bigger than zero. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. ... Properties In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. ...

Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Atoms can be classed into elements, which are a useful tool for predicting chemical reactivity. Chemistry (in Greek: χημεία) is the science of matter that deals with the composition, structure, and properties of substances and with the transformations that they undergo. ... This article needs to be cleaned up to conform to a higher standard of quality. ... Chemical reactions are also known as chemical changes. ... Generally, an element is a basic part that is the foundation of something. ...

Elements 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. The chemical elements labelled as synthetic are unstable, with a half-life so short (ranging from a fraction of millisecond to a few million years) relative to the age of the Earth that any atoms of that element that may have been present when the Earth formed have long since... Radioactive decay is the set of various processes by which unstable atomic nuclei (nuclides) emit subatomic particles (radiation). ...

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 2 hydrogen and one oxygen atom. A chemical bond is the phenomenon of atoms being held together in molecules or crystals. ... A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ...

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. A Bose-Einstein condensate is a gaseous superfluid phase formed by atoms cooled to temperatures very near to absolute zero. ...

Atomic theory

Main article: Atomic theory

The atomic theory is a theory of the nature of matter. It states that all matter is composed of atoms. The atomic theory is a theory of the nature of matter. ... Theory has a number of distinct meanings, depending on the context. ... Matter is commonly referred to as the substance of which physical objects are composed. ...

Atom sizes

The size of an atom is not easily defined since the electron orbitals 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 1.2×10^-10m. Compare this to the size of the proton which is the only particle in the nucleus of the hydrogen atom which is approximately 0.87×10^-15m. 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. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Properties In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... Plural: nuclei In chemistry and physics, the nucleus (atomic nucleus) is the collection of protons and neutrons in the center of an atom that carries the bulk of the atoms mass and positive charge. ... Generally, an element is a basic part that is the foundation of something. ...

Elements and isotopes

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 6 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 atomic number (Z) is a term used in chemistry and physics to represent the number of protons found in the nucleus of an atom. ... A chemical element, often called simply element, is the class of atoms which contain the same number of protons. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... The periodic table of the chemical elements, also called the Mendeleev periodic table, is a tabular display of the known chemical elements. ...

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 mass number (A), also called atomic mass number or nucleon number, is the number of protons and neutrons in an atomic nucleus. ... The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic masses and molecular masses. ... Isotopes are forms of an element whose nuclei have the same atomic number–-the number of protons in the nucleus--but different atomic masses because they contain different numbers of neutrons. ... Carbon-14 is the radioactive isotope of carbon discovered February 27, 1940, by Martin Kamen and Sam Ruben. ...

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 1 neutron is called deuterium or hydrogen-2; the hydrogen isotope with 2 neutrons is called tritium or hydrogen-3. A hydrogen atom is an atom of the element hydrogen. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance of one atom in 6500 of hydrogen. ... Tritium (symbol T or 3H) is a radioactive isotope of hydrogen. ...

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. The atomic mass of an element (also known as the relative atomic mass or average atomic mass or atomic weight) is the average atomic mass of all the chemical elements isotopes as found in a particular environment, weighted by isotopic abundance. ...

Valence and bonding

see main article valence electrons and chemical bond

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 which is usually visualized as their mean distance 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. In chemistry, valence electrons are the electrons contained in the valence shell (the outermost electron level) of an atom, and which are likely to participate in a chemical reaction through bonding with other atoms, molecules, or ions. ... A chemical bond is the phenomenon of atoms being held together in molecules or crystals. ... Electron configuration is the arrangement of electrons in an atom, molecule or other body. ... In atomic physics, an electron shell is a group of atomic orbitals with the same value of the principal quantum number n. ... In chemistry, valence electrons are the electrons contained in the valence shell (the outermost electron level) of an atom, and which are likely to participate in a chemical reaction through bonding with other atoms, molecules, or ions. ...

The atomic orbital wavefunctions of hydrogen atom hydrogen. 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.

Each shell, numbered from the one closest to the nucleus (lowest in energy), can hold up to a specific number of electrons due to its differing sublevel and orbital capacity: First few hydrogen atom orbitals; cross section showing color-coded probability density for different n=1,2,3 and l=s,p,d; note: m=0 The picture shows the first few hydrogen atom orbitals (energy eigenfunctions). ... Electron atomic and molecular orbitals In quantum mechanics, the states of an atom, i. ... In quantum mechanics, the wavefunction associated with a particle such as an electron, is a complex-valued function ψ defined over a portion of space and normalized in such a way that In Max Borns probabilistic interpretation of the wavefunction, the amplitude squared of the wavefunction |ψ(x)|2 is the... In atomic physics, the Principal quantum number (usually written n) is the first quantum number of an atomic orbital. ... The Azimuthal quantum number (or orbital angular momentum quantum number) l is a quantum number for an atomic orbital which determines its orbital angular momentum. ...

• Shell 1: 2 electron capacity - s sublevel - 1 orbital
• Shell 2: 8 electron capacity - s and p sublevels - 4 orbitals
• Shell 3: 18 electron capacity - s, p, and d sublevels - 9 orbitals
• Shell 4: 32 electron capacity - s, p, d, and f sublevels - 16 orbitals

To determine the electron capacity of a shell, the formula 2n² is used, where n is the shell number or principle quantum number. Electrons fill orbitals and shells from the inside out, beginning with shell one. Whichever occupied shell is currently most outward is the valence shell, even if it only has one electron.

The reason why shells fill up in order is that the energy levels of electrons in the innermost shells are significantly lower than the energy levels of electrons in outer shells. So if the inner shells were not completely full, the electron in an outer shell would quickly "fall" into the inner shell (with the emission of a photon that would carry away the difference in the energy. A quantum mechanic system can only be in certain states, so that only certain energy levels are possible. ... For the Science Fiction weapon, as seen in Star Trek, see Photon torpedo. ...

The number of electrons in an atom's outermost 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. The periodic table of the chemical elements, also called the Mendeleev periodic table, is a tabular display of the known chemical elements. ... Reactivity refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. ... General Name, Symbol, Number caesium, Cs, 55 Chemical series alkali metals Group, Period, Block 1, 6, s Appearance silvery gold Atomic mass 132. ... General Name, Symbol, Number rubidium, Rb, 37 Chemical series alkali metals Group, Period, Block 1, 5, s Appearance silvery white Atomic mass 85. ... General Name, Symbol, Number francium, Fr, 87 Chemical series alkali metals Group, Period, Block 1, 7, s Appearance metallic Atomic mass (223) g/mol Electron configuration [Rn] 7s1 Electrons per shell 2, 8, 18, 32, 18, 8, 1 Physical properties Phase solid Density (near r. ...

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: A chemical bond is the phenomenon of atoms being held together in molecules or crystals. ... A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ...

• ionic bonds;
• covalent bonds;
• coordinate covalent bonds;
• hydrogen bonds; and
• metallic bonds.

An ionic bond can be formed after two or more atoms lose or gain electrons to form an ion. ... Covalently bonded hydrogen and carbon in a molecule of methane. ... A coordinate covalent bond (also known as dative covalent bond) is a special type of covalent bond in which the shared electrons come from one of the atoms only. ... Animation of H-bond formation, from Robert Wyatt, Western Kentucky Univ. ... Metallic bonding is bonding within metals. ...

History

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. Bust of Democritus Democritus was a pre-Socratic Greek philosopher (born at Abdera in Thrace around 460 BC; died in 370 BC). ... This article is about the philosopher. ... Wikipedia does not yet have an article with this exact name. ... (6th century BC - 5th century BC - 4th century BC - other centuries) (2nd millennium BC - 1st millennium BC - 1st millennium AD) The 5th and 6th centuries BC are a period of philosophical brilliance among advanced civilizations. ... Atomism is the theory that all the objects in the universe are composed of very small, indestructible particles. ... John Dalton John Dalton (September 6, 1766 – July 27, 1844) was a British chemist and physicist, born at Eaglesfield, near Cockermouth in Cumberland. ... Amedeo Avogadro Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto (August 9, 1776–July 9, 1856) was an Italian scientist, most noted for his contributions to the theory of molarity and molecular weight. ... Alternative meaning: Nineteenth Century (periodical) (18th century — 19th century — 20th century — more centuries) As a means of recording the passage of time, the 19th century was that century which lasted from 1801-1900 in the sense of the Gregorian calendar. ... The concept of infinite divisibility arises in different ways in philosophy, physics, economics, order theory (a branch of mathematics), and probability theory (also a branch of mathematics). ... Matter is commonly referred to as the substance of which physical objects are composed. ...

The controversy settled down in 1911 by Perrin when he discovered the meta-particle we nowadays call an atom. Jean Perrin thought that he had found the "atomos" that Democritus talked about and so named his particles atoms. 1911 was a common year starting on Sunday (click on link for calendar). ... Jean Baptiste Perrin, generally known as Jean Perrin (Lille, September 30, 1870 – April 17, New York, 1942), was a French physicist. ... Bust of Democritus Democritus was a pre-Socratic Greek philosopher (born at Abdera in Thrace around 460 BC; died in 370 BC). ...

Through this time, atoms were thought to be the smallest possible piece of matter. However, it was later shown that atoms are made up of subatomic particles. Thomson's experiments discovered the electron, the first of the subatomic particles to be discovered. This showed that atoms are actually 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. Matter is commonly referred to as the substance of which physical objects are composed. ... A subatomic particle is a particle smaller than an atom: it may be elementary or composite. ... Sir Joseph John Thomson, OM, PRS (December 18, 1856 – August 30, 1940) often known as J. J. Thomson, was an English physicist, the discoverer of the electron. ... Properties The electron is a fundamental subatomic particle which carries a negative electric charge. ... Bust of Democritus Democritus was a pre-Socratic Greek philosopher (born at Abdera in Thrace around 460 BC; died in 370 BC). ... Particles erupt from the collision point of two relativistic (100 GeV) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ...

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 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. A schematic representation of the plum pudding model of the atom. ... Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. ... An alpha particle is deflected by a magnetic field Alpha particles or alpha rays are a form of particle radiation which are highly ionizing and have low penetration. ... General Name, Symbol, Number polonium, Po, 84 Chemical series metalloids Group, Period, Block 16, 6, p Appearance silvery Atomic mass (209) g/mol Electron configuration [Xe] 4f14 5d10 6s2 6p4 Electrons per shell 2, 8, 18, 32, 18, 6 Physical properties Phase solid Density (near r. ...

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 proved in 1932 by James Chadwick. Properties In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... Properties In physics, the neutron is a subatomic particle with no net electric charge and a mass of 940 MeV/c² (1. ... Sir James Chadwick (October 20, 1897 – July 24, 1974) was an English physicist and Nobel laureate. ...

Later, experiments by Max Planck and Albert Einstein demonstrated that energy is transferred in tiny fixed amounts known as quanta. This led Bohr to propose an updated model, wherein the electrons orbited the nucleus in fixed circles. Because their energy could only change by fixed quantities, they couldn't move closer to or farther from the nucleus in spirals; they could only make quantum leaps from one circle to the next. Max Planck This article is about Planck, the German physicist. ... Albert Einstein, by Yousuf Karsh Albert Einstein (March 14, 1879 – April 18, 1955) was a German-born Jewish theoretical physicist of German, Swiss and American citizenship, who is widely regarded as the greatest scientist of the 20th century or even of all time. ... In physics quanta is the plural of quantum. ... This article is about the TV show. ...

Study of atoms

The study of atoms was done by largely 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, has allowed pictures to be taken of actual, individual atoms. Atomic force microscopy is another technique by which individual atoms can be visualized. Methods also exist to identify atoms and compounds. Elemental analysis allows the exact identification of the types and amounts of atoms in a substance. Alternative meaning: Nineteenth Century (periodical) (18th century — 19th century — 20th century — more centuries) As a means of recording the passage of time, the 19th century was that century which lasted from 1801-1900 in the sense of the Gregorian calendar. ... (19th century - 20th century - 21st century - more centuries) Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s As a means of recording the passage of time, the 20th century was that century which lasted from 1901–2000 in the sense of the Gregorian calendar (1900–1999 in the... A transmission electron microscope. ... 1931 is a common year starting on Thursday. ... The atomic force microscope (AFM) is a very powerful microscope invented by Binnig, Quate and Gerber in 1986. ... Elemental analysis is a process where a sample of some material (eg. ...

See also

• Atomism
• Chemical bond
• Exotic atom
• Individual (same literal meaning)
• Infinite divisibility
• List of particles
• Radioactive isotope
• Superatom
• Super-heavy atom
• Transuranium element

Atomism is the theory that all the objects in the universe are composed of very small, indestructible particles. ... A chemical bond is the phenomenon of atoms being held together in molecules or crystals. ... An exotic atom is the anologue of a normal atom in which one or more of the the electrons are replaced by other negative particles, such as a muon or a pion, or the positively charged nucleus is replaced by other positively charged elementary particles, or both. ... In common speech, the word individual most often refers to a person, or, by analogy, to any specific object in a group of things. ... The concept of infinite divisibility arises in different ways in philosophy, physics, economics, order theory (a branch of mathematics), and probability theory (also a branch of mathematics). ... List of particles in particle physics. ... A radionuclide is an atom with an unstable nucleus. ... Superatoms are clusters of atoms which seem to exhibit some of the properties of elemental atoms. ... In chemistry, transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, the atomic number of uranium. ... In chemistry, transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, the atomic number of uranium. ...

External links and references

• The American Heritage Dictionary entry on atom (contains popups)
• The Universe, at Sacramento Peak
• How Atoms Work by howstuffworks.