Editing Atom (section)

== Properties ==
=== Nuclear properties ===
{{Main|Isotope|Stable isotope|List of nuclides|List of elements by stability of isotopes}}
By definition, any two atoms with an identical number of ''protons'' in their nuclei belong to the same [[chemical element]]. Atoms with equal numbers of protons but a different number of ''neutrons'' are different isotopes of the same element. For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons ([[hydrogen-1]], by far the most common form,<ref name=matis2000 /> also called protium), one neutron ([[deuterium]]), two neutrons ([[tritium]]) and [[isotopes of hydrogen|more than two neutrons]]. The known elements form a set of atomic numbers, from the single-proton element [[hydrogen]] up to the 118-proton element [[oganesson]].<ref name=weiss20061017 /> All known isotopes of elements with atomic numbers greater than 82 are radioactive, although the radioactivity of element 83 ([[bismuth]]) is so slight as to be practically negligible.<ref name=s131>{{cite book|last=Sills|first=Alan D.|year=2003|title=Earth Science the Easy Way|publisher=Barron's Educational Series|isbn=978-0-7641-2146-3|oclc=51543743|pages=[https://archive.org/details/earthscienceeasy00alan/page/131 131–134]|url=https://archive.org/details/earthscienceeasy00alan/page/131}}</ref><ref name=dume20030423 />

About 339 nuclides occur naturally on [[Earth]],<ref name=lidsay20000730 /> of which 251 (about 74%) have not been observed to decay, and are referred to as "[[stable isotope]]s". Only 90 nuclides are stable [[list of nuclides|theoretically]], while another 161 (bringing the total to 251) have not been observed to decay, even though in theory it is energetically possible. These are also formally classified as "stable". An additional 35 radioactive nuclides have half-lives longer than 100 million years, and are long-lived enough to have been present since the birth of the [[Solar System]]. This collection of 286 nuclides are known as [[primordial nuclide]]s. Finally, an additional 53 short-lived nuclides are known to occur naturally, as daughter products of primordial nuclide decay (such as [[radium]] from [[uranium]]), or as products of natural energetic processes on Earth, such as cosmic ray bombardment (for example, carbon-14).<ref name=tuli2005 /><ref group=note>For more recent updates see [[Brookhaven National Laboratory]]'s [http://www.nndc.bnl.gov/chart Interactive Chart of Nuclides] ] {{Webarchive|url=https://web.archive.org/web/20200725182342/https://www.nndc.bnl.gov/nudat2/ |date=25 July 2020 }}.</ref><!-- See article [[list of nuclides]]. The numbers are derived by [[WP:CALC]] (counting the table), which is not [[WP:OR]]-->

For 80 of the chemical elements, at least one [[stable isotope]] exists. As a rule, there is only a handful of stable isotopes for each of these elements, the average being 3.1 stable isotopes per element. Twenty-six "[[monoisotopic element]]s" have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten, for the element [[tin]]. Elements [[technetium|43]], [[promethium|61]], and all elements numbered [[bismuth|83]] or higher have no stable isotopes.<ref name=CRC>CRC Handbook (2002).</ref>{{rp|1–12}}

Stability of isotopes is affected by the ratio of protons to neutrons, and also by the presence of certain "magic numbers" of neutrons or protons that represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the [[Nuclear shell model|shell model]] of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. Of the 251 known stable nuclides, only four have both an odd number of protons ''and'' odd number of neutrons: [[hydrogen-2]] ([[deuterium]]), [[lithium-6]], [[boron-10]], and [[nitrogen-14]]. ([[Tantalum-180m]] is odd-odd and observationally stable, but is predicted to decay with a very long half-life.) Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: [[potassium-40]], [[vanadium-50]], [[lanthanum-138]], and [[lutetium-176]]. Most odd-odd nuclei are highly unstable with respect to [[beta decay]], because the decay products are even-even, and are therefore more strongly bound, due to [[Semi-empirical mass formula#Pairing term|nuclear pairing effects]].<ref>{{cite book |last=Krane |first=K. |year=1988 |title=Introductory Nuclear Physics |url=https://archive.org/details/introductorynucl00kran |url-access=limited |publisher=[[John Wiley & Sons]] |isbn=978-0-471-85914-7 |pages=[https://archive.org/details/introductorynucl00kran/page/n90 68]}}</ref>

=== Mass ===
{{Main|Atomic mass|mass number}}

The large majority of an atom's mass comes from the protons and neutrons that make it up. The total number of these particles (called "nucleons") in a given atom is called the [[mass number]]. It is a positive integer and dimensionless (instead of having dimension of mass), because it expresses a count. An example of use of a mass number is "carbon-12," which has 12 nucleons (six protons and six neutrons).

The actual [[Invariant mass|mass of an atom at rest]] is often expressed in [[dalton (unit)|daltons]] (Da), also called the unified atomic mass unit (u). This unit is defined as a twelfth of the mass of a free neutral atom of [[carbon-12]], which is approximately {{val|1.66|e=-27|u=kg}}.<ref name=iupac /> [[hydrogen atom|Hydrogen-1]] (the lightest isotope of hydrogen which is also the nuclide with the lowest mass) has an atomic weight of 1.007825&nbsp;Da.<ref name=chieh2001 /> The value of this number is called the [[atomic mass]]. A given atom has an atomic mass approximately equal (within 1%) to its mass number times the atomic mass unit (for example the mass of a nitrogen-14 is roughly 14&nbsp;Da), but this number will not be exactly an integer except (by definition) in the case of carbon-12.<ref name=nist_wc /> The heaviest [[stable atom]] is lead-208,<ref name=s131 /> with a mass of {{val|207.9766521|u=Da}}.<ref name=audi2003 />

As even the most massive atoms are far too light to work with directly, chemists instead use the unit of [[Mole (unit)|moles]]. One mole of atoms of any element always has the same number of atoms (about [[Avogadro constant|{{val|6.022|e=23}}]]). This number was chosen so that if an element has an atomic mass of 1&nbsp;u, a mole of atoms of that element has a mass close to one gram. Because of the definition of the [[Atomic mass unit|unified atomic mass unit]], each carbon-12 atom has an atomic mass of exactly 12&nbsp;Da, and so a mole of carbon-12 atoms weighs exactly 0.012&nbsp;kg.<ref name=iupac>{{cite book
|last=Mills
|first=Ian
|author2=Cvitaš, Tomislav
|author3=Homann, Klaus
|author4=Kallay, Nikola
|author5=Kuchitsu, Kozo
|title=Quantities, Units and Symbols in Physical Chemistry
|publisher=[[International Union of Pure and Applied Chemistry]], Commission on Physiochemical Symbols Terminology and Units, Blackwell Scientific Publications
|location=Oxford
|edition=2nd
|year=1993
|isbn=978-0-632-03583-0
|oclc=27011505
|url=https://archive.org/details/quantitiesunitss0000unse/page/70
|page=[https://archive.org/details/quantitiesunitss0000unse/page/70 70]
}}</ref>

=== Shape and size ===
{{Main|Atomic radius}}
Atoms lack a well-defined outer boundary, so their dimensions are usually described in terms of an [[atomic radius]]. This is a measure of the distance out to which the electron cloud extends from the nucleus.<ref name=Ghosh02>{{cite journal | author = Ghosh, D.C. |author2= Biswas, R. | title = Theoretical calculation of Absolute Radii of Atoms and Ions. Part 1. The Atomic Radii | journal = Int. J. Mol. Sci. | volume = 3 |issue= 11 | pages = 87–113 | year = 2002 | doi=10.3390/i3020087| doi-access = free }}</ref> This assumes the atom to exhibit a spherical shape, which is only obeyed for atoms in vacuum or free space. Atomic radii may be derived from the distances between two nuclei when the two atoms are joined in a [[chemical bond]]. The radius varies with the location of an atom on the atomic chart, the type of chemical bond, the number of neighboring atoms ([[coordination number]]) and a [[quantum mechanics|quantum mechanical]] property known as [[Spin (physics)|spin]].<ref name=aca32_5_751 /> On the [[periodic table]] of the elements, atom size tends to increase when moving down columns, but decrease when moving across rows (left to right).<ref name=dong1998 /> Consequently, the smallest atom is [[helium]] with a radius of 32&nbsp;[[Picometre|pm]], while one of the largest is [[caesium]] at 225&nbsp;pm.<ref>{{cite book
|last=Zumdahl|first=Steven S.|year=2002
|title=Introductory Chemistry: A Foundation
|edition=5th|publisher=Houghton Mifflin
|url=http://college.hmco.com/chemistry/intro/zumdahl/intro_chemistry/5e/students/protected/periodictables/pt/pt/pt_ar5.html
|isbn=978-0-618-34342-3
|oclc=173081482| archive-url= https://web.archive.org/web/20080304155935/http://college.hmco.com/chemistry/intro/zumdahl/intro_chemistry/5e/students/protected/periodictables/pt/pt/pt_ar5.html| archive-date= 4 March 2008 | url-status= live}}</ref>

When subjected to external forces, like [[electrical field]]s, the shape of an atom may deviate from [[spherical symmetry]]. The deformation depends on the field magnitude and the orbital type of outer shell electrons, as shown by [[group theory|group-theoretical]] considerations. Aspherical deviations might be elicited for instance in [[crystal]]s, where large crystal-electrical fields may occur at [[crystal symmetry|low-symmetry]] lattice sites.<ref name= Bethe1929>{{cite journal|author = Bethe, Hans|title = Termaufspaltung in Kristallen|journal = Annalen der Physik|volume = 3|issue = 2|pages = 133–208|year = 1929|doi = 10.1002/andp.19293950202|bibcode = 1929AnP...395..133B }}</ref><ref name= ZPB1995a>{{cite journal | author = Birkholz, Mario | title = Crystal-field induced dipoles in heteropolar crystals – I. concept | journal = Z. Phys. B | volume = 96 | issue = 3 | pages = 325–332 | year = 1995 | doi = 10.1007/BF01313054 |bibcode = 1995ZPhyB..96..325B | url=https://www.researchgate.net/publication/227050494| citeseerx = 10.1.1.424.5632 | s2cid = 122527743 }}</ref> Significant [[ellipsoid]]al deformations have been shown to occur for sulfur ions<ref name=pssb2008>{{cite journal | author = Birkholz, M. | author2 = Rudert, R. | title = Interatomic distances in pyrite-structure disulfides – a case for ellipsoidal modeling of sulfur ions | journal = Physica Status Solidi B | volume = 245 | issue = 9 | pages = 1858–1864 | year = 2008 | url = https://www.mariobirkholz.de/pssb2008.pdf | doi = 10.1002/pssb.200879532 | bibcode = 2008PSSBR.245.1858B | s2cid = 97824066 | access-date = 2 May 2021 | archive-date = 2 May 2021 | archive-url = https://web.archive.org/web/20210502151542/https://www.mariobirkholz.de/pssb2008.pdf | url-status = live }}</ref> and [[chalcogen]] ions<ref name=mdpi2014>{{cite journal | author = Birkholz, M. | title = Modeling the Shape of Ions in Pyrite-Type Crystals| journal = Crystals | volume = 4 | issue = 3| pages = 390–403 | year = 2014 | doi = 10.3390/cryst4030390| doi-access = free| bibcode = 2014Cryst...4..390B}}</ref> in [[pyrite]]-type compounds.

Atomic dimensions are thousands of times smaller than the wavelengths of [[light]] (400–700&nbsp;[[nanometre|nm]]) so they cannot be viewed using an [[optical microscope]], although individual atoms can be observed using a [[scanning tunneling microscope]]. To visualize the minuteness of the atom, consider that a typical human hair is about 1&nbsp;million carbon atoms in width.<ref name=osu2007 /> A single drop of water contains about 2&nbsp;[[sextillion]] ({{val|2|e=21}}) atoms of oxygen, and twice the number of hydrogen atoms.<ref>{{cite book
|last=Padilla|first=Michael J.
|author2=Miaoulis, Ioannis|author3= Cyr, Martha|year = 2002
|title = Prentice Hall Science Explorer: Chemical Building Blocks
|publisher = Prentice-Hall, Inc.
|location = Upper Saddle River, New Jersey
|isbn = 978-0-13-054091-1
|oclc=47925884|page=32
|quote=There are 2,000,000,000,000,000,000,000 (that's 2&nbsp;sextillion) atoms of oxygen in one drop of water—and twice as many atoms of hydrogen.}}</ref> A single [[Carat (unit)|carat]] [[diamond]] with a mass of {{val|2|e=-4|u=kg}} contains about 10&nbsp;sextillion (10<sup>22</sup>) atoms of [[carbon]].<ref group=note>A carat is 200&nbsp;milligrams. [[Atomic mass unit|By definition]], carbon-12 has 0.012&nbsp;kg per mole. The [[Avogadro constant]] defines {{val|6|e=23}} atoms per mole.</ref> If an apple were magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple.<ref>{{cite web |url=https://feynmanlectures.caltech.edu/I_01.html#Ch1-S2-p3 |title=The Feynman Lectures on Physics Vol. I Ch. 1: Atoms in Motion |access-date=3 May 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730092955/https://www.feynmanlectures.caltech.edu/I_01.html#Ch1-S2-p3 |url-status=live }}</ref>

=== Radioactive decay ===
{{Main|Radioactive decay}}

[[File:Isotopes and half-life.svg|right|thumb|This diagram shows the [[half-life]] (T<sub>{{frac|1|2}}</sub>) of various isotopes with Z protons and N neutrons.]]

Every element has one or more isotopes that have unstable nuclei that are subject to radioactive decay, causing the nucleus to emit particles or electromagnetic radiation. Radioactivity can occur when the radius of a nucleus is large compared with the radius of the strong force, which only acts over distances on the order of 1&nbsp;fm.<ref name=splung />

The most common forms of radioactive decay are:<ref>{{cite book
|last=L'Annunziata<!-- Note: the single quote mark before the name is correct. -->
|first=Michael F.
|year=2003|title=Handbook of Radioactivity Analysis
|url=https://archive.org/details/handbookradioact00lann|url-access=limited|publisher=Academic Press|isbn=978-0-12-436603-9
|oclc=16212955|pages=[https://archive.org/details/handbookradioact00lann/page/n22 3]–56}}</ref><ref name=firestone20000522 />
* [[Alpha decay]]: this process is caused when the nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. The result of the emission is a new element with a lower [[atomic number]].
* [[Beta decay]] (and [[electron capture]]): these processes are regulated by the [[weak force]], and result from a transformation of a neutron into a proton, or a proton into a neutron. The neutron to proton transition is accompanied by the emission of an electron and an [[antineutrino]], while proton to neutron transition (except in electron capture) causes the emission of a [[positron]] and a [[neutrino]]. The electron or positron emissions are called beta particles. Beta decay either increases or decreases the atomic number of the nucleus by one. Electron capture is more common than positron emission, because it requires less energy. In this type of decay, an electron is absorbed by the nucleus, rather than a positron emitted from the nucleus. A neutrino is still emitted in this process, and a proton changes to a neutron.
* [[Gamma decay]]: this process results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. The excited state of a nucleus which results in gamma emission usually occurs following the emission of an alpha or a beta particle. Thus, gamma decay usually follows alpha or beta decay.

Other more rare types of [[radioactive decay]] include ejection of neutrons or protons or clusters of [[nucleon]]s from a nucleus, or more than one [[beta particle]]. An analog of gamma emission which allows excited nuclei to lose energy in a different way, is [[internal conversion]]—a process that produces high-speed electrons that are not beta rays, followed by production of high-energy photons that are not gamma rays. A few large nuclei explode into two or more charged fragments of varying masses plus several neutrons, in a decay called [[Spontaneous fission|spontaneous nuclear fission]].

Each [[radioactive isotope]] has a characteristic decay time period—the [[half-life]]—that is determined by the amount of time needed for half of a sample to decay. This is an [[exponential decay]] process that steadily decreases the proportion of the remaining isotope by 50% every half-life. Hence after two half-lives have passed only 25% of the isotope is present, and so forth.<ref name=splung />

=== Magnetic moment ===
{{Main|Electron magnetic moment|Nuclear magnetic moment}}

Elementary particles possess an intrinsic quantum mechanical property known as [[Spin (physics)|spin]]. This is analogous to the [[angular momentum]] of an object that is spinning around its [[center of mass]], although strictly speaking these particles are believed to be point-like and cannot be said to be rotating. Spin is measured in units of the reduced [[Planck constant]] (ħ), with electrons, protons and neutrons all having spin {{frac|1|2}}&nbsp;ħ, or "spin-{{frac|1|2}}". In an atom, electrons in motion around the [[Atomic nucleus|nucleus]] possess orbital [[angular momentum]] in addition to their spin, while the nucleus itself possesses angular momentum due to its nuclear spin.<ref name=hornak2006 />

The [[magnetic field]] produced by an atom—its [[magnetic moment]]—is determined by these various forms of angular momentum, just as a rotating charged object classically produces a magnetic field, but the most dominant contribution comes from electron spin. Due to the nature of electrons to obey the [[Pauli exclusion principle]], in which no two electrons may be found in the same [[quantum state]], bound electrons pair up with each other, with one member of each pair in a spin up state and the other in the opposite, spin down state. Thus these spins cancel each other out, reducing the total magnetic dipole moment to zero in some atoms with even number of electrons.<ref name=schroeder2 />

In [[Ferromagnetism|ferromagnetic]] elements such as iron, cobalt and nickel, an odd number of electrons leads to an unpaired electron and a net overall magnetic moment. The orbitals of neighboring atoms overlap and a lower energy state is achieved when the spins of unpaired electrons are aligned with each other, a spontaneous process known as an [[exchange interaction]]. When the magnetic moments of ferromagnetic atoms are lined up, the material can produce a measurable macroscopic field. [[Paramagnetism|Paramagnetic materials]] have atoms with magnetic moments that line up in random directions when no magnetic field is present, but the magnetic moments of the individual atoms line up in the presence of a field.<ref name=schroeder2 /><ref name=goebel20070901 />

The nucleus of an atom will have no spin when it has even numbers of both neutrons and protons, but for other cases of odd numbers, the nucleus may have a spin. Normally nuclei with spin are aligned in random directions because of [[thermal equilibrium]], but for certain elements (such as [[xenon|xenon-129]]) it is possible to [[spin polarization|polarize]] a significant proportion of the nuclear spin states so that they are aligned in the same direction—a condition called [[hyperpolarization (physics)|hyperpolarization]]. This has important applications in [[magnetic resonance imaging]].<ref name=yarris1997 /><ref>{{cite book
|last1=Liang|first1=Z.-P.|last2=Haacke|first2=E.M.
|editor=Webster, J.G.|year=1999
|volume=2
|title=Encyclopedia of Electrical and Electronics Engineering: Magnetic Resonance Imaging
|publisher=John Wiley & Sons
|isbn=978-0-471-13946-1|pages=412–426}}</ref>

=== Energy levels ===
[[File:Atomic orbital energy levels.svg|thumb|right|These electron's energy levels (not to scale) are sufficient for ground states of atoms up to [[cadmium]] (5s<sup>2</sup> 4d<sup>10</sup>) inclusively. The top of the diagram is lower than an unbound electron state.]]
The [[potential energy]] of an electron in an atom is [[negative number|negative]] relative to when the [[distance]] from the nucleus [[limit at infinity|goes to infinity]]; its dependence on the electron's [[position (vector)|position]] reaches the [[minimum]] inside the nucleus, roughly in [[inverse proportion]] to the distance. In the quantum-mechanical model, a bound electron can occupy only a set of [[quantum state|states]] centered on the nucleus, and each state corresponds to a specific [[energy level]]; see [[time-independent Schrödinger equation]] for a theoretical explanation. An energy level can be measured by the [[ionization potential|amount of energy needed to unbind]] the electron from the atom, and is usually given in units of [[electronvolt]]s (eV). The lowest energy state of a bound electron is called the ground state, i.e., [[stationary state]], while an electron transition to a higher level results in an excited state.<ref name=zeghbroeck1998 /> The electron's energy increases along with [[principal quantum number|''n'']] because the (average) distance to the nucleus increases. Dependence of the energy on [[azimuthal quantum number|{{ell}}]] is caused not by the [[electrostatic potential]] of the nucleus, but by interaction between electrons.

For an electron to [[atomic electron transition|transition between two different states]], e.g. [[ground state]] to first [[excited state]], it must absorb or emit a [[photon]] at an energy matching the difference in the potential energy of those levels, according to the [[Niels Bohr]] model, what can be precisely calculated by the [[Schrödinger equation]]. Electrons jump between orbitals in a particle-like fashion. For example, if a single photon strikes the electrons, only a single electron changes states in response to the photon; see [[Atomic orbital|Electron properties]].

The energy of an emitted photon is proportional to its [[frequency]], so these specific energy levels appear as distinct bands in the [[electromagnetic spectrum]].<ref>{{cite book
|last=Fowles|first=Grant R.|year=1989
|title=Introduction to Modern Optics
|url=https://archive.org/details/introductiontomo00fowl_441|url-access=limited|publisher=Courier Dover Publications
|isbn=978-0-486-65957-2
|oclc=18834711|pages=[https://archive.org/details/introductiontomo00fowl_441/page/n233 227]–233}}</ref> Each element has a characteristic spectrum that can depend on the nuclear charge, subshells filled by electrons, the electromagnetic interactions between the electrons and other factors.<ref name=martin2007 />

[[File:Fraunhofer lines.svg|right|thumb|upright=1.5|An example of absorption lines in a spectrum]]

When a continuous [[electromagnetic spectrum|spectrum of energy]] is passed through a gas or plasma, some of the photons are absorbed by atoms, causing electrons to change their energy level. Those excited electrons that remain bound to their atom spontaneously emit this energy as a photon, traveling in a random direction, and so drop back to lower energy levels. Thus the atoms behave like a filter that forms a series of dark [[absorption band]]s in the energy output. (An observer viewing the atoms from a view that does not include the continuous spectrum in the background, instead sees a series of [[emission line]]s from the photons emitted by the atoms.) [[Spectroscopy|Spectroscopic]] measurements of the strength and width of [[atomic spectral line]]s allow the composition and physical properties of a substance to be determined.<ref name=avogadro />

Close examination of the spectral lines reveals that some display a [[fine structure]] splitting. This occurs because of [[spin–orbit interaction|spin–orbit coupling]], which is an interaction between the spin and motion of the outermost electron.<ref name=fitzpatrick20070216 /> When an atom is in an external magnetic field, spectral lines become split into three or more components; a phenomenon called the [[Zeeman effect]]. This is caused by the interaction of the magnetic field with the magnetic moment of the atom and its electrons. Some atoms can have multiple [[electron configuration]]s with the same energy level, which thus appear as a single spectral line. The interaction of the magnetic field with the atom shifts these electron configurations to slightly different energy levels, resulting in multiple spectral lines.<ref name=weiss2001 /> The presence of an external [[electric field]] can cause a comparable splitting and shifting of spectral lines by modifying the electron energy levels, a phenomenon called the [[Stark effect]].<ref>{{cite book
|last1=Beyer|first1=H.F.
|last2=Shevelko|first2=V.P.
|year=2003
|title=Introduction to the Physics of Highly Charged Ions
|publisher=CRC Press|isbn=978-0-7503-0481-8
|oclc=47150433|pages=232–236}}</ref>

If a bound electron is in an excited state, an interacting photon with the proper energy can cause [[stimulated emission]] of a photon with a matching energy level. For this to occur, the electron must drop to a lower energy state that has an energy difference matching the energy of the interacting photon. The emitted photon and the interacting photon then move off in parallel and with matching phases. That is, the wave patterns of the two photons are synchronized. This physical property is used to make [[laser]]s, which can emit a coherent beam of light energy in a narrow frequency band.<ref name=watkins_sjsu />

=== Valence and bonding behavior ===
{{Main|Valence (chemistry)|Chemical bond}}

Valency is the combining power of an element. It is determined by the number of bonds it can form to other atoms or groups.<ref>{{GoldBookRef|title=valence|file=V06588}}</ref> The outermost electron shell of an atom in its uncombined state is known as the [[valence shell]], and the electrons in
that shell are called [[valence electron]]s. The number of valence electrons determines the [[chemical bond|bonding]]
behavior with other atoms. Atoms tend to [[Chemical reaction|chemically react]] with each other in a manner that fills (or empties) their outer valence shells.<ref name=reusch20070716 /> For example, a transfer of a single electron between atoms is a useful approximation for bonds that form between atoms with one-electron more than a filled shell, and others that are one-electron short of a full shell, such as occurs in the compound [[sodium chloride]] and other chemical ionic salts. Many elements display multiple valences, or tendencies to share differing numbers of electrons in different compounds. Thus, [[chemical bond]]ing between these elements takes many forms of electron-sharing that are more than simple electron transfers. Examples include the element carbon and the [[organic compounds]].<ref name=chemguide />

The [[chemical element]]s are often displayed in a [[periodic table]] that is laid out to display recurring chemical properties, and elements with the same number of valence electrons form a group that is aligned in the same column of the table. (The horizontal rows correspond to the filling of a quantum shell of electrons.) The elements at the far right of the table have their outer shell completely filled with electrons, which results in chemically inert elements known as the [[noble gas]]es.<ref name=husted20031211 /><ref name=baum2003 />

=== States ===
{{Main|State of matter|Phase (matter)}}

[[File:Bose Einstein condensate.png|right|thumb|Graphic illustrating the formation of a [[Bose–Einstein condensate]] ]]
Quantities of atoms are found in different states of matter that depend on the physical conditions, such as [[temperature]] and [[pressure]]. By varying the conditions, materials can transition between [[solid]]s, [[liquid]]s, [[gas]]es, and [[plasma (physics)|plasmas]].<ref>
{{cite book
 |last=Goodstein|first=David L.|year=2002
 |title=States of Matter|url=https://archive.org/details/statesmatter00good_082|url-access=limited|publisher=Courier Dover Publications
 |isbn=978-0-13-843557-8|pages=[https://archive.org/details/statesmatter00good_082/page/n445 436]–438
}}</ref> Within a state, a material can also exist in different [[allotropes]]. An example of this is solid carbon, which can exist as [[graphite]] or [[diamond]].<ref name="pu49_7_719" /> Gaseous allotropes exist as well, such as [[dioxygen]] and [[ozone]].

At temperatures close to [[absolute zero]], atoms can form a [[Bose–Einstein condensate]], at which point quantum mechanical effects, which are normally only observed at the atomic scale, become apparent on a macroscopic scale.<ref>
{{cite book
 |last=Myers|first=Richard|year=2003
 |title=The Basics of Chemistry|url=https://archive.org/details/basicschemistry00myer|url-access=limited|publisher=Greenwood Press
 |isbn=978-0-313-31664-7
 |oclc=50164580|page=[https://archive.org/details/basicschemistry00myer/page/n98 85]
}}</ref><ref name=nist_bec /> This super-cooled collection of atoms then behaves as a single [[super atom]], which may allow fundamental checks of quantum mechanical behavior.<ref name=colton_fyffe1999 />
{{clear}}