Editing Atom (section)

=== 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>