Editing Atomic radius (section)

==History==

The first to estimate the radius of an atom was [[Johann Chrysostom Magnenus]] in 1646. He was at [[Mass (liturgy)|Mass]] and noticed the smell of incense permeating the church. He knew the size of the incense and estimated the size of the church. He presumed that he could detect the incense if one atom was in each nostril. He also presumed that the incense was distributed homogenously throughout the church. With these assumptions he was able to estimate the size of an atom to be about 10 to the power of −24 cubic metres. (The units he used have been converted to metric to make comparisons with later estimates easier.) Taking the cube root this gives an estimate of the atomic radius to be about 10 to the power of −8 metres. This is somewhat larger than current estimates but given the assumptions made in the calculation is very good. These calculations were published in his work ''Democritus reviviscens sive de atomis''.   

The concept of atomic radius was preceded in the 19th century by the concept of atomic volume, a relative measure of how much space would on average an atom occupy in a given solid or liquid material.<ref>{{Cite book |last=Knight |first=Charles |url=https://books.google.com/books?id=vHdBAAAAcAAJ&pg=PA711 |title=The English Cyclopaedia: A New Dictionary of Universal Knowledge |date=1859 |publisher=Bradbury and Evans |language=en}}</ref> By the end of the century this term was also used in an absolute sense, as a [[molar volume]] divided by [[Avogadro constant]].<ref>{{Cite journal |last=Fessenden |first=Reginald A. |author-link=Reginald Fessenden |date=1892-07-22 |title=The Laws and Nature of Cohesion |url=https://books.google.com/books?id=7EAwLffl2WEC&pg=PA49 |journal=Science |language=en |volume=ns-20 |issue=494 |pages=48–52 |doi=10.1126/science.ns-20.494.48.b |issn=0036-8075}}</ref> Such a volume is different for different crystalline forms even of the same compound,<ref>{{Cite book |last=Watts |first=Henry |url=https://books.google.com/books?id=noPIn5j1dAMC&pg=PA432 |title=A Dictionary of chemistry and the allied branches of other sciences v. 3, 1882 |date=1882 |publisher=Longmans, Green & Company |language=en}}</ref> but physicists used it for rough, order-of-magnitude estimates of the atomic size, getting 10<sup>−8</sup>–10<sup>−7</sup> cm for copper.<ref>{{Cite book |url=https://books.google.com/books?id=81pPJyOD7dwC&pg=PA157 |title=Electrical World |date=1893 |publisher=McGraw-Hill |language=en}}</ref>

The earliest estimates of the atomic size was made by opticians in the 1830s, particularly [[Augustin-Louis Cauchy|Cauchy]],<ref name=":0">{{Cite journal |last=Fessenden |first=Reginald Aubrey |author-link=Reginald Fessenden |date=February 1900 |title=A Determination of the Nature of the Electric and Magnetic Quantities and of the Density and Elasticity of the Ether, II |url=https://link.aps.org/doi/10.1103/PhysRevSeriesI.10.83 |journal=Physical Review |series=Series I |language=en |volume=10 |issue=2 |pages=83–115 |doi=10.1103/PhysRevSeriesI.10.83 |issn=1536-6065}}</ref><ref>{{Cite journal |last=Thomson |first=W. |author-link=Lord Kelvin |date=1870-07-01 |title=On the size of atoms |url=https://books.google.com/books?id=NQ4zmCTZcR4C&pg=PA38 |journal=American Journal of Science |language=en |volume=s2-50 |issue=148 |pages=38–44 |doi=10.2475/ajs.s2-50.148.38}}</ref> who developed models of light [[Dispersion (optics)|dispersion]] assuming a lattice of connected "molecules".<ref>{{Cite book |last=Darrigol |first=Olivier |url=https://books.google.com/books?id=ImM62wvWE_cC&pg=PA247 |title=A History of Optics from Greek Antiquity to the Nineteenth Century |date=2012 |publisher=OUP Oxford |isbn=978-0-19-162745-3 |language=en}}</ref> In 1857 [[Rudolf Clausius|Clausius]] developed a [[Kinetic theory of gases|gas-kinetic model]] which included the equation for [[mean free path]]. In the 1870s it was used to estimate gas molecule sizes, as well as an aforementioned comparison with [[visible light]] [[wavelength]] and an estimate from the thickness of [[soap bubble]] film at which its contractile force rapidly diminishes.<ref>{{Cite book |url=https://books.google.com/books?id=vnrkYGHPGmoC&pg=PA165 |title=The American chemist: a monthly journal of theoretical, analytical and technical chemistry |date=1877 |publisher=C. F. & W. H. Chandler |language=en}}</ref> By 1900, various estimates of mercury atom diameter averaged around 275±20 pm<ref name=":0" /> (modern estimates give 300±10 pm, see below).

In 1920, shortly after it had become possible to determine the sizes of atoms using [[X-ray crystallography]], it was suggested that all atoms of the same element have the same radii.<ref>
{{cite journal
 |last1=Bragg |first1=W. L.
 |year=1920
 |title=The arrangement of atoms in crystals
 |journal=[[Philosophical Magazine]]
 |volume=40|series=6|issue=236
 |pages=169–189
 |doi=10.1080/14786440808636111
|url=https://zenodo.org/record/1430834
 }}</ref>  However, in 1923, when more crystal data had become available, it was found that the approximation of an atom as a sphere does not necessarily hold when comparing the same atom in different crystal structures.<ref>
{{cite journal
 |last=Wyckoff |first=R. W. G.
 |year=1923
 |title=On the Hypothesis of Constant Atomic Radii
 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]]
 |volume=9 |issue=2 |pages=33–38
 |bibcode = 1923PNAS....9...33W
 |doi=10.1073/pnas.9.2.33
 |pmid=16576657
 |pmc=1085234
|doi-access=free
 }}</ref>