Editing J. J. Thomson (section)

===Isotopes and mass spectrometry===
[[File:Discovery of neon isotopes.JPG|thumb|In the bottom right corner of this photographic plate are markings for the two isotopes of neon: neon-20 and neon-22.]]
In 1912, as part of his exploration into the composition of the streams of positively charged particles then known as [[canal rays]], Thomson and his research assistant [[Francis William Aston|F. W. Aston]] channelled a stream of neon ions through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path.<ref name="ReferenceA" /> They observed two patches of light on the photographic plate (see image on right), which suggested two different parabolas of deflection, and concluded that neon is composed of atoms of two different atomic masses (neon-20 and neon-22), that is to say of two [[isotope]]s.<ref>J.J. Thomson (1912) "Further experiments on positive rays," ''Philosophical Magazine'', series 6, '''24''' (140):  209–253.</ref><ref>J. J. Thomson (1913) "Rays of positive electricity", ''Proceedings of the Royal Society'' A, '''89''': 1–20.</ref> This was the first evidence for isotopes of a stable element; [[Frederick Soddy]] had previously proposed the existence of isotopes to explain the decay of certain [[radioactive]] elements.

Thomson's separation of neon isotopes by their mass was the first example of [[mass spectrometry]], which was subsequently improved and developed into a general method by [[Francis William Aston|F. W. Aston]] and by [[A. J. Dempster]].<ref name="Profile" /><ref name="Jones">{{cite web |author-first=Mark |author-last=Jones|title=Gas Chromatography-Mass Spectrometry  |url=https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/gas-chromatography-mass-spectrometry.html |publisher=American Chemical Society |access-date=19 November 2019}}</ref>

{{external media | width = 180px | float = right | headerimage= [[File:Title page On the Chemical Combination of Gases by Joseph John Thomson 1856-1940.jpg|180px]] | video1 = [https://www.youtube.com/watch?v=WH-U_qCEzT0 ''The Early Life of J. J. Thomson: Computational Chemistry and Gas Discharge Experiments'']}}