Editing Atomic number (section)

=== Moseley's 1913 experiment ===
[[File:Henry Moseley.jpg|thumb|upright|[[Henry Moseley]] in his lab.]]
The experimental position improved dramatically after research by [[Henry Moseley]] in 1913.<ref>[http://www.rsc.org/Education/Teachers/Resources/periodictable/pre16/order.doc Ordering the Elements in the Periodic Table] {{Webarchive|url=https://web.archive.org/web/20160304090148/http://www.rsc.org/Education/Teachers/Resources/periodictable/pre16/order.doc |date=4 March 2016 }}, Royal Chemical Society</ref> Moseley, after discussions with Bohr who was at the same lab (and who had used Van den Broek's hypothesis in his [[Bohr model]] of the atom), decided to test Van den Broek's and Bohr's hypothesis directly, by seeing if [[spectral line]]s emitted from excited atoms fitted the Bohr theory's postulation that the frequency of the spectral lines be proportional to the square of ''Z''.

To do this, Moseley measured the wavelengths of the innermost photon transitions (K and L lines) produced by the elements from [[aluminium]] (''Z''&nbsp;=&nbsp;13) to gold (''Z''&nbsp;=&nbsp;79) used as a series of movable anodic targets inside an [[x-ray tube]].<ref>{{cite journal |url=https://zenodo.org/record/1430926 |doi=10.1080/14786441308635052 |title=XCIII.The high-frequency spectra of the elements |journal=Philosophical Magazine |series=Series 6 |volume=26 |issue=156 |pages=1024–1034 |year=1913 |last1=Moseley |first1=H.G.J. |url-status=live |archive-url=https://web.archive.org/web/20230708085927/https://zenodo.org/record/1430926/files/article.pdf |archive-date=8 July 2023 |df=dmy |access-date=12 December 2023 }}</ref> The square root of the frequency of these photons {{nowrap|(x-rays)}} increased from one target to the next in an [[arithmetic progression]]. This led to the conclusion ([[Moseley's law]]) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) to the calculated [[electric charge]] of the nucleus, i.e. the element number ''Z''. Among other things, Moseley demonstrated that the [[lanthanide]] series (from [[lanthanum]] to [[lutetium]] inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.