Editing Atomic number (section)

=== The periodic table and a natural number for each element ===
[[File:DIMendeleevCab.jpg|thumb|upright|Russian chemist [[Dmitri Mendeleev]], creator of the periodic table.]]
The [[periodic table]] of elements creates an ordering of the elements, and so they can be numbered in order.<ref name=PaisInward>{{Cite book |last=Pais |first=Abraham |title=Inward bound: of matter and forces in the physical world |date=2002 |publisher=Clarendon Press [u.a.] |isbn=978-0-19-851997-3 |edition=Reprint |location=Oxford}}</ref>{{rp|222}}
[[Dmitri Mendeleev]] arranged his first periodic tables (first published on March 6, 1869) in order of [[atomic weight]] ("Atomgewicht").<ref name="dm1869">[https://history.aip.org/exhibits/curie/periodic.htm The Periodic Table of Elements] {{Webarchive|url=https://web.archive.org/web/20230818200557/https://history.aip.org/exhibits/curie/periodic.htm |date=18 August 2023 }}, American Institute of Physics</ref> However, in consideration of the elements' observed chemical properties, he changed the order slightly and placed [[tellurium]] (atomic weight 127.6) ahead of [[iodine]] (atomic weight 126.9).<ref name="dm1869"/><ref>[http://www.rsc.org/chemsoc/visualelements/pages/history_ii.html The Development of the Periodic Table] {{Webarchive|url=https://web.archive.org/web/20120726030605/http://www.rsc.org/chemsoc/visualelements/pages/history_ii.html |date=26 July 2012 }}, Royal Society of Chemistry</ref> This placement is consistent with the modern practice of ordering the elements by proton number, ''Z'', but that number was not known or suspected at the time.

A simple numbering based on atomic weight position was never entirely satisfactory. In addition to the case of iodine and tellurium, several other pairs of elements (such as [[argon]] and [[potassium]], [[cobalt]] and [[nickel]]) were later shown to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties.<ref name=PaisInward/>{{rp|222}} However the gradual identification of more and more chemically similar [[lanthanide]] elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from [[lutetium]] (element 71) onward ([[hafnium]] was not known at this time).