Editing Thorium (section)

===Initial chemical classification===
In the periodic table published by [[Dmitri Mendeleev]] in 1869, thorium and the rare-earth elements were placed outside the main body of the table, at the end of each vertical period after the [[alkaline earth metal]]s. This reflected the belief at that time that thorium and the rare-earth metals were divalent. With the later recognition that the rare earths were mostly trivalent and thorium was tetravalent, Mendeleev moved cerium and thorium to group IV in 1871, which also contained the modern [[carbon group]] (group 14) and titanium group (group 4), because their maximum oxidation state was +4.<ref name="leach">{{cite web |url=http://www.meta-synthesis.com/webbook//35_pt/pt_database.php |title=The Internet Database of Periodic Tables |last=Leach |first=M. R. |access-date=14 May 2012 |archive-date=24 March 2016 |archive-url=https://web.archive.org/web/20160324070522/http://www.meta-synthesis.com/webbook/35_pt/pt_database.php |url-status=live }}</ref><ref name="Jensen">{{cite journal|author1-link=William B. Jensen |last1=Jensen |first1=William B. |title=The Place of Zinc, Cadmium, and Mercury in the Periodic Table |journal=Journal of Chemical Education |date=August 2003 |volume=80 |issue=8 |page=952 |doi=10.1021/ed080p952 |bibcode=2003JChEd..80..952J }}</ref> Cerium was soon removed from the main body of the table and placed in a separate lanthanide series; thorium was left with group 4 as it had similar properties to its supposed lighter congeners in that group, such as [[titanium]] and zirconium.<ref name="Masterton" />{{efn|Thorium also appears in the 1864 table by British chemist [[John Newlands (chemist)|John Newlands]] as the last and heaviest element, as it was initially thought that uranium was a trivalent element with an atomic weight of around 120: this is half of its actual value, since uranium is predominantly hexavalent. It also appears as the heaviest element in the 1864 table by British chemist [[William Odling]] under titanium, zirconium, and [[tantalum]]. It does not appear in the periodic systems published by French geologist [[Alexandre-Émile Béguyer de Chancourtois]] in 1862, German-American musician [[Gustav Hinrichs]] in 1867, or German chemist [[Julius Lothar Meyer]] in 1870, all of which exclude the rare earths and thorium.<ref name="leach" />}}