Editing Thorium (section)

===Further classification===
Up to the late 19th century, chemists unanimously agreed that thorium and uranium were the heaviest members of group 4 and [[group 6 element|group 6]] respectively; the existence of the lanthanides in the sixth row was considered to be a one-off fluke. In 1892, British chemist Henry Bassett postulated a second extra-long periodic table row to accommodate known and undiscovered elements, considering thorium and uranium to be analogous to the lanthanides. In 1913, Danish physicist [[Niels Bohr]] published a [[Bohr model|theoretical model]] of the atom and its electron orbitals, which soon gathered wide acceptance. The model indicated that the seventh row of the periodic table should also have f-shells filling before the d-shells that were filled in the transition elements, like the sixth row with the lanthanides preceding the 5d transition metals.<ref name="leach" /> The existence of a second inner transition series, in the form of the actinides, was not accepted until similarities with the electron structures of the lanthanides had been established;<ref>{{cite book|last=van Spronsen |first=J. W. |year=1969 |title=The periodic system of chemical elements |publisher=Elsevier |pages=315–316 |isbn=978-0-444-40776-4}}.</ref> Bohr suggested that the filling of the 5f orbitals may be delayed to after uranium.<ref name="leach" />

It was only with the discovery of the first [[transuranic element]]s, which from plutonium onward have dominant +3 and +4 oxidation states like the lanthanides, that it was realised that the actinides were indeed filling f-orbitals rather than d-orbitals, with the transition-metal-like chemistry of the early actinides being the exception and not the rule.<ref>{{cite book |last=Rhodes |first=R. |title=The Making of the Atomic Bomb |edition=25th Anniversary |date=2012 |publisher=[[Simon & Schuster]] |isbn=978-1-4516-7761-4 |pages=221–222, 349}}</ref> In 1945, when American physicist [[Glenn T. Seaborg]] and his team had discovered the transuranic elements americium and curium, he proposed the [[actinide concept]], realising that thorium was the second member of an f-block actinide series analogous to the lanthanides, instead of being the heavier congener of [[hafnium]] in a fourth d-block row.<ref name="Masterton">{{cite book |last1=Masterton|first1=W. L. |last2=Hurley|first2=C. N.|last3=Neth|first3=E. J.|title=Chemistry: Principles and reactions|publisher=[[Cengage Learning]]|edition=7th|isbn=978-1-111-42710-8|page=173|year=2011 }}</ref>{{efn|The filling of the 5f subshell from the beginning of the actinide series was confirmed when the 6d elements were reached in the 1960s, proving that the 4f and 5f series are of equal length. [[Lawrencium]] has only +3 as an oxidation state, breaking from the trend of the late actinides towards the +2 state; it thus fits as a heavier congener of [[lutetium]]. Even more importantly, the next element, [[rutherfordium]], was found to behave like hafnium and show only a +4 state.<ref name=johnson/><ref>{{cite journal |doi= 10.1016/S0925-8388(98)00072-3 |title= Evidence for relativistic effects in the chemistry of element 104 |first9= D. |last10= Timokhin |first10= S. N. |last11= Yakushev |first11= A. B. |last12= Zvara |first12= I. |last9= Piguet |first8= V. Ya. |last8= Lebedev |first7= D. T. |last7= Jost |first6= S. |last6= Hübener |first5= M. |last5= Grantz |first4= H. W. |last4= Gäggeler |first3= B. |last3= Eichler |first2= G. V. |date= 1998 |last2= Buklanov |last1= Türler| first1= A. |journal= Journal of Alloys and Compounds |volume= 271–273 |pages= 287–291| display-authors=3}}</ref> Today, thorium's similarities to hafnium are still sometimes acknowledged by calling it a "pseudo group 4 element".<ref name="Pershina">{{cite book |last1=Kratz |first1=J. V. |last2=Nagame |first2=Y. |editor1-last=Schädel |editor1-first=M. |editor2-last=Shaughnessy |editor2-first=D. |chapter=Liquid-Phase Chemistry of Superheavy Elements |date=2014 |edition=2nd |title=The Chemistry of Superheavy Elements |publisher=Springer-Verlag |page=335 |isbn=978-3-642-37465-4 |doi=10.1007/978-3-642-37466-1 |s2cid=122675117 |chapter-url=https://cds.cern.ch/record/643991 |archive-date=17 April 2021 |access-date=21 June 2023 |archive-url=https://web.archive.org/web/20210417211550/http://cds.cern.ch/record/643991 |url-status=live }}</ref>}}