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Since it was unknown how many true rare earth metals there were, research into their chemistry was challenging until the discovery of the last naturally occurring rare-earth element (lutetium) in 1907. Fortunately, this issue was resolved by the research of English physicist Henry Gwyn Jeffreys Moseley and Danish physicist Niels Bohr in 1913–1914.
The existence of only 14 lanthanides was demonstrated by theorists using Bohr's theory of the hydrogen atom. The 13 of these elements were confirmed to exist by Moseley's experimental studies, which also demonstrated that the 14th lanthanide must be element 61 and be located between neodymium and samarium.