The discovery of hafnium, element number 72, provided
a controversial episode in chemistry. G. Urbain, a French
chemist, claimed in 1911 to have isolated an element
number 72 from a sample of rare earth (elements 58–71)
compounds. However, Niels Bohr believed that hafnium
was more likely to be found along with zirconium than
with the rare earths. D. Coster and G. von Hevesy, working
in Bohr's laboratory in Copenhagen, showed in 1922 that
element 72 was present in a sample of Norwegian zircon,
an ore of zirconium. (The name hafnium comes from the
Latin name for Copenhagen, Hafnia).
(d) Using their electron configurations, account for the fact
that Zr and Hf form chlorides MCl4 and oxides MO2.
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