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). (a) How
would you use electron configuration arguments to justify Bohr's prediction?
(b) Zirconium, hafnium's neighbor in group \(4 \mathrm{~B}\), can be produced as
a metal by reduction of solid \(\mathrm{ZrCl}_{4}\) with molten sodium metal.
Write a balanced chemical equation for the reaction. Is this an oxidation-
reduction reaction? If yes, what is reduced and what is oxidized? (c) Solid
zirconium dioxide, \(\mathrm{ZrO}_{2}\), reacts with chlorine gas in the
presence of carbon. The products of the reaction are \(\mathrm{ZrCl}_{4}\) and
two gases, \(\mathrm{CO}_{2}\) and \(\mathrm{CO}\) in the ratio \(1: 2 .\) Write a
balanced chemical equation for the reaction. Starting with a \(55.4-\mathrm{g}\)
sample of \(\mathrm{ZrO}_{2}\), calculate the mass of \(\mathrm{ZrCl}_{4}\)
formed, assuming that \(\mathrm{ZrO}_{2}\) is the limiting reagent and assuming
\(100 \%\) yield.
(d) Using their electron configurations, account for the fact that
\(\mathrm{Zr}\) and Hf form chlorides \(\mathrm{MCl}_{4}\) and oxides
\(\mathrm{MO}_{2}\).
