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 4B, can be produced as a metal by reduction of solid with molten sodium metal. Write a balanced chemical equation for the reaction.

ZrCl4 (s) + 4Na(l) = Zr(s) + 4NaCl(s)

ZrCl4 (s) + Na(l) = Zr(s) + NaCl(s)

ZrCl4 (s) + 3Na(l) = Zr(s) + 2NaCl(s)

ZrCl4 (s) + 2Na(l) = Zr(s) + 3NaCl(s)