Cadmium ionization potential

The melting points, boiling points, and heats of vaporization for the zinc group members are lower than for any other group of metals except la. By the argument presented on page 99, one would expect these metals to have abnormally high" oxidation potentials that is, they should be less noble than other metals with comparable ionization potentials. Thus zinc (ionization potential 9.39 volts) is more easily oxidized than iron (ionization potential 7.90 olts), cadmium (ionization potential 8.9S volts) is more easily oxidized than nickel (ionization potential 7.63 volts), and mercury (ionization potential 10.43 volts) more easily than platinum (ionization potential 8.88 volts). Note that the ionization potentials would favor the opposite order in each case. 

The elements zinc, cadmium, and mercury, which have two electrons outside filled penultimate d shells, are classed in Group 12. Although the difference between the calcium and zinc subgroups is marked, zinc, and to a lesser extent cadmium, show some resemblance to beryllium or magnesium in their chemistry. We discuss these elements separately (Chapter 15), but note here that zinc, which has the lowest second ionization enthalpy in the Zn, Cd, Hg group, still has a value (1726 kJ mol-1) similar to that of beryllium (1757 kJ mol"1), and its standard potential (-0.76 V) is considerably less negative than that of magnesium. 

Zinc and cadmium dissolve in dilute acid to give their -I- 2 ions, but mercury does not dissolve, as indicated by the two positive reduction potentials. Mercury forms the diatomic Hg2 ion, in which the Hg-Hg bond length is 251 pm, consistent with it being a single ct bond formed from the overlap of the two 6s atomic orbitals. The reason for the relatively greater stability of the 6s electrons of Hg is relativistic stabilization which causes the first two ionization energies (1010 and 1810 kJ mol ) to be considerably greater than those of Zn (908 and 1730 kJ mol ) and Cd (866 and 1630 kJ mol ). 

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