Question

The second ionization energy of the following elements follows the order:
A. ${\text{Zn}} > {\text{Cd}} > {\text{Hg}}$
B. ${\text{Zn = Cd}} > {\text{Hg}}$
C. ${\text{Cd}} > {\text{Hg < Zn}}$
D. ${\text{Zn > Cd < Hg}}$

Answer 1

Hint: Ionization energy is a certain amount of energy that is necessary to knock off electrons from an atom to form a positive ion. Second ionization energy is required to remove the second electron from the gaseous atom. It is always higher than first, since the electron being removed is from an inner energy level which has stronger electrostatic force with the nucleus.

Complete step by step answer:
From left to right across a period, ionization energy increases. Here, the atomic number is increased, thereby increasing the nuclear charge. Thus electrons are held tightly with the nucleus hence greater energy is needed to remove electrons.
Cations are always smaller than neutral atoms. Down the group, nuclear charge increases, atomic size increases, and the number of shells increases. Thus electrons are loosely held with the nucleus, hence lesser energy is needed to remove the electrons. Thus ionization energy decreases.
Considering cadmium, zinc and mercury, they are in the same group, i.e. ${12^{{\text{th}}}}$ group. In general case, the order of ionization energy is ${\text{Zn}} > {\text{Cd}} > {\text{Hg}}$, but there is an anomalous behavior. Mercury shows an anomalous behavior.
Relativity is a phenomenon in which when an article moves at a speed which is almost equal to speed of light, its dimension decreases. Similar case happens for electrons in heavier atoms. This is connected with quantum numbers ${\text{n}} + {\text{l}}$. Electronic configuration of mercury is $\left[ {{\text{Xe}}} \right]4{{\text{f}}^{14}}5{{\text{d}}^{10}}6{{\text{s}}^2}$. Here ${\text{n}} + {\text{l}}$ values are $8$ or $7$. These orbital dimensions are very high. This gets disappeared in inner orbitals which do not have this much huge number. Thus it behaves almost like xenon. It behaves like a noble gas. This is the reason why mercury has a different ionization energy.
Hence the correct option is D.

Additional information:
The magnitude of ${{\text{K}}_{\text{c}}}$ will tell about whether the equilibrium reaction favors the reactants or products. If equilibrium constant is greater than one, it favors products. If equilibrium constant is less than one, it favors reactants.

Note:
Factors affecting the ionization energy are:
-Atomic size
-Number of protons
-Nature of orbital
-Shielding effect
Smaller the atom, harder to steal an electron. Larger the atom, the easier it is to steal an electron.
The ionization energy values of cadmium, zinc and mercury are $1631.40{\text{kJ}}.{\text{mo}}{{\text{l}}^{ - 1}},1733.3{\text{kJ}}.{\text{mo}}{{\text{l}}^{ - 1}},1809.76{\text{kJ}}.{\text{mo}}{{\text{l}}^{ - 1}}$ respectively.