Question

(a) Why is third ionization enthalpy of manganese very high ?

(b) Why Zn, Cd and Hg are not regarded as transition elements ?

Hint:

Mn2+ has a stable half-filled 3d5 shell, so the third electron is hard to remove. Zn, Cd and Hg have full d10 shells in the atom and in the +2 ion.

Answer 1

Step 1: Read the two parts.
Part (a) asks why removing the third electron from manganese needs so much energy.
Part (b) asks why zinc, cadmium and mercury are not counted as transition elements.

Step 2: Part (a) - write the configurations.
Manganese (Z = 25) has the configuration \(\mathrm{[Ar]\,3d^5\,4s^2}\).
The first two ionisations remove the two 4s electrons.
This leaves \(\mathrm{Mn^{2+}}\) with the configuration \(\mathrm{[Ar]\,3d^5}\).
The third ionisation enthalpy is the energy needed for the next step.
\[ \mathrm{Mn^{2+}\,(3d^5)\ \longrightarrow\ Mn^{3+}\,(3d^4) + e^-} \]

Step 3: Part (a) - explain the high value.
A half-filled d subshell (\(\mathrm{d^5}\)) is extra stable.
Its five electrons are spread symmetrically and have a large exchange energy.
Removing one electron destroys this stable arrangement.
So a very large amount of energy is needed.
The third ionisation enthalpy of Mn is 3248 kJ \(\mathrm{mol^{-1}}\).
This is higher than that of Fe (2957 kJ \(\mathrm{mol^{-1}}\)), where \(\mathrm{Fe^{2+}}\) (3d6) loses an electron to reach the stable d5 state.

Step 4: Part (b) - recall the definition.
A transition element has a partly filled d subshell in its ground state or in any of its common oxidation states.

Step 5: Part (b) - apply it to Zn, Cd and Hg.
The ground state configurations are:
\[ \mathrm{Zn: [Ar]\,3d^{10}4s^2,\quad Cd: [Kr]\,4d^{10}5s^2,\quad Hg: [Xe]\,4f^{14}5d^{10}6s^2} \]
Their d subshell is already full in the atom.
In the common +2 state they lose only the s electrons, so the d subshell stays \(\mathrm{d^{10}}\).
The d subshell is never partly filled, so they do not fit the definition.

Final Answer:
(a) \(\mathrm{Mn^{2+}}\) has a very stable half-filled 3d5 configuration, so removing a third electron needs a lot of energy.
(b) Zn, Cd and Hg have completely filled d10 configurations in the atom and in the +2 state, so they are not regarded as transition elements.