Question

Gadolinium has a low value of third ionisation enthalpy because of

• A. small size
• B. high exchange enthalpy
• C. high electronegativity
• D. high basic character

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Ionisation enthalpy is the energy required to remove an electron from a gaseous atom or ion.
The stability of electronic configurations, particularly half-filled and fully-filled subshells, significantly influences these values.
Gadolinium (\(Z = 64\)) is a member of the Lanthanide series.
Step 2: Detailed Explanation:
The electronic configuration of neutral Gadolinium (\(Gd\)) is \([Xe] 4f^7 5d^1 6s^2\).
1. To find the third ionisation enthalpy (\(IE_3\)), we look at the energy required to remove an electron from the \(Gd^{2+}\) ion.
2. The electronic configuration of \(Gd^{2+}\) (after losing the two \(6s\) electrons) is \([Xe] 4f^7 5d^1\).
3. Removing the third electron from the \(5d\) subshell results in the \(Gd^{3+}\) ion.
4. The configuration of \(Gd^{3+}\) is \([Xe] 4f^7\).
5. The \(4f^7\) state represents an exactly half-filled \(f\)-subshell.
6. Half-filled subshells are exceptionally stable due to high exchange energy (enthalpy) and symmetrical distribution of electrons.
7. Because the resulting ion (\(Gd^{3+}\)) achieves this highly stable state, the energy required to remove that third electron is relatively low.
Step 3: Final Answer:
Gadolinium has a low value of third ionisation enthalpy because losing the third electron leads to a stable \(4f^7\) configuration supported by high exchange enthalpy.