Kerala KEAM 5 June 2024 Solved Paper

Step 1: The lanthanides are the series of elements from $Ce$ to $Lu$ in the periodic table, and they involve filling the 4f orbitals. In the +3 oxidation state, these elements generally lose their 5s and 4d electrons, but the 4f electrons remain.Step 2: In the case of gadolinium ($Gd$), the electronic configuration in the ground state is:$$Gd (Z = 64): \, [Xe] \, 4f^7 5s^2 5d^1.$$When it loses three electrons to form the +3 state, the electron configuration becomes: $$Gd^{3+}: [Xe] \, 4f^7.$$Thus, the 4f orbitals in gadolinium’s +3 state are half-filled ($4f^7$), which is a stable configuration due to the special stability of half-filled subshells.Step 3: This half-filled configuration is a notable characteristic of gadolinium in its +3 oxidation state.Thus, the correct answer is option (B).Quick Tip: In transition and lanthanide elements, half-filled and fully-filled d or f orbitals are especially stable due to exchange energy and symmetry considerations. This often leads to unusual electron configurations in their ionic states.