Np = 1s² 2² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s²

4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹

Total No. of 'f' electron = 14e^$$-$$ + 4e^$$-$$ = 18

The electronic configuration of

$$_{64}Gd:[Xe]4{f^7}5{d^1}6{s^2}$$

So, the electronic configuration of

$$_{64}G{d^{2 + }}:[Xe]4{f^7}5{d^1}6{s^0}$$

i.e. the number of 4f electrons in the ground state electronic configuration of $$G{d^{2 + }}$$ is 7.

Sublimation enthalpy, Ionisation enthalpy and hydration enthalpy affect the reduction potential.

Ho = [Xe]4f¹¹6s²

Ho³⁺ = [Xe] 4f¹⁰

So number of e^$$-$$ present in 4f is 10.