To identify the ion having the 4f6 electronic configuration, we need to look at the electron configurations of the given ions. The f-block elements, also known as the lanthanides, typically fill their 4 f orbitals as we move across the series. The atomic numbers and electron configurations of some relevant lanthanides are as follows: Samarium(Sm) : Atomic number =62 , Configuration: [Xe]4f66s2 Gadolinium ( Gd) : Atomic number =64 , Configuration: [Xe]4f75d16s2 Terbium (Tb) : Atomic number =65 , Configuration: [Xe]4f96s2 Now let's examine the electronic configurations of the ions given in the options: square Gd3+ : Gadolinium loses three electrons to form the ion. This typically means losing two 6 s electrons and one 5d electron: [Xe]4f7 [Xe]4f7 Sm3+ : Samarium loses three electrons to form the ion, usually removing the two 6 s electrons and one 4 f electron: [Xe]4f5 Sm2+ : Samarium loses two electrons to form the ion, typically losing only the two 6 s electrons: [Xe]4f6 Tb3+ : Terbium loses three electrons to form the ion, generally removing the two 6 s electrons and one 4 f electron: [Xe]4f8
Among the options, the ion with the 4f6 electronic configuration is Sm2+. This configuration is obtained after Samarium loses its two 6 s electrons. Therefore, the correct answer is: Option C: Sm2+