【正确答案】The lanthanoid 3 + ions have incomplete 4f subshells of electrons. The orbital angular momentum of these electrons contributes to the o verall magnetic moment. (In transition metal ions,the orbital angular momentum of the d electrons is more or less "quenched" by the interaction of these electrons and the atoms with which bonding takes place. )Gd³⁺ has a half-filled f subshell,with a total orbital angular momentum of zero. Since there is no orbital angular momentum, it cannot contribute to the moment of the ion, and the "spin only" formula works.