\(64 Gd ^{3+}=[ Xe ] 4 f ^{7}\)
\(Gd ^{3+}\) having \(7\) unpaired electrons.
Magnetic moment \((\mu)=\sqrt{n(n+2)}\) \(B.M.\)
\(\mu=\sqrt{7(7+2)} B \cdot M\)
\(=7.9 B . M\)
\(n \Rightarrow\) Number of unpaired electrons.
($Zn =30,\,Ni =28$ અને $Cr =24$ના પરમાણ્વીય ક્રમાંક )