Question

Sol. [Key Concept: Write the configurations: Eu and Gd are both $4f^{7}$, and Yb and Hf are both $4f^{14}$ — so the matching pairs are A and C.]

$${Gd - \lbrack Xe\rbrack 4f^{7}5{\text{ }d}^{1}6{\text{ }s}^{2} }{Eu - \lbrack Xe\rbrack 4f^{7}6{\text{ }s}^{2} }$$Dy $- \lbrack Xe\rbrack 4f^{10}6{\text{ }s}^{2}$

$${Ho - \lbrack Xe\rbrack 4f^{11}6{\text{ }s}^{2} }{Hf - \lbrack Xe\rbrack 4f^{14}6{\text{ }s}^{2}5{\text{ }d}^{2} }{Yb - \lbrack Xe\rbrack 4f^{14}6{\text{ }s}^{2} }{Lu - \lbrack Xe\rbrack 4f^{14}5{\text{ }d}^{1}6{\text{ }s}^{2} }{Tm - \lbrack Xe\rbrack 4f^{13}6{\text{ }s}^{2} }$$