Question

The electronic configurations of chromium (at. no. $24$ ) and copper (at. no. $29$ ) are

• A. $[ Ar ] 4 s ^{1} 3 d ^{5}$ and $[ Ar ] 4 s ^{1} 3 d ^{10}$
• B. $[ Ar ] 4 s ^{2} 3 d ^{4}$ and $[ Ar ] 4 s ^{1} 3 d ^{10}$
• C. $[ Ar ] 4 s ^{1} 3 d ^{5}$ and $[ Ar ] 4 s ^{2} 3 d ^{9}$
• D. $[ Ar ] 4 s ^{2} 3 d ^{4}$ and $[ Ar ] 4 s ^{2} 3 d ^{9}$

Answer 1

Answer: (A)

Chromium and copper have five and ten electrons in $3 d$-orbitals rather than four and nine because fully-filled and half-filled orbitals have lower energy and thus have extra stability. Therefore, chromium and copper have $d^{5}$ and $d^{10}$-configurations.