Question

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

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

Answer 1

The correct option is A $\left[Ar\right]4s^{1}3d^5$ and $\left[Ar\right]4s^{1}3d^{10}$

Chromium and copper have five and ten electrons in 3d-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.