WAEC Chemistry 2018 Paper

Concept:Chromium is a transition element with atomic number $24$, and it shows an exceptional electron configuration due to the extra stability of a half-filled $3d$ subshell.Explanation:According to the Aufbau principle, the expected filling order for $24$ electrons is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{4}$.However, the $3d$ and $4s$ energy levels are very close in energy.A half-filled $3d$ subshell ($3d^{5}$) and a half-filled $4s$ subshell ($4s^{1}$) provide greater exchange energy and stability.So one electron is promoted from $4s^{2}$ to the $3d$ subshell, changing $3d^{4}$ into $3d^{5}$.Thus, the actual electronic configuration of Chromium is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^{5}$.Answer:Option D: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^{5}$