WAEC Chemistry 2019 Paper

Concept:Chromium has a special electronic configuration due to extra stability from a half-filled $3d$ subshell.Explanation:For ${}_{24}\mathrm{Cr}$, the total number of electrons is 24.Following the normal filling order, the expected configuration is:$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2\,3d^4$However, a half-filled $3d^5$ configuration is more stable than $3d^4$.So, one electron from the $4s$ orbital moves to the $3d$ subshell.Therefore, the actual ground state configuration is:$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^1\,3d^5$Thus, the $3d$ subshell contains 5 electrons.Answer:D. 5