WAEC Chemistry 2012 Paper

Concept:An atom is in its ground state when its electrons occupy the lowest available energy levels in the correct order.Explanation:The correct filling order is $1s < 2s < 2p < 3s < 3p$.No higher subshell should be filled while a lower subshell is still incomplete.Option A: $1s^{2}2s^{1}2p^{1}$ has one electron in $2p$ while $2s$ is not full. This is an excited state.Option C: $1s^{2}2s^{1}2p^{3}$ also occupies $2p$ before completing $2s$. This is an excited state.Option B: $1s^{2}2s^{2}2p^{6}3s^{1}3p^{1}$ has one electron in $3p$ while $3s$ is not full. This is an excited state.Option D: $1s^{2}2s^{2}2p^{6}3s^{0}$ has the $1s$, $2s$, and $2p$ subshells completely filled and no electron in a higher orbital. This is the lowest-energy arrangement, so it represents the ground state.Answer:The correct answer is D: $1s^{2}2s^{2}2p^{6}3s^{0}$.