WAEC Chemistry 2011 Paper

Concept:An inert element is a noble gas with a completely filled outermost shell, usually ending in $ns^2np^6$.Explanation:For each configuration, check whether the outermost subshells are fully filled.Option A: $1s^2\,2s^2\,2p^4$ has a partially filled $2p$ subshell, so it is not inert.Option B: $1s^2\,2s^2\,2p^6\,3s^2$ has $3s^2$ filled but the $3p$ subshell is empty, so it is not inert.Option C: $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$ has a partially filled $3p$ subshell, so it is not inert.Option D: $1s^2\,2s^2\,2p^6$ has both $2s$ and $2p$ subshells completely filled.This configuration represents neon, which is a noble gas and therefore an inert element.Answer:D. $1s^2\,2s^2\,2p^6$