WAEC Chemistry 2010 Paper

Concept:Elements are classified into blocks based on the subshell that receives the last electron. If the last electron enters a $d$-orbital, the element belongs to the $d$-block.Explanation:Consider the electronic configurations of the given elements.Calcium has $[\mathrm{Ar}]\,4s^2$, so its last electron enters the $4s$ orbital. Hence, it is an $s$-block element.Iron has $[\mathrm{Ar}]\,3d^6\,4s^2$, so its last electron enters the $3d$ orbital. Hence, it is a $d$-block element.Lithium has $[\mathrm{He}]\,2s^1$, so its last electron enters the $2s$ orbital. Hence, it is an $s$-block element.Silicon has $[\mathrm{Ne}]\,3s^2\,3p^2$, so its last electron enters the $3p$ orbital. Hence, it is a $p$-block element.Among the four options, only iron satisfies the condition for being a $d$-block element.Answer:Iron is the $d$-block element.