JAMB Chemistry 2020 Paper

Concept:The oxidation number of an element depends on how many electrons it can gain or lose to achieve a stable configuration.Explanation:The element with atomic number $17$ is chlorine, $\mathrm{Cl}$.Its electronic configuration is $1s^2\,2s^2\,2p^6\,3s^2\,3p^5$, meaning it has $7$ electrons in its outermost shell.Chlorine needs only one electron to complete its octet, so it commonly gains one electron and exhibits an oxidation number of $-1$ (for example, in $\mathrm{NaCl}$).However, when chlorine bonds with more electronegative elements like oxygen or fluorine, it can lose all seven outer electrons and show its highest oxidation number of $+7$ (for example, in $\mathrm{HClO}_4$).Therefore, the possible oxidation numbers of the element are $-1$ and $+7$.Answer:A. $-1$ and $7$