Which of the following elements would form anions? Why?
$(i)$ $Na$ $(ii)$ $O$ $(iii)$ $Mg$ $(iv)$ $Al$ $(v)$ $Cl$

(II, V) Oxygen $(O)$ and chlorine $(Cl)$ would form anions.
$O + 2e^- \rightarrow O^{2-}$ (Oxide ion)
$Cl + e^- \rightarrow Cl^-$ (Chloride ion)
Reason: Anions are negatively charged ions formed by the gain of electrons. Non-metals like oxygen and chlorine gain electrons to achieve a stable noble gas electronic configuration (octet rule).