WAEC Chemistry 2018 Paper

Concept:Hydrogen forms an ionic compound when it combines with a highly electropositive metal by gaining one electron to form a hydride ion, $\mathrm{H}^-$.Explanation:In $\mathrm{CH}_4$, $\mathrm{HCl}$, and $\mathrm{NH}_3$, hydrogen shares electrons with non-metals such as carbon, chlorine, and nitrogen. These shared electron pairs form covalent bonds.In sodium hydride, $\mathrm{NaH}$, sodium is a metal with very low electronegativity, so it readily transfers its valence electron to hydrogen.Sodium becomes $\mathrm{Na}^+$, while hydrogen becomes $\mathrm{H}^-$.The oppositely charged ions are held together by a strong electrostatic force of attraction. This is the basis of an ionic bond.Therefore, among the given compounds, only $\mathrm{NaH}$ shows hydrogen forming an ionic compound.Answer:D. $\mathrm{NaH}$