WAEC Chemistry 2011 Paper

Concept:Element ${}_{20}\mathrm{A}$ is a metal in Group 2, while element ${}_{8}\mathrm{Y}$ is a non-metal in Group 6, so they combine by electron transfer to form an ionic compound.Explanation:${}_{20}\mathrm{A}$ has the electronic configuration $2, 8, 8, 2$.As a Group 2 metal, it loses its two outermost electrons easily.This loss forms the cation $\mathrm{A}^{2+}$.${}_{8}\mathrm{Y}$ has the electronic configuration $2, 6$.As a Group 6 non-metal, it gains two electrons to complete its octet.This gain forms the anion $\mathrm{Y}^{2-}$.The charges are equal in magnitude: $+2$ and $-2$.Thus, one $\mathrm{A}^{2+}$ ion combines with one $\mathrm{Y}^{2-}$ ion.The neutral compound formed has the formula $\mathrm{AY}$.The bond is ionic because it forms between a metal and a non-metal through the transfer of electrons.Answer:B. an ionic compound AY is formed