BECE Science 2020 Paper

Concept:In an ionic compound, the total positive charge must balance the total negative charge.Explanation:Aluminium has atomic number 13 and electronic configuration $2, 8, 3$.It can lose 3 electrons to become stable, so its valency is 3, written as $\mathrm{Al}^{3+}$.Sulphur has atomic number 16 and electronic configuration $2, 8, 6$.It needs to gain 2 electrons to complete its octet, so its valency is 2, written as $\mathrm{S}^{2-}$.In the formula $\mathrm{Al}_y \mathrm{S}_3$, the subscript 3 on sulphur means there are 3 sulphide ions.The total negative charge is therefore $3 \times (-2) = -6$.Each aluminium ion carries a charge of $+3$.To balance the compound, the total positive charge must be $+6$.So, $y \times (+3) = +6$, which gives $y = 2$.Hence, the correct formula is $\mathrm{Al}_2 \mathrm{S}_3$.Answer:The value of $y$ is 2, which is option B.