WAEC Chemistry 2010 Paper

Concept:The atomic number is equal to the number of protons, and the ionic charge is found by comparing protons and electrons.Explanation:M has 13 protons, so its atomic number is $Z = 13$.It has 10 electrons, while a neutral atom of M would have 13 electrons.Net charge $= +13 + (-10) = +3$.So the ion has a $3+$ charge and should be written with the atomic number 13.This matches the symbol $\mathrm{^{3+}_{13}M}$.Answer:C. $\mathrm{^{3+}_{13}M}$