WAEC Chemistry 2014 Paper

Concept:Elements in the same group have the same number of valence electrons.Explanation:$_{11}\mathrm{X}$ has electronic configuration $2, 8, 1$, so its valence electrons are $1$.$_{19}\mathrm{Y}$ has electronic configuration $2, 8, 8, 1$, so its valence electrons are also $1$.Because both atoms have the same valence electron count, they are placed in the same group.The other options, such as ionization energy, number of shells, and atomic size, vary down a group.Answer:A. valence electrons