WAEC Chemistry 2018 Paper

Concept:Valence electrons are the electrons in the outermost shell of an atom.Explanation:Magnesium has atomic number $12$, so it contains $12$ electrons.Its electronic configuration is $1s^2\,2s^2\,2p^6\,3s^2$.The electrons are arranged as $2, 8, 2$ across the shells.The outermost shell is the third shell, which contains $2$ electrons.These $2$ valence electrons occupy the $3s$ orbital.Hence, the valence electrons are in the $3s$ orbital.Answer:A. $3s$ orbital