JAMB Chemistry 2020 Paper

Concept:Valence electrons are the electrons found in the highest occupied energy level of an atom.They take part in chemical bonding, and for main-group elements their number matches the group number.Explanation:In $^{31}_{15}\mathrm{P}$, the atomic number is $Z = 15$, so a neutral phosphorus atom contains 15 electrons.These electrons are arranged in the shells $2, 8, 5$, written as $1s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{3}$.The outermost shell is the third shell, $n = 3$.The electrons there are the $3s^{2}$ and $3p^{3}$ electrons, giving $2 + 3 = 5$ valence electrons.Phosphorus is found in Group V (Group 15) of the periodic table, which confirms that it has 5 valence electrons.Its common valencies of 3 and 5 come from this same valence-shell arrangement, but the actual number of valence electrons is 5.Answer:$^{31}_{15}\mathrm{P}$ contains 5 valence electrons.