JAMB Chemistry 2022 Paper

Concept:Electrons fill orbitals in order of increasing energy and occupy equal-energy orbitals singly before pairing them.Explanation:The orbital energies increase in the order $1s < 2s < 2p < 3s$.So a $3s$ orbital should receive electrons only after the lower-energy $2p$ orbitals are correctly filled.Options C and D place an electron in $3s$ while the $2p$ subshell is not properly filled, so they are incorrect.The three $2p$ orbitals, $2p_x$, $2p_y$ and $2p_z$, have the same energy.According to Hund's rule, each $2p$ orbital must receive one electron before any of them is paired.For four electrons in the $2p$ subshell, the correct arrangement is $2p_x^2\,2p_y^1\,2p_z^1$.This is shown only by option B.Answer:B. $1s^2\,2s^2\,2p_x^2\,2p_y^1\,2p_z^1\,3s^0$