WAEC Chemistry 2024 Paper

Concept:Hund's rule governs how electrons fill orbitals of the same subshell.Explanation:The configuration $1s^2\,2s^2\,2p_x^2$ places two electrons in the same $p_x$ orbital.According to Hund's rule, electrons must occupy each degenerate orbital singly with parallel spins before any pairing occurs.In the $2p$ subshell, the $p_x$, $p_y$, and $p_z$ orbitals should each receive one electron first.Thus, placing both $2p$ electrons in only $p_x$ violates Hund's rule.It does not violate Pauli's exclusion principle because the pairing has opposite spins implied by $^2$.It also does not violate Aufbau's principle since lower-energy orbitals are filled first.Answer:D. Hund's rule