Q1(a). Quantum numbers of oxygen electrons

Answer in brief

Oxygen has the ground-state configuration 1s² 2s² 2p⁴, with two unpaired electrons; one valid set of quantum numbers is tabulated.

Key points

• The allowed values are n = 1, 2, …; l = 0, …, n − 1; mₗ = −l, …, +l and mₛ = ±½.
• Apply the Aufbau principle, Pauli exclusion and Hund's rule: 1s and 2s electrons take (n, 0, 0, ±½); 2p electrons take n = 2, l = 1.
• Three p orbitals are singly occupied with parallel spins before the fourth p electron pairs. No two rows have all four quantum numbers identical.
• The paired p orbital may equally be assigned mₗ = 0 or +1 in a degenerate orbital description. Reversing all spin projections is also equivalent.