Energy levels and electronic configuration

Combining two 1s atomic orbitals gives the pair σ1s (bonding) and σ1s (antibonding); combining two 2s orbitals similarly gives σ2s and σ2s; and the three 2p orbitals on one atom combine with the three 2p orbitals on the other atom to give SIX molecular orbitals in total — three bonding (σ2pz, πpx, πpy) and three antibonding (σ2pz, πpx, π*py). Crucially, the experimentally measured ENERGY ORDER of these orbitals is not identical across every second-row diatomic molecule. For every second-row diatomic EXCEPT O2 and F2, the order is $\sigma1s<\sigma^*1s<\sigma2s<\sigma^*2s<(\pi2p_x=\pi2p_y)<\sigma2p_z<(\pi^*2p_x=\pi^*2p_y)<\sigma^*2p_z$ — the degenerate πpx/πpy PAIR sits BELOW σ2pz. For O2 and F2 SPECIFICALLY, the order instead has σ2pz sitting BELOW the degenerate πpx/πpy pair: $\sigma1s<\sigma^*1s<\sigma2s<\sigma^*2s<\sigma2p_z<(\pi2p_x=\pi2p_y)<(\pi^*2p_x=\pi^*2p_y)<\sigma^*2p_z$. Filling a molecule's total electron count into these levels, lowest energy first, gives its molecular-orbital electronic configuration, from which three important things follow directly: (a) STABILITY — a molecule is stable if it has more electrons in BONDING molecular orbitals than in ANTIBONDING ones; (b) MAGNETIC NATURE — a molecule is diamagnetic if every one of its molecular orbitals is completely filled (all electrons paired), and paramagnetic if at least one molecular orbital is only half-filled (carrying a …