Electronic Configuration of Molecules — Prelims Strategy

Prelims Strategy

To excel in NEET questions on the electronic configuration of molecules, a systematic approach is vital. First, memorize the two distinct MO energy level orders: one for diatomic molecules with $\le 14$ electrons (where $\pi_{2p}$ is below $\sigma_{2p_z}$ due to s-p mixing) and another for those with $> 14$ electrons (where $\sigma_{2p_z}$ is below $\pi_{2p}$).

This is the most common point of error. Second, practice counting total valence electrons accurately, especially for charged species. Third, apply the Aufbau principle, Pauli's exclusion principle, and Hund's rule meticulously when filling electrons into the MOs.

Pay special attention to Hund's rule for degenerate orbitals (like $\pi_{2p}$ and $\pi^*_{2p}$) as it dictates magnetic properties. For numerical problems involving bond order, use the formula $BO = \frac{1}{2}(N_b - N_a)$ correctly.

For conceptual questions, understand the direct relationship between bond order and stability/bond strength, and the inverse relationship with bond length. Always double-check your electron count and MO filling.

A quick trick for isoelectronic species (same total electrons) is that they often have the same bond order and similar magnetic properties, which can save time.