Formation of Molecular Orbitals — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on Molecular Orbital Theory, a systematic approach is crucial. First, thoroughly understand the LCAO principle and the conditions for effective atomic orbital overlap (comparable energy, proper symmetry, maximum overlap).

Second, memorize the two main MO energy level diagrams: one for molecules with $\le 14$ electrons (where $\pi_{2p}$ is lower than $\sigma_{2p}$ due to s-p mixing) and another for molecules with $> 14$ electrons (where $\sigma_{2p}$ is lower than $\pi_{2p}$).

Practice writing MO electronic configurations for various homonuclear and simple heteronuclear diatomic species and their ions.

For numerical problems involving bond order, always start by calculating the total number of electrons. Then, apply the correct energy order to fill the MOs according to Aufbau, Pauli, and Hund's rules. Finally, use the bond order formula: Bond Order = $\frac{1}{2} (N_b - N_a)$. Pay close attention to the sign of the charge for ions.

For conceptual questions, visualize the overlap of atomic orbitals to understand $\sigma$ and $\pi$ bond formation. Remember that bonding MOs stabilize (lower energy, increased electron density) and antibonding MOs destabilize (higher energy, nodal plane). For magnetic properties, count unpaired electrons. If there's an odd number of total electrons, the species must be paramagnetic. Be wary of trap options that confuse bonding and antibonding characteristics or misapply energy orders.