$word_{count}$: 280

MOT treats molecules as a whole, combining atomic orbitals to form molecular orbitals that belong to the entire molecule. Key principles: (1) Number of MOs = number of AOs combined. (2) Bonding MOs are lower in energy (constructive interference); antibonding MOs are higher (destructive interference, denoted with ). (3) Electrons fill MOs following Aufbau, Pauli exclusion, and Hund's rules. Bond order = $\frac{Nb - Na}{2}$. If BO = 0, the molecule does not exist (He2, Be2). If BO > 0, the molecule is stable. Higher bond order means shorter, stronger bonds. The critical detail for JEE is the MO energy ordering switch: for Z <= 7 (Li2 through N2), s-p mixing pushes sigma(2p) above pi(2p); for Z >= 8 (O2, F2), sigma(2p) falls below pi(2p). This switch explains why B2 is paramagnetic (2 unpaired electrons in degenerate pi bonding MOs) and why O2 is paramagnetic (2 unpaired electrons in degenerate pi antibonding MOs). MOT's triumph is explaining O2's paramagnetism, which VBT cannot. Key species to know: O2 (BO=2, paramagnetic), O2+ (BO=2.5), O2- (BO=1.5), $O2^{2-}$ (BO=1), N2 (BO=3, diamagnetic), NO (BO=2.5, paramagnetic), CO (BO=3, isoelectronic with N2). Isoelectronic species (same electron count) have identical MO configurations: N2, CO, NO+, CN- all have 14 electrons and BO=3.