Chemical Bonding & Molecular Structure — Quick Review (10 Sentences) — Summary

Chemical bonding describes atom combinations to form stable, lower-energy species through ionic (electron transfer) or covalent (electron sharing) mechanisms.
Lattice enthalpy, calculated via the Born-Haber cycle ( $\Delta H_f = \Delta H_{sub} + \tfrac{1}{2}\Delta H_{diss} + IE + EA + U$ ), measures the stability of ionic compounds.
Fajan's rules state that covalent character in ionic bonds increases with a small, highly charged cation and a large, polarizable anion.
The steric number (sigma bonds + lone pairs on the central atom) directly determines hybridization: SN 2→sp, 3→ $sp^{2}$ , 4→ $sp^{3}$ , 5→ $sp^{3}$ d, 6→ $sp^{3}$$d^{2}$ .
Lone-pair repulsion (lp-lp > lp-bp > bp-bp) compresses bond angles, giving $H_{2}O$ (104.5°) < $NH_{3}$ (107°) < $CH_{4}$ (109.5°).
Symmetric molecules ( $CO_{2}$ , $BF_{3}$ , $CCl_{4}$ , $SF_{6}$ ) have zero dipole moment because individual bond dipoles cancel by vector addition.
Molecular orbital theory constructs bonding and antibonding MOs from atomic orbitals; bond order = (Nb − Na)/2.
For Z ≤ 7 ( $B_{2}$ , $C_{2}$ , $N_{2}$ ), π2p orbitals fill before σ2p (mixing); for Z > 7 ( $O_{2}$ , $F_{2}$ ), σ2p fills first (normal order).
$O_{2}$ is paramagnetic with bond order 2 due to two unpaired electrons in the degenerate π*2p MOs — MOT's triumph over VBT.
Resonance (electron delocalization) and hydrogen bonding (H–F, H–O, H–N) are key stabilizing phenomena affecting molecular properties and reactivity.