Chemical Bonding & Molecular Structure — Essential NEET Facts — Summary

Steric number rule: SN = sigma bonds + lone pairs on central atom; SN directly gives hybridization (2→sp, 3→ $sp^{2}$ , 4→ $sp^{3}$ , 5→ $sp^{3}$ d, 6→ $sp^{3}$$d^{2}$ )
Lone-pair repulsion order: lp-lp > lp-bp > bp-bp → each lone pair compresses bond angles by ~2.5°
Bond angles: $CH_{4}$ = 109.5°, $NH_{3}$ = 107°, $H_{2}O$ = 104.5° (lone pairs reduce angle)
$XeF_{2}$ : SN = 5, $sp^{3}$ d, 3 lone pairs equatorial → linear (not bent)
$XeF_{4}$ : SN = 6, $sp^{3}$$d^{2}$ , 2 lone pairs both axial → square planar
$XeF_{6}$ : Distorted octahedral (1 lone pair)
Zero dipole moment: $CO_{2}$ (linear), $BF_{3}$ (trigonal planar), $CCl_{4}$ (tetrahedral), $SF_{6}$ (octahedral) — all symmetric
Non-zero dipole moment despite similar formula: $NH_{3}$ ≠ $BF_{3}$ ( $NH_{3}$ has lone pair contributing to resultant dipole); $H_{2}O$ ≠ $CO_{2}$
Bond order formula: BO = (Nb − Na)/2 in MOT
$O_{2}$ is paramagnetic — 2 unpaired $e^{-}$ in degenerate π*2p; bond order = 2
MO mixing rule: Z ≤ 7 ( $B_{2}$ , $C_{2}$ , $N_{2}$ ) → π2p filled BEFORE σ2p; Z > 7 ( $O_{2}$ , $F_{2}$ ) → σ2p before π2p
$N_{2}$ : BO = 3, diamagnetic, shortest bond among diatomics; electronic config: σ1 $s^{2}$ σ1 $s^{2}$ σ2 $s^{2}$ σ2 $s^{2}$ π2 $p^{4}$ σ2 $p^{2}$
$O_{2}^{-}$ (superoxide): BO = 1.5, paramagnetic; $O_{2}^{2-}$ (peroxide): BO = 1, diamagnetic
Bond properties: Bond order ↑ → bond length ↓ → bond energy ↑
Born-Haber equation: $\Delta H$ _f = $\Delta H$ _sub + ½ $\Delta H$ _diss + IE + EA + U
Fajan's rules: Small cation + large anion + high charge = more covalent character
Sigma bond: head-on overlap, free rotation, in every covalent bond; Pi bond: lateral overlap, restricts rotation, weaker
Resonance: actual structure is hybrid; bond length intermediate (benzene C–C = 1.40 Å, between single 1.54 Å and double 1.34 Å)
H-bonding: only when H is directly bonded to F, O, or N (highly electronegative, small atoms)
Dipole moment unit: 1 Debye = $3.336 \times 10^{-30}$ C·m