Summary & Key Concepts Cheat-Sheet

The Whole Chapter in One Page

Let's tie everything together. Atoms bond to reach a stable noble-gas configuration (octet/duplet), because the bonded system is lower in energy.

The three bond types:

• Ionic (electrovalent): complete electron transfer (metal + non-metal). Stability set by lattice enthalpy (↑ with higher charge, smaller size). Non-directional; high m.p.; conducts when molten/aqueous.
• Covalent: electron sharing (non-metal + non-metal). Directional → definite shapes.
• Coordinate (dative): shared pair donated entirely by one atom (e.g. $NH_4^+$, $O_3$).

The toolkit we built, in order: Lewis structures → formal charge → octet exceptions → bond parameters → dipole moment → resonance → VSEPR shapes → VBT/overlap → hybridization → MOT → hydrogen bonding.

Key Point: Ionic = transfer; covalent = sharing; both chase the octet. Everything else in the chapter explains how strong, what shape, and why.

Master Shortcut Formulas

Keep these at your fingertips — they solve most numericals:

• Quantisation of charge: $Q = ne$
• Formal charge: $FC = V - L - \dfrac{B}{2}$
• Hybridization steric number: $H = \dfrac{1}{2}(V + M - C + A)$ — V = central-atom valence e⁻, M = monovalent atoms, C = cation charge (subtract), A = anion charge (add)
• Bond order (MOT): $\dfrac{1}{2}(N_b - N_a)$
• Reaction enthalpy: $\Delta_r H = \sum BE_{broken} - \sum BE_{formed}$
• Dipole moment: $\mu = q \times d$ (unit: Debye, $1\,D = 3.336\times10^{-30}$ C m)
• % ionic character: $\dfrac{\mu_{obs}}{\mu_{ionic}} \times 100$
• Resonance bond order: $\dfrac{\text{total bonds among equivalent positions}}{\text{number of positions}}$

Key Point: The steric-number formula $H = \tfrac{1}{2}(V+M-C+A)$ is the single most time-saving tool in the chapter — it gives hybridization and (after subtracting lone pairs) the shape.

Hybridization & VSEPR Quick-Table

Steric No.	Hybridization	Electron geometry	0 lp	1 lp	2 lp	3 lp
2	$sp$	linear	linear	—	—	—
3	$sp^2$	trigonal planar	trigonal planar	bent	—	—
4	$sp^3$	tetrahedral	tetrahedral	pyramidal	bent	—
5	$sp^3d$	trigonal bipyramidal	TBP	see-saw	T-shape	linear
6	$sp^3d^2$	octahedral	octahedral	square pyramidal	square planar	—
7	$sp^3d^3$	pentagonal bipyramidal	PBP	—	—	—

Bond-angle quick facts: $CH_4$ 109.5°, $NH_3$ 107°, $H_2O$ 104.5°, $CO_2$/$BeCl_2$ 180°, $BF_3$ 120°.

s-character → angle: $sp$ (50%, 180°) > $sp^2$ (33%, 120°) > $sp^3$ (25%, 109.5°).

Key Point: Memorise this table — identifying hybridization + shape is the single most-tested skill in the chapter.

MOT Reference & Magnetism

Energy order (up to $N_2$): $\sigma1s, \sigma^*1s, \sigma2s, \sigma^*2s, (\pi2p_x = \pi2p_y), \sigma2p_z, (\pi^*2p_x=\pi^*2p_y), \sigma^*2p_z$.

From $O_2$ onward: $\sigma2p_z$ drops below the $\pi2p$ pair.

Molecule	Bond order	Magnetism
$H_2$	1	diamagnetic
$He_2$	0	does not exist
$B_2$	1	paramagnetic
$C_2$	2	diamagnetic
$N_2$	3	diamagnetic
$O_2$	2	paramagnetic
$F_2$	1	diamagnetic

$O_2$ family: $O_2^+$ (2.5), $O_2$ (2.0), $O_2^-$ (1.5), $O_2^{2-}$ (1.0). Stability/bond strength: $O_2^+ > O_2 > O_2^- > O_2^{2-}$.

Key Point: $B_2$ and $O_2$ are paramagnetic; $C_2$ and $N_2$ are diamagnetic. $O_2$'s paramagnetism is MOT's headline success.

Exceptions, Trends & Top Exam Mistakes

Octet exceptions: incomplete ($BeCl_2$, $BCl_3$, $AlCl_3$), odd-electron ($NO$, $NO_2$, $ClO_2$), expanded ($PCl_5$, $SF_6$, $IF_7$, $XeF_4$).

Fajans' rules (covalent character ↑): smaller/higher-charge cation, larger anion → more covalent (e.g. $LiI > LiCl$; $AlCl_3 > NaCl$).

Hydrogen bonding: only H–F, H–O, H–N. Intermolecular ↑ b.p.; intramolecular (o-nitrophenol) ↓ b.p.

Top mistakes to avoid:

1. Confusing resonance (↔) with equilibrium (⇌).
2. Forgetting lone pairs count toward hybridization (NH₃, H₂O are still $sp^3$).
3. Using the wrong MO order for $O_2$ vs $N_2$.
4. Claiming $CO_2$/$BF_3$/$CCl_4$ are polar (they are non-polar by symmetry).
5. Wrapping plain numbers/units in LaTeX, or forgetting the charge adjustment in Lewis/hybridization counts.

Key Point — 60-second revision card: Transfer vs share → lattice enthalpy & bond order → formal charge & exceptions → μ & symmetry → resonance → VSEPR shape → hybridization (steric number) → MOT (bond order + magnetism) → H-bonding (water's anomalies). Master this flow and the chapter is yours.