The complete lecture — bonds form and molecules take shape in the live panel as you read. Scroll down; the animation keeps pace.
1 — Why atoms combine: the octet rule
Atoms bond to reach a stable noble-gas arrangement — usually an octet (8 valence electrons). They can transfer electrons (ionic) or share them (covalent).
- Octet rule — atoms gain/lose/share to get 8 in the valence shell (2 for H).
2 — The covalent bond
- Covalent bond — mutual sharing of one or more electron pairs.
Bondssingle = 1 shared pair · double = 2 · triple = 3
3 — Coordinate (dative) bond
- Coordinate bond — both shared electrons come from one atom (the donor). e.g. NH₃ + H⁺ → NH₄⁺.
4 — Valence Bond Theory
- σ bond — head-on (axial) overlap; every single bond.
- π bond — sideways overlap of parallel p-orbitals; the extra bond in double/triple.
Countingsingle 1σ · double 1σ+1π · triple 1σ+2π
5 — Hybridisation
| Type | Geometry · angle | Example |
|---|
| sp³ | tetrahedral · 109.5° | CH₄ |
| sp² | trigonal planar · 120° | BF₃ |
| sp | linear · 180° | BeCl₂ |
6 — VSEPR theory
Electron pairs around the central atom push as far apart as possible. Lone pairs repel hardest, squeezing bond angles. Set the sliders in the live panel to build a shape.
Repulsion orderlone–lone > lone–bond > bond–bond
7 — Shapes of molecules
| BP | LP | Shape | Angle |
|---|
| 2 | 0 | linear | 180° |
| 3 | 0 | trigonal planar | 120° |
| 4 | 0 | tetrahedral | 109.5° |
| 3 | 1 | pyramidal | 107° |
| 2 | 2 | bent | 104.5° |
8 — Bond parameters
- Bond length — triple < double < single.
- Bond energy — triple > double > single.
9 — Polar & non-polar bonds
Shape decides molecular polarity: equal but opposite dipoles cancel in a symmetric molecule.
Dipole momentμ = q × d (debye)
10 — Molecular Orbital Theory
Bond order½ (bonding e⁻ − antibonding e⁻)
MOT explains why O₂ is paramagnetic — VBT cannot.
11 — Ionic vs covalent
| Property | Ionic | Covalent |
|---|
| m.p./b.p. | high | low |
| conducts? | molten/aqueous | no |
12 — Worked questions
shape of NH₃
3 bond pairs + 1 lone pair (sp³) → trigonal pyramidal, 107°
bond order of N₂
½(10 − 4) = 3
13 — Exam recap
- Octet rule; ionic vs covalent vs coordinate.
- Single/double/triple; Lewis structures.
- VBT σ vs π; bond counting.
- Hybridisation sp³/sp²/sp.
- VSEPR — the five shapes; lone pairs shrink angles.
- Polarity & shape; MOT bond order.