The ionic model

Ionic bonding is the electrostatic attraction between oppositely charged ions, formed by transfer of electrons from metal to non-metal (e.g. Na⁺ and Cl⁻).

• Forms a giant ionic lattice.
• High melting points (strong forces throughout).
• Conducts electricity when molten or dissolved (ions free to move), not when solid.
• Often brittle and soluble in water.

Ionic properties

The covalent model

A covalent bond is a shared pair of electrons between non-metal atoms. Atoms can share one, two or three pairs (single, double, triple bonds); a coordinate (dative) bond has both electrons from one atom.

Lewis (electron-dot) structures show bonding and lone pairs. Small covalent molecules have weak forces between molecules, so low melting points; giant covalent solids (diamond, SiO₂) are very hard with high melting points.

VSEPR — molecular shapes

Electron domains around a central atom repel and spread out as far as possible. Lone pairs repel a little more strongly, closing bond angles slightly.

• 2 domains → linear, 180° (CO₂)
• 3 domains → trigonal planar, 120° (BF₃)
• 4 domains → tetrahedral, 109.5° (CH₄)
• 3 bonds + 1 lone pair → trigonal pyramidal, ~107° (NH₃)
• 2 bonds + 2 lone pairs → bent, ~104.5° (H₂O)

Bond angle in methane

Bond angle in water

Polarity

Electronegativity is an atom's pull on a shared pair. A big difference gives a polar bond (δ+ / δ−). A molecule is polar overall only if the bond dipoles do not cancel by symmetry.

CO₂ has polar bonds but is non-polar (linear, dipoles cancel); H₂O is polar (bent, dipoles add).

Polar or not?

Intermolecular forces

Between molecules, three forces (weakest → strongest) set melting/boiling points:

• London (dispersion) forces — in all molecules; stronger for bigger electron clouds.
• Dipole–dipole — between polar molecules.
• Hydrogen bonding — strongest; when H is bonded to N, O or F.

Stronger intermolecular forces → higher boiling point.

Boiling points

Metallic bonding and materials

The metallic model is a lattice of cations in a 'sea' of delocalised electrons. This explains electrical conductivity, thermal conductivity, and malleability (layers slide).

From models to materials: real bonding sits on a continuum; alloys (e.g. brass) mix metals for strength; polymers are long covalent chains whose properties depend on their structure.

Sigma & pi bonds, hybridisation

A single bond is a sigma (σ) bond (end-on overlap). Double and triple bonds add pi (π) bonds (side-on overlap). Carbon hybridises its orbitals: sp³ (4 σ, tetrahedral), sp² (3 σ + 1 π, trigonal planar), sp (2 σ + 2 π, linear).

AHL also covers formal charge, resonance and delocalisation — e.g. the six delocalised π electrons in benzene. With 5 or 6 electron domains the shapes are trigonal bipyramidal (90°/120°) and octahedral (90°).

Hybridisation

Octahedral angle

Which bonding model?

Tap a substance, then its bonding type.

Match the shape to its bond angle

Tap an item on the left, then its match on the right.

The big ideas to know

You've covered Models of bonding and structure for IB Diploma Chemistry HL. Press Finish to see your score.