IB Chemistry HL — Structure 2.2: The Covalent Model

This is the Higher Level lesson. The core S2.2.1–S2.2.10 (bonds, VSEPR, polarity, forces, networks) is shared with SL; the HL-only content — sigma & pi bonds, resonance, formal charge and VSEPR for 5–6 electron domains — carries a purple HL badge.

Core — SL & HL

HL AHL only

👆 explore shapes · sigma/pi bonds · resonance

1. Covalent bonds & Lewis formulas S2.2.1–3

A covalent bond is the electrostatic attraction between a shared pair of electrons and the two nuclei. Atoms share to reach a full outer shell (the octet rule). Sharing more pairs gives double and triple bonds — which are shorter and stronger. A coordinate (dative) bond is a covalent bond where both shared electrons come from the same atom.

Bond	Shared pairs	Example	Length	Strength
Single C–C	1	ethane	longest	weakest
Double C=C	2	ethene	shorter	stronger
Triple C≡C	3	ethyne	shortest	strongest

More shared pairs pull the nuclei closer, so as bond order rises the bond gets shorter and stronger.

2. Molecular shapes — VSEPR S2.2.4

The VSEPR model says electron domains (bonding pairs and lone pairs) around a central atom repel and spread out as far as possible. Lone pairs repel a little more strongly, squeezing bond angles slightly. Pick a molecule:

VSEPR shape explorer

Electron domains

Shape

Linear

Bond angle

180°

CO₂: 2 bonding domains (the double bonds count as one domain each), no lone pairs → linear, 180°.

3. Electronegativity & polarity S2.2.5–6

Electronegativity is how strongly an atom pulls the shared electrons. If the two atoms differ, the bond is polar — the more electronegative atom gets a partial negative charge (δ−), the other δ+. Whether the whole molecule is polar depends on the shape: if equal bond dipoles point in opposite directions, they cancel.

Bond dipole & molecular polarity

HCl has one polar bond → polar molecule. CO₂ has two polar bonds but they point in opposite directions and cancel → the molecule is non-polar overall.

4. Forces between molecules & networks S2.2.7–9

Covalent bonds are strong, but separate molecules are held to each other by weaker intermolecular forces. These decide melting/boiling points. Click each type:

Some substances aren't made of small molecules at all: covalent network solids like diamond and silicon dioxide are giant lattices of atoms held entirely by strong covalent bonds — so they have very high melting points and are hard. Graphite is a network too, but its layers slide (soft, a lubricant) and its delocalised electrons let it conduct.

5. Sigma (σ) and pi (π) bonds HL

Every covalent bond is one of two types. A sigma (σ) bond forms from head-on overlap of orbitals along the bond axis — strong, and free to rotate. A pi (π) bond forms from sideways overlap of p orbitals above and below the axis. The rule: a single bond is 1 σ; a double bond is 1 σ + 1 π; a triple bond is 1 σ + 2 π.

σ bonds

π bonds

A single bond is one σ bond (head-on overlap) — atoms can rotate freely around it.

6. Shapes with 5 & 6 electron domains HL

At HL the VSEPR model extends to five and six domains — and to shapes where some domains are lone pairs. Explore:

HL VSEPR explorer

Electron domains

Shape

Trigonal bipyramidal

Bond angles

90° & 120°

7. Resonance & formal charge HL

Sometimes one Lewis structure isn't enough. In ozone (O₃) the two oxygen–oxygen bonds are actually identical, midway between a single and a double bond. We draw two resonance structures linked by a double-headed arrow; the real molecule is a delocalised average of them. Benzene (C₆H₆) is the classic case — six delocalised π electrons spread evenly, drawn as a ring.

Neither structure is real on its own — the true ozone molecule is the resonance hybrid, with two equal bonds.

Formal charge helps choose the best Lewis structure. For each atom: FC = (valence electrons) − (non-bonding electrons) − ½(bonding electrons). The most likely structure is the one where formal charges are closest to zero. For example, the central O in ozone has FC = 6 − 2 − ½(6) = +1.

Common mistakes examiners see

What is the shape and bond angle of a water molecule?✗ Linear, 180°. ✓ Bent, ~104.5°. Oxygen has 4 domains (2 bonding + 2 lone pairs); the lone pairs push the bonds closer together.

Is CO₂ a polar molecule?✗ Yes, because the C=O bonds are polar. ✓ No — the two polar bonds point in exactly opposite directions (linear) and cancel, so the molecule is non-polar.

When ice melts, which forces are overcome?✗ The covalent O–H bonds break. ✓ Only the weaker hydrogen bonds between molecules are overcome — the covalent bonds within each water molecule stay intact.

Why does diamond have such a high melting point?✗ Strong intermolecular forces. ✓ It is a covalent network — every carbon is covalently bonded to four others, so melting means breaking strong covalent bonds throughout the lattice.

How many σ and π bonds are in a C≡C triple bond?✗ 3 σ bonds. ✓ 1 σ and 2 π. Every multiple bond has exactly ONE σ bond; the extra pairs are π bonds. HL

What does resonance mean for the bonds in ozone?✗ The molecule flips between two structures. ✓ It does not flip — the real molecule is a single delocalised hybrid, so both O–O bonds are identical (equal length). HL

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The Covalent Model