Ionic, covalent, dative and metallic bonding, electronegativity and polarity, intermolecular forces, and the shapes of simple molecules from electron-pair repulsion.

What this dot point is asking

WJEC wants you to describe the four bonding types, explain electronegativity and polarity, identify the intermolecular forces between molecules, and predict molecular shapes and bond angles using electron-pair repulsion theory.

The answer

The bonding types

Electronegativity and polarity

Electronegativity is the power of an atom to attract the bonding electrons in a covalent bond. It increases across a period and up a group, with fluorine the most electronegative. When two bonded atoms differ in electronegativity, the bond is polar, carrying partial charges $\delta+$ and $\delta-$.

Intermolecular forces

Three forces act between molecules, in increasing strength: London (dispersion) forces from instantaneous dipoles, present in all molecules and stronger with more electrons; permanent dipole-dipole forces between polar molecules; and hydrogen bonds, the strongest, between an $\text{H}$ attached to $\text{N}$, $\text{O}$ or $\text{F}$ and a lone pair on an adjacent $\text{N}$, $\text{O}$ or $\text{F}$.

Predicting molecular shapes

Electron-pair repulsion theory states that electron pairs around a central atom repel and arrange themselves as far apart as possible. Lone pairs repel more strongly than bonding pairs (lone-lone > lone-bond > bond-bond).

Common shapes: 2 pairs linear $180$ degrees; 3 pairs trigonal planar $120$ degrees; 4 pairs tetrahedral $109.5$ degrees; 5 pairs trigonal bipyramidal; 6 pairs octahedral $90$ degrees.

Examples in context

Hydrogen bonding and ice. Water expands on freezing because hydrogen bonds hold molecules in an open hexagonal lattice, making ice less dense than liquid water, which is why ice floats and lakes freeze from the top down. Dative bonds in catalysis. Transition-metal complexes form dative bonds from ligand lone pairs into empty metal $d$ orbitals, the basis of much of inorganic and biological chemistry covered later in Unit 3.

Try this

Q1. State the shape and bond angle of a methane molecule. [1 mark]

• Cue. Tetrahedral, $109.5$ degrees.

Q2. Name the strongest intermolecular force present in liquid hydrogen fluoride. [1 mark]

• Cue. Hydrogen bonding.

Q3. Explain why carbon dioxide is non-polar but sulfur dioxide is polar. [2 marks]

• Cue. $\text{CO}_2$ is linear so bond dipoles cancel; $\text{SO}_2$ is bent (a lone pair on sulfur) so dipoles do not cancel.