Intermolecular Forces & Molecular Shapes

Intermolecular forces are the attractive forces that exist between neighbouring molecules, and they're absolutely crucial for determining physical properties like melting and boiling points. Think of them as the "glue" that holds molecules together in liquids and solids.

There are three main types you need to know: hydrogen bonds (the strongest), dipole-dipole forces (medium strength), and Van der Waals forces (the weakest). Understanding these will help you predict why some substances are gases at room temperature whilst others are solids.

Dipole-dipole forces occur between polar molecules that have permanent dipoles due to differences in electronegativity. For example, in HCl, the hydrogen becomes slightly positive (δ+) whilst the chlorine becomes slightly negative (δ−), creating an attraction between neighbouring molecules.

Van der Waals forces are fascinating because they occur even between non-polar molecules. They arise from temporary, instantaneous dipoles created as electrons move around - these temporary dipoles then induce dipoles in neighbouring molecules. This explains why larger molecules with more electrons (higher Mr) have stronger Van der Waals forces and higher boiling points.

Key Pattern: As you go down the halogens (Cl₂ → Br₂ → I₂), the boiling points increase dramatically due to stronger Van der Waals forces from more electrons.

Hydrogen bonding is special - it only occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. This creates exceptionally strong intermolecular attractions that give water its unique properties, including its surprisingly high boiling point of 100°C compared to H₂S at -60°C.

The VSEPR theory (Valence Shell Electron Pair Repulsion) helps predict molecular shapes by assuming electron pairs repel each other and arrange themselves as far apart as possible. Lone pairs repel more strongly than bonding pairs, which explains why bond angles decrease from methane (109.5°) to ammonia (107°) to water (104.5°).

Molecular shape determines whether a molecule is polar or non-polar. Even if individual bonds are polar, symmetric shapes like linear, tetrahedral, or octahedral can result in non-polar molecules because the dipoles cancel out.