Comprehensive A-Level Chemistry Notes on Bonding

Bond Polarity

Electronegativity is basically how greedy an atom is for electrons in a covalent bond. Scientists measure this on the Pauling scale from 0-4, with fluorine being the greediest at 4.0.

The usual suspects for high electronegativity are F, O, N, and Cl - remember these! As you move across a period, atoms get more electronegative because they have more protons pulling on the same shell of electrons. Going down a group, electronegativity decreases because the outer electrons are further from the nucleus.

Bonding exists on a spectrum rather than rigid categories. Pure covalent bonds (like F₂) have identical electronegativity, whilst polar covalent bonds form when the electronegativity difference is 0.3-1.7. When it's above 1.7, you get ionic bonding.

Key Tip: In polar covalent bonds like HCl, the more electronegative atom (Cl) becomes slightly negative (δ⁻) whilst the other becomes slightly positive (δ⁺). However, symmetrical molecules with identical bonds cancel out their dipoles, making them non-polar overall.

Intermolecular Forces

Van der Waals forces are the weakest intermolecular attractions, occurring between all molecules (but not ionic compounds). They happen because electrons are constantly moving around, creating temporary dipoles that influence neighbouring molecules.

More electrons mean stronger van der Waals forces, which is why larger molecules have higher boiling points. Shape matters too - molecules with greater surface area can get closer together, strengthening these forces.

Permanent dipole-dipole forces occur between polar molecules and are stronger than van der Waals. These molecules line up so their opposite charges attract, like HCl molecules arranging δ⁺ to δ⁻.

Hydrogen bonding is the strongest intermolecular force, forming when hydrogen bonds to nitrogen, oxygen, or fluorine. The huge electronegativity difference creates such strong attractions that substances like water have unusually high boiling points.

Real-world Connection: Hydrogen bonding explains why water expands when it freezes - the bonds hold molecules in a rigid structure that's less dense than liquid water, which is why ice floats!

Water's Special Properties

Here's a fascinating fact that demonstrates hydrogen bonding in action: ice floats on water because hydrogen bonds create a structured lattice in solid ice. This arrangement actually holds the water molecules further apart than in liquid water, making ice less dense.

This unique property is crucial for life on Earth - if ice sank, lakes and oceans would freeze from the bottom up, potentially killing aquatic life and dramatically altering our climate.

Exam Success: Understanding why ice floats is a classic exam question that combines electronegativity, hydrogen bonding, and density concepts - perfect for showing off your chemical knowledge!