Unit 2
Bonding and structure.
How atoms hold together: ionic, metallic and covalent bonds, lattices and crystals, and why structure decides how a substance behaves.
Before this unit
The unit, lesson by lesson
Lessons
6 lessons · 39 practice questions · about 3 hours
Ions and ionic bonding
Why atoms give or take electrons, how positive and negative ions form, what sets their charge, and how to write the formula of an ionic compound.
Ionic and metallic lattices
Inside a salt crystal and a lump of copper: a lattice of ions each pulled by all its neighbours, and a lattice of positive ions in a sea of electrons, and why salt cracks while a metal bends and conducts.
Molecules and covalent bonds
When non-metals meet they share electrons: a shared pair, double and triple bonds, and lone pairs, and how the repulsion between electron pairs sets a molecule’s shape and its bond angles.
Giant structures and small molecules
One carbon, four solids: diamond, graphite, graphene and buckyballs. Giant covalent structures against molecular substances, and how structure explains melting point, hardness and conduction.
Crystals
Why a grain of salt is a cube and a snowflake has six sides: a unit cell that repeats, and the shape it gives the crystal. Seeds and growth, and why slow cooling grows big crystals and fast cooling a crowd of tiny ones.
Alloys
Why an 18-carat gold ring is harder than pure gold: layers of atoms that slip over each other, the yield strength, and odd-sized atoms that buckle the slip plane. And why silver hardly helps.