Concepts, explained clearly
Short, exam-focused explanations of the core concepts across A-Level, AP, IB and IGCSE — with diagrams, worked examples and the exact definitions examiners want. Pick a topic to dive in.
Chemistry
Ionization Energy TrendsEnergy to remove an electron from a gaseous atom — increases across a period, decreases down a group.Solubility RulesGuidelines predicting whether an ionic compound dissolves in water or forms a precipitate.Electron ConfigurationHow an atom's electrons fill shells, sub-shells and orbitals — built by the Aufbau, Pauli and Hund rules.ElectronegativityAn atom's ability to attract the shared pair in a covalent bond — increases across a period, decreases down a group.Van der Waals ForcesWeak intermolecular attractions (London dispersion and dipole–dipole) that set melting and boiling points.Formal ChargeA bookkeeping charge assuming equal electron sharing, used to compare and choose Lewis structures.Covalent vs Ionic BondingIonic = electrons transferred to form ions; covalent = electrons shared — and how that drives their properties.Limiting ReactantThe reactant that runs out first and controls how much product forms; found by comparing moles, not masses.Atomic Radius and Periodic TrendsAtomic radius describes the effective size of an atom.Shells, Sub-shells and OrbitalsElectron shells are labelled by the principal quantum number n.Hydrogen Bonding and Intermolecular ForcesHydrogen bonding is an attraction that helps explain water's relatively high boiling point.