Cambridge IGCSE·Co-ordinated Sciences 0654·Chemistry
Atoms Elements Compounds
Key knowledge — Atoms Elements Compounds
Formulas & equations
Mass number = protons + neutronsAtomic number = number of protons = number of electrons (neutral atom)
Key facts & rules
- Protons: +1 charge, in nucleus. Neutrons: neutral, in nucleus. Electrons: −1 charge, in shells.
- Isotopes: same element (same proton number), different mass number (different neutrons). Same chemical properties.
- Electronic configuration: fill shells 2, 8, 8, 18 … (first shell max 2, second max 8).
- Group number = number of outer electrons. Period number = number of occupied shells.
- Ion: atom that has gained or lost electrons. Cation (+): lost electrons. Anion (−): gained electrons.
- Relative atomic mass (Ar): weighted average mass of an atom compared to 1/12 of ¹²C.
- Compound: two or more elements chemically bonded — fixed ratio, different properties from elements.
- Mixture: can be separated by physical means (filtration, distillation, chromatography).
IGCSE Chemistry — Atoms, elements & compounds
2447 Cambridge IGCSE 0654 chemistry past paper questions on atoms, elements & compounds, organised by subtopic. Pick a subtopic to start practising.
About atoms, elements & compounds on IGCSE 0654
Atomic structure, protons, neutrons and electrons, electron arrangement, isotopes, and the difference between elements, compounds and mixtures.