This guide covers IGCSE Chemistry topic 3: atoms, elements and compounds. You will revise atomic structure (proton, neutron, electron), isotopes, elements vs compounds vs mixtures, and chemical formulae.

Key concepts to revise

• Atomic structure – Nucleus (protons, neutrons); electrons in shells. Proton number = atomic number; mass number = protons + neutrons.
• Isotopes – Same element; same number of protons; different number of neutrons; different mass number.
• Element – One type of atom; cannot be broken down by chemical means.
• Compound – Two or more elements chemically combined; fixed ratio; properties different from elements.
• Mixture – Two or more substances not chemically combined; can be separated by physical means.

Exam tips and command words

• State – The number of protons, neutrons, electrons in an atom or ion from the periodic table.
• Define – Isotope (same protons, different neutrons); element; compound; mixture.
• Compare – Element vs compound (one type of atom vs chemically combined); compound vs mixture (fixed ratio vs not).
• Write – Chemical formula from name (e.g. sodium chloride NaCl); state number of atoms of each element.

Common mistakes

• Confusing mass number (protons + neutrons) with atomic number (protons only).
• Saying isotopes have “different atomic number” (they have the same atomic number; different mass number).
• Not stating that a compound has different properties from its elements.

Revision checklist

• Draw the structure of an atom (nucleus, shells, protons, neutrons, electrons).
• Calculate protons, neutrons, electrons for an atom and for a simple ion (e.g. Na+, O2-).
• Define isotope and give one example; define element, compound, mixture.
• Write the formula for simple compounds (e.g. water, carbon dioxide, sodium chloride).

Next steps

Book a free trial with an IGCSE Chemistry tutor to practise atomic structure and formulae, or explore Tutopiya’s learning portal for more revision resources.