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ChemistryInternational GCSEEdexcel & Cambridge IGCSEYears 10–11

IGCSE Chemistry Principles of Chemistry Notes

Chemistry starts with the atom. This unit sets out the particles inside atoms, the difference between elements, compounds and mixtures, and how to describe them for IGCSE.

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Key concepts

Element
A substance made of only one type of atom; cannot be broken down chemically.
Compound
Two or more elements chemically bonded in fixed proportions, e.g. H₂O, CO₂.
Mixture
Two or more substances not chemically bonded, in variable proportions; can be separated physically.
Atomic structure
Protons and neutrons in a tiny nucleus, with electrons in shells around it (2, 8, 8 for first 20 elements).
  • Atomic number = protons; mass number = protons + neutrons.
  • Atoms are neutral, so electrons = protons.
  • Isotopes have the same protons but different neutrons.
  • Relative atomic mass (Aᵣ) is a weighted average of isotope masses.

Worked example

Chlorine has two isotopes: ³⁵Cl (75%) and ³⁷Cl (25%). Calculate its relative atomic mass.

  1. 1Aᵣ = (35 × 75 + 37 × 25) ÷ 100
  2. 2= (2625 + 925) ÷ 100
  3. 3= 3550 ÷ 100 = 35.5

Answer: Aᵣ(Cl) = 35.5

Common mistakes

✗ Calling a compound a mixture because it contains different elements.

✓ A compound is chemically bonded in fixed ratio; a mixture is not bonded and has variable composition.

✗ Saying isotopes are different elements.

✓ Isotopes have the same number of protons, so they are the same element.

✗ Using mass number as the number of neutrons.

✓ Neutrons = mass number − atomic number.

Quick check

Try these, then reveal the answers.

What defines the element an atom belongs to?Reveal answer

The number of protons (its atomic number).

How many electrons are in a neutral atom of magnesium (Z = 12)?Reveal answer

12 electrons, arranged 2,8,2.

Is air an element, compound or mixture?Reveal answer

A mixture (mainly nitrogen and oxygen, not chemically bonded).

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Exam tips

  • Learn the sub-atomic particle table (charge and relative mass) — it is asked almost every year.
  • Show working for relative atomic mass calculations; final answer marks depend on method marks.
  • Use 'chemically bonded' vs 'not chemically bonded' to distinguish compound and mixture.

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Related topics

Bonding and StructureStoichiometry

Key terms explained

Short, exam-ready definitions with a formula, a worked example and an examiner tip.

What is catalyst?What is concentration?What is covalent bond?What is dynamic equilibrium?What is electronegativity?What is ionic bond?Full glossary

Frequently asked questions

What is the difference between an atom and an ion?+

An atom is neutral. An ion is an atom or group of atoms that has lost or gained electrons, giving it an overall charge.

Why can mixtures be separated physically?+

Because the substances in a mixture are not chemically bonded, so techniques like filtration, distillation or chromatography can pull them apart.

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