Atoms, Elements and Compounds

This guide covers subtopics 2.1 to 2.3 of topic 2 for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Bonding (2.4 to 2.7) is covered in a separate guide.

0620 separates Core and Extended; Extended candidates study the Core plus the Supplement. 5070 has no such split — all of its stated outcomes are required. The isotopes section below is where the two qualifications differ most.

2.1 Elements, compounds and mixtures

An element contains only one type of atom. A compound contains two or more different elements chemically combined in a fixed ratio. A mixture contains two or more substances that are not chemically combined, so its composition can vary and its components keep their own properties.

The practical consequence is separation. Mixtures can be separated by physical means such as filtration or distillation. Compounds cannot — separating them requires a chemical reaction.

CORE (0620) · REQUIRED (5070) — describe the differences between elements, compounds and mixtures.

2.2 Atomic structure and the Periodic Table

Both qualifications require the same five outcomes here, and 0620 lists all of them as Core. This is genuinely shared material.

Structure of the atom

An atom has a central nucleus containing protons and neutrons, surrounded by electrons arranged in shells.

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	−1	1/1840

Because electrons have negligible mass, almost all of an atom’s mass is in the nucleus. An atom is neutral overall because it has equal numbers of protons and electrons.

Proton number and mass number

Proton number (atomic number) is the number of protons in the nucleus. It defines the element — every atom with 6 protons is carbon.

Mass number (nucleon number) is the total number of protons and neutrons in the nucleus.

Number of neutrons = mass number − proton number.

Electronic configuration

You need to determine the electronic configuration of elements and their ions for proton numbers 1 to 20. Shells fill 2, 8, 8 for these elements.

Sodium (proton number 11) is 2,8,1. The sodium ion Na⁺ has lost one electron, giving 2,8. Chlorine (17) is 2,8,7; the chloride ion Cl⁻ has gained one electron, giving 2,8,8.

The number of outer-shell electrons corresponds to the group in the Periodic Table, and the number of occupied shells corresponds to the period. That link is why electronic configuration explains chemical behaviour rather than just describing it.

CORE (0620) · REQUIRED (5070) — all of section 2.2.

2.3 Isotopes — the qualifications differ here

Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons.

CORE (0620) · REQUIRED (5070) — define isotopes, and interpret and use symbols for atoms.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — state that isotopes of the same element have the same chemical properties because they have the same number of outer-shell electrons; and calculate the relative atomic mass of an element from the relative masses and abundances of its isotopes.

So an O Level candidate must be able to do the relative atomic mass calculation. An IGCSE Core candidate is not required to. If you are taking IGCSE Extended, you do need it.

Calculating relative atomic mass

Multiply each isotope’s relative mass by its percentage abundance, add the results, and divide by 100.

Chlorine exists as ³⁵Cl (75%) and ³⁷Cl (25%):

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

This is why relative atomic masses are often not whole numbers — they are weighted averages across isotopes, not the mass of any single atom.

Isotopes have the same chemical properties because chemical behaviour is determined by outer-shell electrons, and isotopes of an element have identical electronic configurations. Their physical properties, which depend on mass, can differ.

Common mistakes

• Confusing mass number with relative atomic mass. Mass number is a whole number for one atom; relative atomic mass is a weighted average for the element.
• Forgetting to adjust electrons when writing ion configurations. Positive ions have lost electrons; negative ions have gained them.
• Saying isotopes differ in protons. They differ in neutrons; differing protons would make them a different element.
• Core candidates memorising the Ar calculation unnecessarily — or Extended and O Level candidates skipping it because a generic guide called it optional.

Quick revision checklist

• Element, compound and mixture — definitions and how each can be separated
• Relative charges and masses of proton, neutron and electron
• Proton number and mass number defined; calculating neutrons
• Electronic configuration for proton numbers 1–20, including ions
• Isotopes defined
• (0620 Extended, 5070 required) why isotopes share chemical properties
• (0620 Extended, 5070 required) calculating relative atomic mass from abundances

Related resources

• States of Matter and Kinetic Particle Theory
• Atomic Structure — revision notes on the same material
• Ionic, Covalent and Metallic Bonding
• Formulae, Equations and the Mole
• The Periodic Table: Groups and Trends — where electronic configuration explains group properties
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.