2.3Atoms, Elements and Compounds

Isotopes — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 7 parts Lesson 3 of 7 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision

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Lesson summary

2.3 Isotopes — Notes — IGCSE Chemistry notes: Define isotopes, use atomic symbols, and explain why isotopes share chemical properties and how relative atomic mass is calculated.Isotopes — different atoms of the same element that have the same number of protons but different numbe…

Not different elements
Same chemical properties
Different physical properties
Ion formation
Chloride ions
Relative atomic mass Ar

Part 1 of 5Defining isotopes

1Isotopes

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

They have the same proton number and different mass numbers.

3Symbol

Symbol the chemical symbol or name followed by a dash and the mass number, so carbon-14 or C-14.

It can be written with the mass number at the top left and the proton number at the bottom left of the symbol.

Carbon-12 has 6 protons and 6 neutrons.

Carbon-14 has 6 protons, 6 electrons and 14 − 6 = 8 neutrons.

Ions can be written the same way, such as chlorine-35 as 35Cl− with 17 protons.

8Not different elements

Not different elements changing the neutron number does not change the proton number, so the element stays the same.

Part 2 of 5Isotopes of hydrogen

Isotope	Protons	Neutrons	Electrons
Hydrogen-1	1	0	1
Hydrogen-2 (deuterium)	1	1	1
Hydrogen-3 (tritium)	1	2	1

Part 3 of 5Properties of isotopes (Supplement)

1Same chemical properties

Same chemical properties isotopes have the same number of electrons in their outer shell, which decides chemistry.

They have the same electronic configuration, and chemical reactions involve electrons.

3Different physical properties

Different physical properties the extra neutrons are neutral and add only mass, so density, boiling point and melting point differ.

Isotopes look identical, so a sample of carbon-14 looks like carbon-12.

Part 4 of 5Isotopes and ions

Ion formation

Ion formation gaining or losing electrons changes only the number of electrons.

The identity of an isotope depends on its protons and neutrons.

Chloride ions

Chloride ions the ions of chlorine-35 and chlorine-37 each have 18 electrons but keep different numbers of neutrons (18 and 20).

Part 5 of 5Relative atomic mass (Supplement)

Relative atomic mass (Supplement)

Relative atomic mass Ar

Relative atomic mass Ar atoms are too small for grams,
so the average mass of an element's isotopes is compared with 1/12 of the mass of a carbon-12 atom, and there are no units.

Hydrogen has Ar 1, so 12 hydrogen atoms have the same mass as one carbon atom.

Calculation

Multiply each isotope's mass number by its percentage abundance.

Add the products.

Divide by 100.

Ar is a weighted average that allows for isotopes, so it is often not a whole number and is often rounded on the Periodic Table.

Lithium has Ar 6.94, which rounds to 7.

Chlorine has Ar 35.5, a weighted average of chlorine-35 (75%) and chlorine-37 (25%), and no single atom has this mass.

Key terms

Term	Meaning
Isotopes	atoms of the same element with the same number of protons but different numbers of neutrons.
Relative atomic mass	the average mass of an element's isotopes compared with 1/12 of the mass of a carbon-12 atom.
Nucleon number	the total number of protons and neutrons, also called the mass number.
Deuterium	hydrogen-2, an isotope with one proton and one neutron.
Weighted average	an average that allows for the abundance of each isotope.

Avoid these mistakes

Isotopes have different numbers of protonsthey have the same protons and different neutrons.

Isotopes react differentlytheir chemical properties are the same.

Ar is the mass number of the commonest isotope: it is a weighted average of all isotopes.

Mass number is the number of neutrons — it is protons plus neutrons

Last updated: 18 October 2026