GlossariesGCSE GlossariesGCSE Chemistry GlossaryIsotope

Isotope - GCSE Chemistry Definition

Reviewed by: Alexandra Brennan

Published

What is an isotope?

In GCSE chemistry, isotopes are atoms of the same element that have the same numbers of protons, but different numbers of neutrons. They therefore have the same atomic number, but different mass numbers. 

For example, hydrogen has three isotopes, shown below. Each isotope has one proton and one electron, but different numbers of neutrons. They all have an atomic number of 1, but mass numbers of 1, 2 and 3.

Diagram showing atomic structures of hydrogen isotopes: hydrogen (1 proton), deuterium (1 proton, 1 neutron), tritium (1 proton, 2 neutrons).
Isotopes of hydrogen

Isotopes share the same chemical properties because they still have the same number of electrons in their outer shells. 

The abundance and mass of each isotope of an element are used to calculate its relative atomic mass (Ar). 

To do this, the following equation is used:

A subscript r equals space fraction numerator left parenthesis space percent sign space o f space i s o t o p e space 1 cross times space m a s s space n u m b e r space o f space i s o t o p e space 1 space right parenthesis plus space left parenthesis space percent sign space o f space i s o t o p e space 2 space cross times space m a s s space n u m b e r space o f space i s o t o p e space 2 space right parenthesis over denominator 100 end fraction


The mass number and abundance for two isotopes of rubidium are shown below: 

Isotope

Mass Number

Percentage Abundance

1

85

72

2

87

28


To calculate the relative atomic mass of rubidium:

R A M equals fraction numerator open parentheses 85 cross times 72 close parentheses plus open parentheses 87 cross times 28 close parentheses over denominator 100 end fraction equals 85.6

Isotope Revision Resources to Ace Your Exams

Most exam boards require you to be able to define isotopes and use them to calculate relative atomic mass. You can revise everything on isotopes using our relative atomic mass revision notes. To make sure you smash this topic in your exams, you can also practice our  model of the atom topic questions.

Explore Our GCSE Chemistry Revision Resources

Examiner-written GCSE Chemistry revision resources that improve your grades 2x

  • Written by expert teachers and examiners
  • Aligned to exam specifications
  • Everything you need to know, and nothing you don’t
GCSE Chemistry revision resources

Share this article

Alexandra Brennan

Reviewer: Alexandra Brennan

Expertise: Chemistry Content Creator

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

The examiner written revision resources that improve your grades 2x.

Join now