Explain why atoms of the same element can have different mass numbers.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.3 Atomic structure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Carbon has an atomic number of 6. One atom of carbon has a mass number of 12 and another has a mass number of 14.

Model answer (4 marks)

Atoms of the same element have the same number of protons, so their atomic number is unchanged. However, the two atoms have different mass numbers because they contain a different number of neutrons. Mass number is the sum of protons and neutrons in the nucleus, so a difference in neutrons gives a different mass number. Atoms of the same element with different mass numbers are called isotopes.

Examiner tips

Mention the constant proton number first, then the variable neutron number, and define isotopes.
Use the term ‘mass number’ and explain it as protons + neutrons.
Show the link between neutron number and mass number clearly.

Common mistakes

Confusing atomic number with mass number.
Failing to mention that the difference is due to neutrons.
Calling the atoms ‘different elements’ instead of ‘isotopes’.

Mark scheme (4 marks)

Atoms of the same element always have the same number of protons (atomic number stays the same)
The atoms have different numbers of neutrons
Mass number is the total number of protons and neutrons (in the nucleus)
These atoms are called isotopes

Key terms in this question

mass number