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

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.2 Atoms · Explain · 4 marks · View as Markdown

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

Carbon-12 and carbon-14 are both atoms of the element carbon, yet they have different mass numbers.

Model answer (4 marks)

Atoms of the same element have the same number of protons, but they can have different numbers of neutrons. The mass number is the sum of protons and neutrons in the nucleus. Therefore, if the neutron number differs, the mass number differs. Atoms with the same element but different mass numbers are called isotopes.

Examiner tips

State that protons are the same for a given element
Explain that neutrons can vary
Define mass number as protons + neutrons
Mention isotopes as the result

Common mistakes

Confusing mass number with atomic mass
Saying the element itself changes
Forgetting to mention neutrons

Mark scheme (4 marks)

Atoms of the same element always have the same number of protons (atomic number)
These atoms have a different number of neutrons
Mass number is the total number of protons and neutrons (in the nucleus)
Because the number of neutrons differs, the total (mass number) differs — these are called isotopes

Key terms in this question

mass number