Describe what is meant by the term 'isotope' and explain why two isotopes of the same element have the same atomic number but different mass numbers.

AQA GCSE Chemistry (8462) — 4.1.1 A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes · Describe · 4 marks · View as Markdown

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

Silicon has three naturally occurring isotopes: silicon-28, silicon-29 and silicon-30. The relative atomic mass of silicon is 28.1.

Model answer (4 marks)

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

The atomic number of an element is the number of protons in the nucleus, so all isotopes of an element have the same atomic number.

The mass number is the total number of protons and neutrons in the nucleus.

Because the number of neutrons differs between isotopes, the mass number (protons + neutrons) is different for each isotope.

Examiner tips

Use the definition of isotope first, then explain atomic number and mass number separately.
Show that atomic number is fixed by protons, mass number varies because neutrons vary.

Common mistakes

Confusing atomic number with mass number.
Saying isotopes have different numbers of protons instead of neutrons.

Mark scheme (4 marks)

Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
The atomic number is determined by the number of protons, which is the same in all isotopes of an element.
The mass number is the total number of protons and neutrons in the nucleus.
Because the number of neutrons differs between isotopes, the mass number (protons + neutrons) is different for each isotope.

Key terms in this question

isotope · atomic number · mass number

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