Explain why two atoms that have the same atomic number but different mass numbers are classed as isotopes of the same element.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lithium-6 and lithium-7 are both naturally occurring forms of lithium.
Model answer (4 marks)
Lithium‑6 and lithium‑7 have the same atomic number, meaning they each contain six protons. The atomic number determines the element, so both atoms are lithium.
The mass number is the sum of protons and neutrons. Lithium‑6 has a mass number of 6 (6 protons + 0 neutrons), while lithium‑7 has a mass number of 7 (6 protons + 1 neutron). The different mass numbers show that the two atoms contain a different number of neutrons.
Because they share the same number of protons but differ in neutron number, they are classified as isotopes of the same element.
The mass number is the sum of protons and neutrons. Lithium‑6 has a mass number of 6 (6 protons + 0 neutrons), while lithium‑7 has a mass number of 7 (6 protons + 1 neutron). The different mass numbers show that the two atoms contain a different number of neutrons.
Because they share the same number of protons but differ in neutron number, they are classified as isotopes of the same element.
Examiner tips
- Use the command word ‘explain’ – describe the role of atomic number and mass number and link them to the definition of isotopes.
- Show the relationship: same Z → same element; different A → different neutron count → isotope.
Common mistakes
- Confusing mass number with atomic mass; students write the wrong numbers for lithium‑6 and lithium‑7.
- Failing to mention that the atomic number defines the element, or omitting the neutron difference.
Mark scheme (4 marks)
- The atomic number is the number of protons in an atom / both atoms have the same number of protons
- Atoms of the same element must have the same atomic number / the number of protons determines which element an atom belongs to
- The mass number is the total number of protons and neutrons / a different mass number means a different number of neutrons
- Isotopes have the same number of protons but different numbers of neutrons (linking both features together to justify the classification)
Key terms in this question
atomic number · mass number · isotopes
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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