# Explain why two atoms with the same mass number can belong to different elements, and identify the subatomic property that determines which element an atom belongs to.

> IB DP Chemistry Higher Level (2023 syllabus) — S1.2 The nuclear atom · Explain · 4 marks

## Mark scheme (4 marks)

1. Mass number is the total number of protons and neutrons (nucleons) in the nucleus.
2. Two atoms with the same mass number can have different numbers of protons and different numbers of neutrons that sum to the same total, so they belong to different elements.
3. The number of protons (atomic number / proton number) determines which element an atom belongs to.
4. Atoms that have the same mass number but different atomic numbers (and therefore belong to different elements) are called isobars.

## Key terms

- [mass number](https://www.gradenine.co.uk/glossary/mass-number)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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