# Explain why atoms of the same element can have different masses, and outline how the relative atomic mass of an element is calculated from its isotopic data.

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

## Mark scheme (4 marks)

1. Atoms of the same element have the same number of protons (atomic/proton number) but can differ in the number of neutrons in the nucleus.
2. These different forms of the same element are called isotopes, and the differing neutron number gives each isotope a different mass number (nucleon number).
3. Relative atomic mass is calculated as the weighted mean/average of the masses (relative isotopic masses) of all naturally occurring isotopes of the element.
4. Each isotopic mass is multiplied by its fractional (or percentage) abundance, and the results are summed; the scale is relative to 1/12 the mass of carbon-12.

## Key terms

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

## 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)
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