# Explain why atoms of the same element can have different masses, and outline how this affects the relative atomic mass (Ar) value shown on the periodic table.

> IB DP Chemistry Standard 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 (same atomic number) but a different number of neutrons.
2. These atoms are called isotopes; they have different mass numbers (nucleon numbers) and therefore different masses.
3. The relative atomic mass is a weighted mean / average of the masses of all naturally occurring isotopes of the element.
4. Because isotopes occur in different proportions (relative abundances), the Ar value is rarely a whole number.

## Key terms

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

## Related

- [Revision notes for IB DP Chemistry Standard 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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