# Explain why the mass number of an atom is always a whole number, yet the relative atomic mass of most elements is not a whole number.

> IB DP Chemistry Standard 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 of one atom.
2. Since the number of protons and neutrons must be whole numbers, mass number is always a whole number / integer.
3. Most elements exist as a mixture of isotopes, which have the same number of protons but different numbers of neutrons / different mass numbers.
4. Relative atomic mass is the weighted mean / average of the masses of all naturally occurring isotopes of the element, taking into account their relative abundances, so it is rarely a whole number.

## Key terms

- [mass number](https://www.gradenine.co.uk/glossary/mass-number)
- [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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-mass-number-of-c4db0b18) · Published by Druglandscape Ltd.