# Explain why the atomic number of an element uniquely identifies that element, and distinguish between the terms atomic number and mass number.

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

## Mark scheme (4 marks)

1. The atomic number is the number of protons in the nucleus of an atom.
2. Each element has a unique/fixed number of protons, so the atomic number uniquely identifies the element; changing the proton number would create a different element.
3. The mass number is the total number of protons AND neutrons in the nucleus.
4. Unlike atomic number, the mass number can vary for atoms of the same element because the number of neutrons can differ (isotopes), whereas the atomic number remains constant for all atoms of the same element.

## Key terms

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

## Related

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

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-atomic-number-of-1bd210e1) · Published by Druglandscape Ltd.