# Mass number

The total number of protons plus neutrons in the nucleus of an atom. Electrons are too light to count.

Chemistry (school) · Part 01: Atoms and matter · Card 9 of 10 · Last updated 2026-10-19

## Analogy

It is like counting the heavy bricks in a wall and ignoring the dust.

## Example

**Counting neutrons**

Chlorine-35
Mass number = 35
Protons = 17
Neutrons = 35 - 17 = 18

## In practice

To find neutrons, take the atomic number away from the mass number.

## Practice

### Flashcards

- **How do you work out the number of neutrons?** Mass number minus atomic number.

## Questions people ask

### Why is the mass number always a whole number?

It counts particles, and you cannot have half a proton or neutron. So the mass number of a single atom is always a whole number. The relative atomic mass on the periodic table can be a decimal because it is an average over many isotopes.

### Do electrons count towards mass number?

No. An electron has about 1/2000 the mass of a proton or neutron, so its contribution is tiny. Mass number is only the protons plus neutrons in the nucleus. That is why losing or gaining electrons never changes the mass number of an atom.

## Related concepts

- [Atomic number](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-1/chemistry-school-part-1-concept-8)
- [Neutron](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-1/chemistry-school-part-1-concept-6)

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