# Atomic number and mass number

Atomic number is the number of protons. Mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons.

Physics (school) · Part 08: Atoms and radioactivity · Card 2 of 8 · Last updated 2026-10-19

## Analogy

Atomic number is an element's ID card. Mass number is its weight class.

## Example

**Carbon-14**

Protons = 6 (atomic number)
Mass number = 14
Neutrons = 14 - 6 = 8

## In practice

Carbon-12 and carbon-14 both have 6 protons, but carbon-14 has two more neutrons.

## Practice

### Flashcards

- **What does the atomic number tell you?** The number of protons.
- **What is an isotope?** An atom with the same number of protons but a different number of neutrons.

## Questions people ask

### How do you find the number of neutrons?

Subtract the atomic number from the mass number. Carbon-14 has a mass number of 14 and an atomic number of 6, so it has 14 - 6 = 8 neutrons. The atomic number gives the protons, which also decides which element the atom belongs to.

### What is an isotope?

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They behave almost identically in chemical reactions. Some isotopes are stable, while others, such as carbon-14, are unstable and decay by giving out radiation.

## Related concepts

- [Structure of the atom](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-1)
- [Nuclear fission](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-7)
- [Nuclear fusion](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-8)

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