# Atomic number

The number of protons in an atom. It is the number that identifies the element.

Chemistry (school) · Part 01: Atoms and matter · Card 8 of 10 · Last updated 2026-10-19

## Analogy

Like a house number, it tells you exactly which element you are looking at.

## Example

**Sodium atom**

Atomic number = 11
Protons = 11
Electrons = 11
Charge = 0

## In practice

Sodium has atomic number 11, so every sodium atom has 11 protons and, if neutral, 11 electrons.

## Practice

### Flashcards

- **What does the atomic number tell you?** The number of protons in the atom, which identifies the element.
- **How do you work out the number of neutrons?** Mass number minus atomic number.

## Questions people ask

### Where do you find the atomic number on the periodic table?

On most periodic tables the atomic number is the whole number next to the symbol, and it is usually the smaller of the two numbers. The elements are arranged in order of increasing atomic number, so it also tells you the element's position in the table.

### Can two different elements have the same atomic number?

No. The atomic number is a unique label. Each element has its own number of protons, so no two elements share one. If an atom has 8 protons it is oxygen, always. That is why the periodic table is ordered by atomic number.

## Related concepts

- [Mass number](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-1/chemistry-school-part-1-concept-9)
- [Neutron](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-1/chemistry-school-part-1-concept-6)

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