# Isotope

Atoms of the same element with the same number of protons but different numbers of neutrons. They behave the same in reactions but differ in mass.

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

## Analogy

Isotopes are like two bags of the same sweets, one packed with a heavier wrapper.

## Example

**Two chlorine isotopes**

Chlorine-35: 17 protons, 18 neutrons
Chlorine-37: 17 protons, 20 neutrons

## In practice

Carbon-12 and carbon-14 are both carbon. Carbon-14 is used to date ancient objects.

## Practice

### Flashcards

- **What makes two atoms isotopes of the same element?** Same number of protons, different number of neutrons.

### Interview questions

**Explain what is the same and what is different about two isotopes of an element.**

- Same number of protons
- Same number of electrons when neutral
- Different number of neutrons
- Different mass number

## Questions people ask

### Why are isotopes of an element chemically the same?

Chemical reactions depend on electrons, and isotopes of an element have the same number of electrons when neutral. The extra neutrons add mass but not charge. So two isotopes of chlorine react in the same way, though one is a little heavier.

### Why is relative atomic mass often not a whole number?

Most elements are a mixture of isotopes with different masses. Relative atomic mass is a weighted average of these. Chlorine is about three parts chlorine-35 to one part chlorine-37, so the average comes out around 35.5 rather than a whole number.

## Related concepts

- [Proton](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-1/chemistry-school-part-1-concept-5)
- [Neutron](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-1/chemistry-school-part-1-concept-6)

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