# Relative atomic mass

The average mass of an atom of an element compared with carbon-12 (which is 12). It is the larger number next to each symbol on the periodic table. It has no units.

Chemistry (school) · Part 05: Moles and calculations · Card 1 of 9 · Last updated 2026-10-19

## Analogy

It is a score showing how heavy an atom is compared with a standard atom.

## Example

**Looking up Ar**

H = 1
C = 12
O = 16
Cl = 35.5

## In practice

Oxygen has a relative atomic mass of 16. Magnesium has 24, so a magnesium atom is one and a half times as heavy.

## Practice

### Flashcards

- **What is the Mr of water, H2O?** 18 (2 x 1 + 16).

## Questions people ask

### Why does chlorine have a relative atomic mass of 35.5?

Chlorine is a mixture of two isotopes, chlorine-35 and chlorine-37, in a ratio of about 3 to 1. The relative atomic mass is the average, weighted by how common each is. The result lies between 35 and 37, nearer 35, so 35.5.

### Which number on the periodic table is the relative atomic mass?

It is the larger of the two numbers next to the symbol, and it is often a decimal. The smaller whole number is the atomic number. For sodium, 23 is the relative atomic mass and 11 is the atomic number. Check the labels on your own table.

## Related concepts

- [Relative formula mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-2)
- [Conservation of mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-4/chemistry-school-part-4-concept-2)

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