# Formula Mass of an Atom Relative atomic mass    Value of the unified atomic mass unit ## Atomic mass

Unit
Absolute mass of an atom. For example, a helium atom has the following (absolute) mass in kilograms:$$m_{\text a} ~=~ 4 \cdot 1.66 \cdot 10^{-27} \, \mathrm{kg} ~=~ 6.64 \cdot 10^{-27} \, \mathrm{kg}$$

Here for the relative atomic mass $$A_{\text r} \approx 4$$ only the nucleon number was taken (i.e. the number of protons and neutrons), because the electrons are much lighter and thus are not considered here.

## Relative atomic mass

Unit
Relative atomic mass is the ratio of the absolute atomic mass (in kg) to the value of the unified atomic mass unit $$u$$ in kilograms. So the relative atomic mass is the value of the absolute atomic mass in the unit $$u$$: $$m_{\text a} ~=~ A_{\text r} \, u$$.

## Value of the unified atomic mass unit

Unit
Atomic mass unit $$1 \, u$$ corresponds to $$1.66 \cdot 10^{-27} \, \mathrm{kg}$$. So we can specify the absolute mass either in atomic mass unit: $$A_{\text r} \, u$$ or in kilograms, as shown in the example above.