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

## Atomic mass

`$$ m_{\text a} $$`Unit

`$$ \mathrm{kg} $$`

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

`$$ A_{\text r} $$`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

`$$ u $$`Unit

`$$ \mathrm{kg} $$`

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.