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

Formula
Formula: Mass of an Atom
Electron Shell Model of Lithium

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.