Atomic mass unit

Beginner

Unit of mass for atoms.

Key relation1 u = 931.5 MeV/c²

1 u = 1/12 of the mass of carbon-12 = 1.6605×10⁻²⁷ kg = 931.5 MeV/c². A proton is about 1.0073 u.

Worked example

Helium-4 has mass 4.0026 u.

Common mistake

Not exactly integer because of binding energy.

In more detail

The atomic mass unit, u, is one twelfth of the mass of a carbon-12 atom, about 1.66 × 10⁻²⁷ kg. It is used to express masses of atoms and nuclei.

Chemists express molar masses using atomic mass units.
Nuclear physicists quote masses of particles in atomic mass units.
Chemists and nuclear physicists use the atomic mass unit to compare the masses of atoms and molecules.

Why use u instead of kilograms?

Atomic masses are tiny, so u gives convenient numbers.

How does u relate to molar mass?

One mole of a substance has mass in grams equal to its mass in u.

What is one atomic mass unit in kilograms?

About 1.66 times ten to the power minus 27 kilograms, one twelfth of the mass of a carbon-12 atom.