Mass and energy (A-level only): Atomic mass unit, u.

The idea

The atomic mass unit, u, is a non-SI unit of mass defined as one twelfth of the mass of a carbon-12 atom in its nuclear and electronic ground state. It provides a convenient scale for atomic and nuclear masses because these are far too small for kilograms to be practical. One u is approximately 1.661 × 10⁻²⁷ kg. In nuclear physics, masses are often quoted in u, and mass differences in nuclear reactions are converted to energy using Einstein’s mass–energy equivalence.

For example: The mass defect of a helium-4 nucleus is about 0.0304 u, which corresponds to a binding energy of roughly 28.3 MeV.

What earns the marks

• State that the atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom.
• Recall that 1 u ≈ 1.661 × 10⁻²⁷ kg.
• Use u as a unit of mass in nuclear calculations, such as mass defect.
• Convert mass in u to energy using the equivalence 1 u = 931.5 MeV.
• Explain why u is used instead of kg for atomic and nuclear masses.

Common mistakes and how to put them right

• Confusing the atomic mass unit with the relative atomic mass.

  Put it right: u is a unit of mass, while relative atomic mass is a dimensionless ratio.

• Using 1 u = 1.66 × 10⁻²⁷ kg without recognising it is an approximation.

  Put it right: Use the more precise value 1.661 × 10⁻²⁷ kg when needed.

• Forgetting to convert mass from u to kg before using SI equations.

  Put it right: Either convert to kg or use the energy equivalent 1 u = 931.5 MeV directly.

Exam tips

• Learn the definition of u precisely, including the carbon-12 reference.
• In calculations, keep track of units and convert to kg if the equation requires SI units.
• Use the conversion 1 u = 931.5 MeV to simplify energy calculations in nuclear reactions.