Atomic Mass Unit (Edexcel International A Level Physics)

Atomic Mass Unit

• The unified atomic mass unit (u or sometimes a.m.u) is roughly equal to the mass of one proton or neutron:

  • 1 u = 1.66 × 10⁻²⁷ kg

  • This value is provided on the exam data sheet 

• The a.m.u is commonly used in nuclear physics to express the mass of subatomic particles. It is defined as

The mass of exactly one-twelfth of an atom of carbon-12

• Therefore, one atom of carbon-12 has a mass of exactly 12 u

• Since mass and energy are interchangeable, the a.m.u can also be expressed in MeV

  • 1 u is equivalent to 931.5 MeV

Table of common particles with mass in a.m.u

• The mass of an atom in a.m.u is roughly equal to the sum of its protons and neutrons (nucleon number)

  • For example, the mass of Uranium-235 is roughly equal to 235u

  • However, note that the actual mass is slightly lower than the expected mass, due to mass-energy equivalence

• a.m.u might be quoted in kg or MeV since mass and energy are equivalent via 

  • MeV is a unit of energy whilst kg is a unit of mass