Atomic Mass Unit (Edexcel International A Level Physics)

Revision Note

Ann H


Ann H



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

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 capital delta E space equals space capital delta m c squared
    • MeV is a unit of energy whilst kg is a unit of mass

Worked example

Estimate the mass of the nucleus of the element copernicium-285 in kg. Give your answer to 2 decimal places.

WE - amu answer image

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Ann H

Author: Ann H

Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students no matter their schooling or background.