11.2 Atomic Mass Unit

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

Worked example

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

Get unlimited access

to absolutely everything:

• Unlimited Revision Notes
• Topic Questions
• Past Papers