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Lesson Summary. The atomic mass unit (AMU) is used by scientists to describe the mass of atoms. If you want to convert a mass in grams to AMU, all you need to do is divide by 1.660599 x 10^-24 g/AMU. To unlock this lesson you must be a Study.com Member. Create your account.
The mass number is very close to the actual mass of the atom in amu but it is not exactly equal to the mass for three reasons: 1  The electrons, though very light, do weigh something. 2  The mass of the proton and the neutron arenât exactly 1 amu. The proton weighs 1.0073 amu and the neutron weighs 1.0087 amu.