Simple Description

The number of protons and neutrons in the nucleus of an atom.

Further Detail

The atomic mass number tells you how many protons (relative mass of 1) and neutrons (relative mass of 1) are in one atom. For example, lithium has an atomic mass of 7 and a proton number of 3. Therefore one atom of lithium contains 3 protons and 4 neutrons (a total mass of 7).

Related Words:

• Relative atomic mass
• Ar

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