Mass number (A) — Science definition

The total count of protons plus neutrons in an atom's nucleus.

Where it is taught

• Atomic Models & Subatomic Particles — Atoms, Electrons & Periodicity, Physical Science: Chemistry, Grade 11

Taught alongside

• atom
• proton
• neutron
• electron
• nucleus
• atomic number (Z)
• ion
• atomic mass unit (amu)

Elsewhere

• All science terms
• All courses and reading

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