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Atomic mass number is the sum of number of protons (p) and neutrons (n) in the nucleus. It is represented by the symbol ‘A’.

Atomic mass number = Number of protons + Number of neutrons

A = p + n

By the above formula, we can easily calculate the atomic mass number of any element.

Example:-

i. A lithium atom contains 3 protons and 4 neutrons. Its atomic mass number is A = 3 + 4 = 7

ii. In a sodium atom, there are 11 protons and 12 neutrons. Thus, its atomic number is A = 11 + 12 = 23

iii. A magnesium atom contains 12 protons and 12 neutrons. Its atomic mass number is A = 12 + 12 = 24

iv. A hydrogen atom contains 1 proton and 0 neutron. Its atomic mass number is A = 1 + 0 = 1