The number of nucleons in an atom is the atom's (atomic, mass) number.

It determines the mass number of the atom.

Explanation:

Nucleons are the subatomic particles that are present in nucleus of an atom. The particles which are present in the nucleus of an atom are protons and neutrons.

Mass number is defined as the sum of number of protons and number of neutrons. It is represented as A.

A = Number of neutrons + Number of protons

Atomic number is defined as the number of protons or number of electrons that are present in an atom. It is represented as Z.

Z = Number of protons = Number of electrons

From the above information, we conclude that the number of nucleons determines the mass number of an atom.

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