The_ of an element tells how many particles are in the nucleus of each atom.

The mass number of an element tells us about the number of particles present in the nucleus of an atom

Explanation:

Nucleons are defined as the sub-atomic particles which are present in the nucleus of an atom. Nucleons are protons and neutrons.

Atomic number is defined as the number of protons or electrons that are present in a neutral atom.

Atomic number = number of protons = number of electrons

Mass number is defined as the sum of number of protons and neutrons that are present in an atom.

Mass number = Number of protons + Number of neutrons

For Example:  Taking an isotope of chlorine, Cl-35

Atomic number = Number of protons = 17

Mass number = Nucleons = 35

Hence, the mass number of an element tells us about the number of particles present in the nucleus of an atom

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