Three subatomic particles make up all atoms: protons electrons and neutrons. Each atom has a fixed number of protons electrons and neutrons. One way to describe an atom is by its mass number (A) which is the sum of the number of protons and neutrons in its nucleus. These two particles are collectively called nucleons.
A= the number of protons + the number of neutrons in the nucleus
Atomic number, Z, on the other hand, is the number of protons in the nucleus. Note that the numbe of electrons in an atom is equal to the number of protons in the nucleus.
Z= the number of protons in the nucleus=the number of electrons orbiting in the extranuclear shells.
Therefore, the given sentence when completed should read:
The mass number is used to calculate the number of nucleons in one atom of an element.