Final answer:
The number of protons in an atom is its atomic number, and the total number of protons and neutrons is its mass number. The number of neutrons can be calculated by subtracting the atomic number from the mass number.
Step-by-step explanation:
The number of protons in the nucleus of an atom is called its atomic number (Z). This is the defining trait of an element, and it determines the identity of the atom. The total number of protons and neutrons in an atom is called its mass number (A).
The number of neutrons can be calculated by subtracting the atomic number from the mass number: number of neutrons = mass number (A) - atomic number (Z).