The number of protons determines an element's atomic number and is used to distinguish one element from another. ... Together, the number of protons and the number of neutrons determine an element's mass number.
The number of electrons in a neutral atom is equal to the number of protons. The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus. The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z).
