Answer: atomic number
Step-by-step explanation:
Atomic number is defined as the number of protons or number of electrons that are present in an atom.
Atomic number = Number of electrons = Number of protons
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
Atomic weight is the weight of an atom. It is equal to the number of protons and neutrons, with a little extra added by the electrons.
Thus the number of protons in the nucleus of an atom equivalent to atomic number