The atomic number of an atom is determined by the number of protons present in the atom that is the atomic number and number of protons are same for an atom. It is unique for every atom.
Atomic mass: It is nearly equals to the sum of mass of subatomic particles present in the atom that is sum of proton and neutron.
Number of neutrons: in an atom, the number of neutrons are always equal to the number of protons. If the number of neutrons are different for a number of proton then it is said be an isotope of that atom.
Protons plus neutrons: Sum of number of protons and number of neutrons is said to be the mass number of that atom.
Hence, the atomic number is the number of protons.
