The identity of an atom, or what element it is, depends on the number of protons in its nucleus. This is because the number of protons defines the atomic number, which is unique to each element. For example, an atom with 6 protons is always carbon, and an atom with 8 protons is always oxygen.
While the mass number (which is the sum of protons and neutrons) and the number of neutrons and electrons can vary, the number of protons remains constant for a given element. This is why the number of protons is the key factor in determining an atom’s identity.