what makes one atom different from another?

One atom is different from another primarily because of the number of protons in its nucleus. This number of protons, called the atomic number, uniquely defines each chemical element and thus distinguishes one atom from another. Atoms with the same number of protons but differing numbers of neutrons are called isotopes of that element. Electrons around the nucleus influence chemical behavior, but it is the proton count that makes one atom fundamentally different from another.