* Protons: Protons are positively charged particles found in the nucleus of an atom. The number of protons in an atom defines its atomic number.

* Atomic Number: This number is unique to each element. For example, all carbon atoms have 6 protons, all oxygen atoms have 8 protons, and all gold atoms have 79 protons.

Therefore, the number of protons is the key difference between atoms of different elements.

While atoms of different elements may have differing numbers of neutrons (which are neutral particles also found in the nucleus) and electrons (which are negatively charged particles orbiting the nucleus), the number of protons is the defining factor that determines the element.