The identity of an element is determined by the number of protons in its nucleus. Protons are subatomic particles with a positive charge, and each element has a unique number of protons, known as the atomic number. For example, hydrogen has 1 proton, helium has 2 protons, and so on.
This means that if you change the number of protons in an atom, you change the element itself. The number of neutrons and electrons can vary without changing the element’s identity, but the number of protons remains constant for a given element.
In summary, the proton is the subatomic particle that determines the identity of an element.