Final answer:
The identity of an element is determined by its number of protons, also known as the atomic number. Isotopes have the same number of protons but different numbers of neutrons.
Step-by-step explanation:
The identity of an element is defined by its atomic number (Z), which corresponds to the number of protons in the nucleus of an atom. This defining characteristic differentiates one element from another. For instance, carbon always has six protons. Additionally, the number of protons also determines the number of electrons in a neutral atom, where the numbers are equal. Isotopes of an element have identical numbers of protons but vary in the number of neutrons. The mass number (A) of an atom is the total number of protons and neutrons.