What defines the atomic number of an element?

Neutrons are subatomic particles located in the nucleus alongside protons, but they do not define the atomic number. Instead, the number of neutrons can vary among isotopes of the same element, leading to differences in mass but not in elemental identity. Thus, while neutrons contribute to atomic mass, they do not determine the atomic number.

Electrons are negatively charged particles that orbit the nucleus of an atom and balance the positive charge of protons in a neutral atom. While the number of electrons equals the number of protons in neutral atoms, it does not define the atomic number and can vary in ions, which alter the electrical charge of the atom without changing its identity.

Isotopes refer to different forms of the same element that have the same number of protons but differing numbers of neutrons. The number of isotopes does not influence the atomic number, as it is solely determined by the proton count. Therefore, the existence of isotopes showcases the diversity of an element but does not define its atomic number.