What determines the chemical identity of an atom?

The chemical identity of an atom is determined by the number of protons in its nucleus, also known as the atomic number. Each element on the periodic table is defined by this unique number; for example, hydrogen has one proton, whereas helium has two protons. The number of protons establishes not only the element itself but also its properties, such as its reactivity and how it bonds with other elements.

While the arrangement of electrons and the configuration of the outer electron cloud influence how an atom interacts and bonds with others, they do not change the fundamental identity of the atom. Neutrons, while significant to the stability of an atom and the formation of isotopes, do not determine the chemical identity either. Thus, the definitive characteristic that identifies an atom as a specific element is its proton count.