How does atomic number identify a chemical element?

The atomic number (denoted by the letter Z) is the fundamental characteristic that identifies a chemical element. It is defined as the number of protons present in the nucleus of an atom.123

Because the number of protons determines the number of electrons in a neutral atom, and these electrons dictate how an atom interacts with others, the atomic number ultimately determines the chemical properties of an element. Consequently, elements are completely distinguishable from one another by their atomic numbers; a substance is defined as a chemical element if all of its atoms possess the same atomic number.32

While atoms of the same element must have the same atomic number, they can contain different numbers of neutrons in their nuclei. These variations are known as isotopes, which share the same position in the periodic table and exhibit nearly identical chemical behavior despite having different physical properties and atomic masses.4