What identifies the quantity of protons in an element?

The quantity of protons in an element is identified by the atomic number. The atomic number is a fundamental property of an element that defines its identity in the periodic table. Each element has a unique atomic number, which corresponds directly to the number of protons found in the nucleus of its atoms. This is crucial because the number of protons not only determines the element's identity but also affects its chemical behavior and properties.

Other concepts may relate to an element's characteristics but do not provide the specific count of protons. Atomic mass, for example, represents the average mass of an element's isotopes, reflecting both protons and neutrons. Atomic radius refers to the size of the atom and varies between elements, but does not provide information about the proton count. Mass number is the sum of protons and neutrons in an atom's nucleus, which can vary among isotopes of the same element. Hence, it is the atomic number that distinctly identifies the number of protons in an element.