The atomic number is a fundamental property of an element that quantifies the number of protons found within the nucleus of an atom. This count serves as the defining characteristic of the element, meaning that changing the number of protons fundamentally changes the element's identity. In a neutral atom, the atomic number determines the number of electrons, which contributes to the element's characteristic electron configuration and thus its chemical properties. The atomic number is crucial for classifying matter on the periodic table and is the primary factor used in chemical reactions and nuclear physics to predict how an element will interact with others.