The atomic number of an atom is based upon the number of protons in its nucleus. In an atom, the number of protons is equal to the atomic number, and it uniquely identifies an element. For example, hydrogen has an atomic number of 1 because it has one proton in its nucleus, while carbon has an atomic number of 6 because it has six protons. The atomic number determines an element's position in the periodic table and is crucial in defining its chemical properties and behavior.