- Neutrons
- Protons
- Electrons
- Isotopes
The atomic number (Z) is a fundamental property of an element, representing the number of protons found in the nucleus of its atom. This number is unique for each element and determines its identity—for example, any atom with 6 protons is carbon, while 8 protons indicates oxygen. In a neutral atom, the number of electrons equals the number of protons, maintaining electrical balance. The atomic number also dictates the element’s position in the periodic table, arranged by Mendeleev and Meyer in increasing order of this value. It is distinct from the mass number, which sums protons and neutrons. Understanding atomic numbers is essential for predicting chemical behavior, as elements with similar valence electron configurations (determined indirectly by atomic number) exhibit similar properties. The concept was refined through experiments like Moseley’s X-ray studies in 1913, which confirmed that atomic number, not atomic mass, is the proper organizing principle. Isotopes of an element share the same atomic number but differ in neutron count, affecting mass number without changing chemical identity.
