The atomic number is the number of protons found in the nucleus of an atom. It is typically denoted by the symbol Z.

The atomic number uniquely identifies a chemical element. In other words, the number of protons determines what element an atom is. For example, all atoms with an atomic number of 1 are hydrogen atoms, and all atoms with an atomic number of 6 are carbon atoms.

Because an atom is electrically neutral, the number of protons is equal to the number of electrons. The number of neutrons, however, can vary, which creates different isotopes of the same element.

The atomic number is usually indicated along with the element symbol in the periodic table.