The atomic number (Z) is the number of protons possessed by the nucleus of the atom of a chemical element. If the atom is neutral, it also coincides with the number of electrons it has in the shell. For example, oxygen has atomic number 8 (Z=8), this tells us that it has 8 protons in its nucleus and 8 electrons in its crust.

In 1913, HGJ Moseley (1887-1915) found that the wavelength of X-rays emitted by an element was directly related to the element's atomic number.

The number of protons in the nucleus of an atom determines its identity, this number is known as the atomic number of the element.