The atomic number (Z) of a specific chemical element indicates the number of protons in the nucleus of an atom of the chemical element. In the neutral state of an atom, the atomic number also equals the number of electrons.
The atomic number is fixed for every element. In contrast, the number of neutrons may vary. Together, protons and neutrons make up the atomic mass.
Figure 1: Explanation of the superscripts and subscripts in the atom of Lithium.