An atomic number is the number given to an element indicating not only its position on the periodic table, but also the number of protons contained by one atom of the element. For example, Oxygen has the atomic number 8, indicating that it has 8 protons.

The atomic number can be obtained by simply looking at an elements entry on the periodic table, since it is the number above and to the left of the atomic symbol on a standard periodic table.

Its symbol is $ Z $, and a proton's is $ p $.