The atomic number of an element is equal to the number of protons in the nucleus of an atom of the element. Protons are positively charged particles found in the center of every atom. Each element has its own unique number and is differentiated from one another by the number of protons it has. The nucleus of an atom may also be home to neutrons, but the number of neutrons has no bearing on the element’s atomic number. Electrons reside just outside of the nucleus and also have no bearing on the number.

Some confusion may exist between the atomic number and atomic mass of an element, but these two terms should not be confused. While atomic number is the number of protons an atom has, the atomic mass is the sum of number of protons and neutrons in the atom. Hydrogen, for example, has an atomic number of one because it has one proton. Deuterium, an isotope of hydrogen, has one proton and one neutron. If the atom had two protons, it would not be hydrogen but helium.

Atomic number can be found on the periodic table of elements, a list of the world’s known elements, both natural and man-made. The atomic number is always listed on the upper left hand side of each elements’ entry. In the past, the elements of the periodic table used to be listed in terms of atomic weight. However, Dimitri Mendelev, the designer of the table, found that listing them this way led to inconsistencies. He found that some elements that were correctly ordered in terms of atomic weight were not being grouped together with others of similar chemical properties.

The reason for this inconsistency was not revealed until discoveries were made while experimenting with X-rays. Scientists discovered that elements bombarded with X-rays behaved in a predictable way as they moved up the table of elements. H.G.J. Moseley assigned numbers to the elements equal to the number of protons each element has. The modern periodic table is now ordered according to atomic number rather than according to atomic weight. Ordering the table this way allows elements of similar chemical properties to be grouped together.