The atomic number of an element is defined by the number of protons in its nucleus. Protons carry a positive charge and are responsible for determining the identity of an element. Therefore, if you were to remove a proton from an atom of an element, the number of protons would decrease by one, and as a result, the atomic number would also decrease by one. The element would become a different element altogether, as the atomic number determines the unique identity of an element in the periodic table.