Atomic number (Z) refers to the total protons residing in an atom’s nucleus. The number is also called nuclear charge because protons determine an atom’s electrical charge, along with neutrons. On a periodic table, atomic number is mentioned on top of a particular element. All elements on the periodic table have their own atomic numbers, and the number is what that defines the element.

For example, a carbon atom will always have six protons. If the proton count increases or decreases, the atom won’t be a carbon atom anymore. If the number of protons decreases by one, the atom becomes boron (5 protons). Similarly, if the proton count increases to seven, the atom becomes a nitrogen atom. No atom sees alterations in its proton count. The above example was just to illustrate how protons are critical to an atom’s identity. A carbon atom will remain carbon throughout its life. Similar is the case with other elements and their atoms. Since the protons in an atom’s nucleus never change, an element’s atomic number doesn’t change either.