The Atomic number (symbol Z) uniquely identifies an element and is equal to the number of protons present in the nucleus of an atom.
For example, atomic number 6 identifies the Carbon atom with 6 protons in its nucleus. An uncharged Carbon atom will also have 6 electrons in the outer nuclear region, equal to its atomic number.
The atomic number is different for every element present in a periodic table. Although an atom can gain electrons and become negatively charged or lose electrons and become positively charged, the number of protons in an atom and, therefore, its atomic number remains unchanged in a chemical reaction.
However, in nuclear reactions, the atomic number changes. In the atom undergoing nuclear change, the element is represented in AZE notation with the atomic number written as a subscript and the mass number in the superscript.