The atomic number indicates the number of protons present in the nucleus of an atom, which is used to uniquely identify the atom, as well as its chemical properties. The atomic number is denoted by the universal symbol "Z."
An atom is the smallest fundamental unit that retains a chemical element's properties. It is composed three primary subatomic particles called protons, electrons and neutrons. A particular type of atom known as a "nuclide" is represented in the periodic table of elements, where each nuclide is denoted by an atomic symbol (E), atomic number (Z) and mass number (A). The atomic number is typically written as a subscript on the lower left-hand side of the symbol, while the mass number is written as a superscript on the upper left-hand side. The elements in the periodic table are organized in increasing atomic numbers from left to right.
The atomic number is fixed for all atoms of an element. In a neutral atom, the atomic number also specifies the number of electrons. When atoms become electrically charged by gaining or losing electrons, the atomic number is not affected. However, the mass number, which refers to the total number of protons and neutrons present in the nucleus, may vary between atoms of the same element. One form of atom called an isotope may contain the same number of protons but a different number of neutrons.