Atomic Number And Mass Number
The atomic number denotes the number of protons in an atom's nucleus. The mass number denotes the total number of protons and neutrons. Protons and neutrons are often called nucleons. By convention, the atomic number is usually written to the left of the elemental notation, and the mass number to the left above the elemental notation as represented by the example below. The element shown is aluminum.
Some atoms have the same atomic number, but different mass numbers. This means different number of neutrons. Such atoms are called isotopes.