How do isotopes differ and why is the atomic number of elements not a whole number?

Isotopes differ from each other by having different numbers of neutrons but the same number of protons since they are only the same element if they have the same number of protons.

Atomic numbers aren't whole because their mass is based of the mass of hydrogen and amu's (Atomic Mass units) and due to how elements are formed, minute amounts of mass are lost in order to form the nuclear bonds resulting in not whole numbers.