Isotopes are atoms of the same element that have the same no. of protons but a different number of neutrons. They also have different masses, but retain the same chemical properties. NB: It is the atomic no. that defines an element.
eg. 50% of all Br atoms are 79Br, the other 50% are 81Br. The average is 80Br, as shown in the periodic table.
eg. The average mass for Chlorine is 35.5 Cl. So, the 35Cl atom is more common than 37Cl atom. 75% are 35Cl, the other 25% are 37Cl.