what are isotopes?? expl. dat chlorine one...how did we get 35.5u as its atomic no.

Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons. Another example is of Chlorine-35 and chlorine-37.Since the atomic mass of elements is the average mass of stable isotopes present in nature, Therefore chlorine has atomic mass of 35.5amu .