What are isotopes?
As far as we know, all matter is made out of very small particles that are called atoms. Atoms themselves are made out of three even smaller particles, called electrons, protons and neutrons. Every chemical element has the same number of protons and electrons, but may have a different number of neutrons. For example, hydrogen has always one electron and one proton but can have different number so neutrons. H-1 (also known as protium) has no neutron and H-2 (also known as deuterium) has one neutron.
H-1 and H-2 are called isotopes of the same chemical element, in this case hydrogen. It is also possible to create H-3 (also known as tritium) that has two neutrons, but this is unstable and will fall apart after some time in smaller stable parts. Isotopes that do not fall apart after some time are called stable isotopes.
For example, the only stable isotopes of oxygen are O-16, O-17 and O-18. Isotopes have the same chemical characteristics and therefore behave almost identically. The mass differences, caused by the difference in the number of neutrons, have an effect on the physical behaviour of isotopes. Isotopes of hydrogen and oxygen together make water molecules, but the different isotopes behave differently as they move around in the water cycle.