OXYGEN ISOTOPES
The chemical elements are often found in more than one form.  The number of protons in the atomic nucleus is constant for any given element, but the number of neutrons in the nucleus can vary.  The chemical behavior of isotopes of the same element is very similar, the main difference being their weight. Oxygen primarily comes in two isotopes: Oxygen 16 and Oxygen 18.  Oxygen 18 has two extra neutrons, making it about 10% heavier than Oxygen 16.