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Heavy water (deuterium oxide (2H 2O) or D 2O) is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (2H or D, also known as heavy hydrogen), rather than the common hydrogen-1 isotope (1H or H, also called protium) that makes up most of the hydrogen in regular water.
Molecules are formed by atoms in a  multitude of different arrangements.   - Most atmospheric oxygen forms diatomic molecules (O2),  but also ozone (O3)   - One molecule of water has one atom of hydrogen and two atoms of  oxygen. So-called heavy water contains deuterium, which is hydrogen  that has neutrons.