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1.
D2O, 2 Deuterium, and 1 Oxygen atom. Deuterium is an isotope of hydrogen; it has one extra neutron in the nucleus. Neutrons are what add weight to an atom; this substance is called "Heavy Water" as compared to normal "Light Water." Heavy water exists is most all water, but only in very small amounts (1 in 20 million).

Heavy water is essential to nuclear power using non-enriched uranium. Without it, all uranium must be enriched before use. In WW-II, Hitler attacked Norway for it’s supply of heavy water.
"Heavy water aids in cooling nuclear reactors.
by Contraceptive SpongeBob March 16, 2006
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