Heavy Water

Heavy water is chemically the same as regular water, but with the two hydrogen atoms (as in H2O) replaced with heavy hydrogen atoms called deuterium, hence the symbol D2O of heavy water. Deuterium is an isotope of hydrogen and it has one extra neutron. Thus the deuterium atom consists of one proton and one neutron in the atomic nucleus with one electron orbiting around. It is the extra neutron that makes heavy water "heavy", about 10% heavier in fact.

Hydrogen / Deuterium / Tritium

Ordinary water / Heavy water