Water molecules are composed of two hydrogen atoms and one oxygen atom (H₂O). If the hydrogen atoms of a water molecule are replaced by deuterium atoms, the result is heavy water (D₂O). Deuterium differs from hydrogen by having one neutron in the nucleus of the atom. There is approx. one part in 5000 D₂O in normal water and it can be concentrated by electrolysis. Heavy water has a higher boiling point (101.4 °C) and melts at 3.6 °C. Heavy water is 20/18=1.11 times heavier than ordinary water.