Normal water is made of two hydrogen atoms and an oxygen atom covalently bonded
together. This gives the famous H20 formula.
Heavy water has exactly the same structure, except the hydrogen atoms are isotopes
of hydrogen called 'Deuterium'. Standard hydrogen has one proton in its nucleus
(Relative Molecular Mass = 1), deuterium has one proton AND one neutron in it
nucleus, (RMM = 2) hence it is 'heavier' than normal hydrogen. This leads to the
water being heavier. Mass of water = 18, mass of heavy water = 20.
Answered by: Simon Hooks, Physics A-Level Student, Gosport, UK
'The difference between what the most and the least learned people know is inexpressibly trivial in relation to that which is unknown.'