This is the space-filling model of heavy water.
Question: What Is Heavy Water?
You may have heard of heavy water and wondered how it was different from ordinary water. Here's a look at what heavy water is and some heavy water facts.
Answer: Heavy water is water that contains heavy hydrogen or deuterium. Deuterium differs from the hydrogen usually found in water, protium, in that each atom of deuterium contains a proton and a neutron. Heavy water may be deuterium oxide, D2O or it may be deuterium protium oxide, DHO. Heavy water occurs naturally, although it is much less common than regular water. Approximately one water molecule per twenty million water molecules are heavy water.