Editing Deuterium (section)

==Physical properties==

The physical properties of deuterium compounds can be different from the hydrogen analogs; for example, D2O is more viscous than H2O. Deuterium behaves chemically similarly to ordinary hydrogen, but there are differences in bond energy and length for compounds of heavy hydrogen isotopes which are larger than the isotopic differences in any other element. Bonds involving deuterium and tritium are somewhat stronger than the corresponding bonds in light hydrogen, and these differences are enough to make significant changes in biological reactions (see heavy water).

Deuterium can replace the normal hydrogen in water molecules to form heavy water (D2O), which is about 10.6% denser than normal water (enough that ice made from it sinks in ordinary water). Heavy water is slightly toxic in eukaryotic animals, with 25% substitution of the body water causing cell division problems and sterility, and 50% substitution causing death by cytotoxic syndrome (bone marrow failure and gastrointestinal lining failure). Prokaryotic organisms, however, can survive and grow in pure heavy water (though they grow more slowly). Consumption of heavy water would not pose a health threat to humans unless very large quantities (in excess of 10 liters) were consumed over many days.[citation needed] Small doses of heavy water (a few grams in humans, containing an amount of deuterium comparable to that normally present in the body) are routinely used as harmless metabolic tracers in humans and animals.