Different isotopes of chemical elements have slightly different chemical behaviors, but for most elements the differences are far too small to have a biological effect. In the case of hydrogen, larger differences in chemical properties among protium (light hydrogen), deuterium, and tritium occur, because chemical bond energy depends on the reduced mass of the nucleus–electron system; this is altered in heavy-hydrogen compounds (hydrogen-deuterium oxide is the most common species) more than for heavy-isotope substitution involving other chemical elements. The isotope effects are especially relevant in biological systems, which are very sensitive to even the smaller changes, due to isotopically-influenced properties of water when it acts as a solvent.