A Closer Inspection of Hydrogen IsotopesThe mass of an atom is determined by the nucleus. Since the weight of a proton and that of a neutron are nearly identical, deuterium atoms have a mass essentially double that of ordinary hydrogen. Hence one name for deuterium oxide is heavy water.
Heavy Water Laboratory ObservationsAlan Soper of Rutherford Appleton Laboratory has found that the D-O bonds in heavy water are about 3% shorter than H-O bonds. A shorter bond is a stronger bond. This stronger direct D-O bonding in deuterium oxide logically decreases the strength of deuterium to oxygen intermolecular bonds (bonds between different molecules). They are about 4% longer than intermolecular H-O bonds.
Linearity – Going StraightThe D-O-D bond angle appears to be about 106° in the liquid phase. This makes heavy water a slightly more linear structure than ordinary water at 104.5°. The number of intermolecular hydrogen bonds for D₂O are somewhat fewer in number than for H₂O. Heavy water is denser than regular water. Finally, the freezing point of deuterium oxide is 4°C and it boils at 101°C.
Final CommentsInterestingly, intermolecular hydrogen bonding is a major factor in H₂O being a liquid rather than a gas at ordinary temperatures. Heavy water has fewer and weaker hydrogen bonds, so one might expect it to have a lower boiling point. However, the increased mass, coupled with influences from its other physical differences actually raises its boiling point a little higher than regular water.
A practical joke some find amusing is to put deuterium ice cubes in a glass of water. After asking those who are present if ice sinks or floats, and receiving the answer “it floats,” he or she then puts a heavy water ice cube in a glass and the cube plummets to the bottom of the glass.
Note: You might also enjoy “Three Hydrogen Isotopes: Protium, Deuterium, Tritium”
- Argonne National Laboratory: Quantum Physics Makes Water Different (August 2008)
- D-O-D Bond Angle: K. Ichikawa, Y. Kameda, T. Yamaguchi, H. Wakita and M. Misawa, Neutron-diffraction investigation of the intramolecular structure of a water molecule in the liquid-phase at high-temperatures, Mol. Phys. 73 (1991) 79-86.