When water is frozen into ice, the change in temperature creates excess hydrogen bonds between water molecules that increase the space between the molecules. The additional space created reduces the density of the water as it freezes, making ice less dense than water. Rather than the molecules packing more tightly together and creating a heavier density, as water freezes the hydrogen bonds increase from a 3.4-molecule bond to a 4-molecule bond, which creates the excess space and reduced density in ice.
The crystal structure of ice is actually created and maintained by the same hydrogen bonds that decrease the density of the water molecules. The decreased density of molecules allows ice to float in liquid water rather than sink. In this way, water and ice behave differently from most other liquids and solids of the same volume. Generally, when a liquid solidifies, the kinetic energy is reduced along with the temperature. This packs the molecules more closely together and makes the solid version more dense than the liquid version of the same volume. When the solid is more dense than the liquid, the solid sinks when placed in the liquid. Water is the only substance that loses density when frozen.