Frozen water (image 1 of 3). Molecular graphic of the hexagonal crystal structure of water ice. Water molecules have one oxygen atom (light blue) & 2 hydrogen atoms (white). Each molecule is neutral, but the oxygen has a small negative charge & the hydrogens are slightly positive. The hydrogens are attracted to oxygens in nearby molecules, forming weak hydrogen bonds (dark blue). The molecules have so little energy below 0 degrees celsius that the hydrogen bonds force them to form a crystal. Its open structure means that ice floats because it is less dense than liquid water. (See also A504/063 for liquid water & A504/064 for water vapour).