In liquid water each molecule is hydrogen bonded to approximately 3.4 other water molecules. In ice each each molecule is hydrogen bonded to 4 other molecules.
Compare the two structures below. Notice the empty spaces within the ice structure. Click on the images to view larger gif files.
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In ice Ih, each water forms four hydrogen bonds with O---O distances of 2.76 Angstroms to the nearest oxygen neighbor. The O-O-O angles are 109 degrees, typical of a tetrahedrally coordinated lattice structure. The density of ice Ih is 0.931 gm/cubic cm. This compares with a density of 1.00 gm/cubic cm. for water.