How do hydrogen bonds give ice its open crystal structure?

The water molecule features two lone-pair orbitals that carry a negative electrical charge. These orbitals act as magnets for the hydrogen nuclei of neighboring water molecules, facilitating the formation of hydrogen bonds.1

In the solid state, these hydrogen bonds force water molecules into a highly ordered, rigid lattice structure. Specifically, each oxygen atom collects its four nearest neighbors and arranges them in a tetrahedral configuration. In this arrangement, each oxygen atom is surrounded by four hydrogen atoms: two are held by strong covalent bonds, while the other two are linked via hydrogen bonds to the oxygen atom’s unshared electron pairs.12

This specific orientation creates a spacious, open assembly of molecules. Because this crystalline structure is less compact than the arrangement of molecules in liquid water, ice possesses a lower density, which is why it floats.12