How are water molecules arranged in an ice crystal?

In the solid state, water molecules form a highly ordered, rigid crystal lattice characterized by an open structure. Each oxygen atom is surrounded by four hydrogen atoms: two are covalently bonded to the oxygen, while the other two are linked via hydrogen bonds to the oxygen atom’s unshared electron pairs.12

At standard atmospheric pressure and temperatures near 0 °C, this arrangement typically manifests as sheets of oxygen atoms organized into open hexagonal rings. This configuration is more spacious than the molecular packing found in liquid water, which explains why ice has a lower density and floats.312

Because this structure is open and rigid, it is sensitive to changes in temperature and pressure. Under varying conditions, water can form at least eighteen different crystalline structures. When the crystal undergoes deformation, such as gliding, the hydrogen bonds may temporarily break and reform, occasionally creating sites of electrical charge within the lattice.13