The three-dimensional crystal structure of H2
O ice Ih (c) is composed of bases of H2
O ice molecules (b) located on lattice points within the two-dimensional hexagonal space lattice (a). The values for the H–O–H angle and O–H distance have come from Physics of Ice
with uncertainties of ±1.5° and ±0.005 Å, respectively. The black box in (c) is the unit cell defined by Bernal and Fowler.