Molecular Association in Water : Molecular association in water is due to presence of inter- molecular hydrogen bonding.
Structure of Water : A molecule of water consists of two hydrogen atoms joined to an oxygen atom by covalent bonds. The oxygen atom has six electrons in its outermost shell. The ‘s’ and ‘p orbital of the valence shell are sp3 hybridized to form four sp3 hybrid orbital oriented tetrahedrally around the oxygen atom. Two of the hybrid orbitals are singly occupied while the lone pairs of electrons occupy the other two. Each singly occupied sp3 orbital overlaps with the half filled 1s orbital of ‘H’ atom.
Hence, oxygen is bonded to the two hydrogen atoms by two O-H covalent bonds and two lone pairs of electrons are present on oxygen atom. Due to presence of lone pairs of electrons on the O-atom, the H-O-H bond angle is 104.5°, which is slightly less than the tetrahedral angle of 109° 28′. The structure of water molecule is an angular or bent structure.
Structure of Ice : Ice is crystalline form of water. It has highly ordered three-dimensional structure with hydrogen bonds. X-ray studies show that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm. One hydrogen atom lies in between each pair of oxygen atoms. So, each and every hydrogen atom is covalently bonded to an oxygen atom and linked to another oxygen atom by a hydrogen bond. This arrangement leads to a packing with large open spaces.