CHEM-GUIDE: Structure of water

In the Earth's biosphere, water is essential to all forms of life. Interestingly, it is also the biospheres most abundant compound and a unique substance from the chemical point of view.

About 75% of the Earth's surface consists of water. Water is present as either (a) Solid - ice, snow etc., (b) Liquid - water in the lakes, rivers and oceans etc., (c) Water vapour - as clouds or moisture in the air.

The Human body contains as much as 70% water and all animals and vegetable matter contain water; plants contain up to 50-70% of water.

Of the total estimated global water supply of 1.4 x 10⁹km³, fresh water sources amount to only 2.7% of the total. Most fresh water is not readily available, being under the ground, or in the form of glaciers and frozen lakes. The fraction of water available for human civilization is estimated at less than 0.003% of the total global water availability. Aquatic marine life in oceans, seas and other saline inland water bodies, enjoy 97.3% of the total global supply of water supply.

Structure of water molecule in the gas phase

A molecule of water consists of two hydrogen atoms joined to an oxygen atom by covalent bonds.

The oxygen atom has six electrons (1s² 2s² 2p⁴) in its outermost shell. The 's' and 'p' orbitals of the valence shell are sp³ hybridized to form four sp³ hybrid orbitals oriented tetrahedrally around the oxygen atom. Two of the hybrid oribtals 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. Thus, oxygen is bonded to the two hydrogen atoms by two O-H covalent bonds, and there are two lone-pairs of electrons on the oxygen atom. Due to the presence of two 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'. Therefore, the structure of water molecule is an angular or bent structure.

Polarity of water molecule

Oxygen is more electronegative than hydrogen, being only second to fluorine in its electronegativity. Its high electronegativity causes the oxygen atom to pull the shared pairs of electrons more towards itself. As a result, the O-H bond acquires polarity. Since the two O-H bonds in water are inclined at an angle, hence the net dipole moment of water molecule is not zero. The actual dipole moment of water molecule is 1.84 debye, (denoted as D) is the unit of dipole moments.

Structure of water molecule in the liquid state

In liquid water, water molecules are held together by intermolecular hydrogen bonds. Each oxygen atom can form two hydrogen bonds utilizing both the lone pairs on it. Experimental studies suggest that liquid water consists of aggregates of varying number of water molecules held together by hydrogen bonds and 'free' water molecules in a dynamic equilibrium. The aggregates continually forming, collapsing and reforming. Thus, in liquid water, the equilibrium

Hydrogen bonding has very profound effect on the properties of water.

Structure of water in solid state (ice)

Depending on the conditions for freezing of water, the solid form of water i.e., ice, can exist in different crystalline forms. In 'normal' hexagonal ice, four other oxygen atoms tetrahedrally surround each oxygen atom. One hydrogen atom lies in between each pair of oxygen atoms. Thus, each and every hydrogen atom is covalently bonded to a oxygen atom and linked to another oxygen atom by a hydrogen bond. This arrangement leads to a packing with large open spaces and results in a lower density of ice than that of liquid water.

When ice melts some of the hydrogen bonds are broken and the water molecules become more closely packed. It results in an increase in the density of water above its melting points (273 K). Density of water attains a maximum value of 1 g/mL at 277 K (4°C); above 277 K, the density decreases due to the normal temperature effects.