SAGAR SINGH - IIT DELHI
Last Activity: 14 Years ago
Dear nitin,
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.
Fig: 11.4 - Structure of normal hexagonal ice
There is one hydrogen along each oxygen - oxygen axis closer to one of the two oxygen atoms.
Significance of lower density of ice in nature
Ice floats on water because of its lower density when compared to liquid water. During winter, this floating block of ice prevents or delays the freezing of the underlying water. Thus, when the temperature falls below zero degree Celsius, the upper layer of water in most aquatic bodies such as lakes etc., get frozen, but the underlying water remains liquid. This enables the aquatic life to survive during the winter months.
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Sagar Singh
B.Tech, IIT Delhi
