Why are water molecules polar and how does this make them bond to each other?

Answer 1

So familiar is water, that we tend to forget how unusual a substance it really is. Of all water's properties, the most significant is its ability to form weak chemical associations, known as hydrogen bonds, with about 5-10% of the strength of covalent bonds. This ability stems from the fact that water atoms slight positive charges. The polarity of water causes it to be attracted to other polar molecules. When these other molecules are also water, the attraction is referred to as cohesion; when they are different, it is known as adhesion. The cohesiveness of water is what allows it to remainconsists of polar molecules, in each of which the oxygen atom has a slight negative charge and the two hydrogen a liquid at moderate temperatures (whereas hydrogen sulfide, for example, which is chemically similar, exists as a gas). Water molecules always tend to form the maximum possible number of hydrogen bonds. When nonpolar molecules such as oils, which do not form hydrogen bonds, are placed in water, the water molecules act to exclude them. This forces the nonpolar molecules into association with one another: a tendency to aggregate known as hydrophobic exclusion. Hydrophobic forces arising from the polar nature of water are thought to have been of central importance in the evolution of life. Firstly, they seem to have encouraged the formation of protocells and, in particular, the boundaries which enclosed them (see cell membranes, origin). Secondly, their influence is evident in the fact that some of the exterior portions of many of the molecules on which life came to be based are nonpolar. By forcing the hydrophobic portions of molecules into proximity with one another, water caused such molecules to assume particular shapes.

Answer 2

Water molecules are polar because of the difference in electronegativities of the hydrogen and oxygen atoms. Electronegativity is the tendency of an atom to attract electrons to itself. Oxygen atoms are more electronegative than hydrogen atoms, therefore, the oxygen has a greater attraction for the electrons shared by it and the hydrogen atom. Because of this, the electrons are shared unevenly, and the oxygen atom develops a partial negative charge (), and the hydrogen atom develops a partial positive charge (). Because of their polarity, water molecules attract one another. The oxygen atom of one water molecule forms a weak bond with a hydrogen atom of another water molecule. These weak bonds are called hydrogen bonds. Hydrogen bonds are not chemical bonds like covalent bonds or ionic bonds.

See the related link for an illustration of hydrogen bonding between water molecules.