Surface Tension

Surface tension is a natural phenomenon observed in any liquid that has an inherent chemical attraction to itself.  Since water is one such liquid, the effects of surface tension are seen in everyday life around the world.

Water is attracted to itself due to a phenomenon known as 'hydrogen bonding'.  Because the oxygen molecule in water has a higher attraction for electrons than the two hydrogen atoms, at any given moment the majority of electrons in a water molecule are more likely to be hovering closer to the oxygen than either of the hydrogens.  This difference in electron densities creates a dipole like charge across the molecule, giving the hydrogens a partial positive charge and the oxygen a partial negative one, since electrons are designated to be negatively charged.  The result is that the hydrogen atoms in a water molecule are highly attracted to the oxygen atoms of other water molecules, and tend to line up in a manner that balances out their charge differences.

In the middle of an amount of water, the atoms are surrounded by other atoms, and so this hydrogen bonding effect is cancelled out.  However, on the surface of water, the atoms have an attraction to the other atoms below, but feel only a negligible attraction to the thinly dispersed water molecules in the air.  Because the forces the atoms feel below the surface are so much greater, atoms on the surface are pulled inward and the outermost layer of atoms forms a tight elastic like sheet.  This inward tension surface atoms experience is referred to as surface tension.

It is surface tension that allows many light insects to walk across water, even though the insects themselves are denser than the liquid.  The insects may be denser, but the force they exert in weight due to gravity is less than the force experienced by the water atoms in the outermost layer due to surface tension.  If such an insect were to become submersed, however, the same tension that allowed it to skate freely along the surface would act against its attempts to return to the surface.

Surface tension is also the reason why many liquids, like water, form beads when spilled across a non-absorbent surface, and tend to fall in spherically shaped drops as opposed to scattering randomly. 

Surface tension is measured in the SI units of newtons per meter (N/m), ergs per centimeter (erg/cm) or dynes per centimeter (dyne/cm).