Explain surface tension in the context of potential energy.

(N/A) The liquid stays together because of the attraction between molecules.
Consider a molecule $A$ well inside a liquid. The intermolecular distances are such that it is attracted to all the surrounding molecules. This attraction results in a negative potential energy for the molecule,which depends on the number and distribution of molecules around the chosen one. The average potential energy of all molecules in the bulk is the same.
Now,consider a molecule near the surface. Only the lower half side of it is surrounded by liquid molecules. There is some negative potential energy due to these,but it is less than that of a molecule in the bulk (approximately half of the latter).
Thus,molecules on a liquid surface have some extra potential energy in comparison to molecules in the interior. $A$ liquid thus tends to have the least surface area that external conditions permit,as this minimizes the total potential energy of the system. This property is known as surface tension.
Since a liquid consists of molecules moving about,there cannot be a perfectly sharp surface. The density of the liquid molecules drops rapidly to zero as we move along the direction perpendicular to the surface over a distance of the order of a few molecular sizes. $A$ layer of thickness $r_0$ below the free surface of the liquid is called the surface layer.