# Explaining negative potential energy involved in Surface Energy of fluids

I was studying about Surface Energy in fluids, and here's what I was reading:
(a)

A liquid stays together because of attraction between molecules. Consider a molecule 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...

How do I know that a negative potential energy has been produced?

Also, I saw this thing right after the above statement:

(b)

The average potential energy of all the molecules is the same. This is supported by the fact that to take a collection of such molecules and to disperse them far away from each other in order to evaporate or vaporise, the heat of evaporation required is quite large.

How does the largeness of heat of evaporation in this example show that the average potential energy of all the molecules is the same?

The implication is that, if there are $$N$$ molecules, and each has a total average interaction potential energy with its neighbours $$-E$$, then it would take an overall energy $$NE$$ to separate all the molecules, and this should correspond to the heat of evaporation. A bit more thought will show that this calculation overestimates the total energy by a factor of $$2$$ (since we would count the average interaction of molecule $$i$$ with neighbour $$j$$, and that of $$j$$ with neighbour $$i$$, separately, but really we should only count it once), so the estimate of the total evaporation energy is $$NE/2$$.