Inside the liquid: Molecules are attracted equally in all directions by neighboring molecules, resulting in a net force of zero. This leads to a lower potential energy state.

At the surface: Molecules experience unbalanced inward forces due to fewer neighboring molecules above. This creates surface tension and requires extra surface energy.

Surface energy is roughly half the energy needed to completely remove a molecule from the liquid (half the heat of evaporation). For water, this is approximately 40 kJ/mol.