Page "Equation of state" Paragraph 61

# While ideal gas molecules do not interact, we consider molecules attracting others within a distance of several molecules ' radii.

It makes no effect inside the material, but surface molecules are attracted into the material from the surface.

We see this as diminishing of pressure on the outer shell ( which is used in the ideal gas law ), so we write ( something ) instead of.

To evaluate this ‘ something ’, let's examine an additional force acting on an element of gas surface.

While the force acting on each surface molecule is ~, the force acting on the whole element is ~~.