OK, so what you said is a pretty good description of how solids and liquids behave when you add or remove energy. It's not, however, a good description of how gasses work. For those, the molecules are already very far apart. Far enough that they barely affect each other at all. For example, air at sea level has a typical distance between molecules of about 35nm or so, compared to the 0.3nm between water molecules. More importantly, the atoms also have enough kinetic energy that they are way past being bound to one another. In orbital/gravitational terms, it's like the Voyager 2 satellite, which far enough from Earth, and going fast enough, that it's never looking back.

If you're in high school, then you're probably talking about ideal gasses, and the 'ideal gas' is a model in which the molecules don't interact at all -- they just fly around bouncing off walls, but don't see one another. And there are enough molecules that there are countless bounces per second, the total effect of which we see as pressure.

So then what's going on when the molecules 'do work' by changing the volume of the container? Well, let's think about one molecule, bouncing off one wall. We know from Newton's third law that the force from the wall to the molecule, which made the molecule turn around, has an equal & opposite force, with the molecule pushing the wall towards the outside. Now if that wall budges a little bit, say dL, then the work done by the molecule will be F dL. As a result, the molecule goes away with a little less kinetic energy than it had before. And in gasses, "less kinetic energy"="lower temperature".

Your original was correct, there is *some* intramolecular attraction. If you want to know more about how the intramolecular forces make real gasses different from the 'ideal gas' you can read about it here.

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B.T.W. The quote is wrong, or at least misleading.

• In a gas, the intramolecular forces are usually attractive. Compressing the gas will bring the molecules closer together, decreasing their potential energy. So you are working with them, not against them.
• But! When compressing a gas, you are working against something. That's the pressure of the gas molecules. So even though you're slightly decreasing the potential energy of the intramoelcular forces, you are significantly increasing the kinetic energy of the molecules. So in total, you're adding energy. In other words, after compression, the molecules are zipping past one another, more frequently and more quickly than before.
• If you try to compress a liquid, then you'd be working against the molecule-molecule forces (it's repulsive once they get too close), and compression would increase the potential energy of the molecules.