'Metals' (any atom with equal or more than three protons) are heavier then hydrogen or helium. So 'metals' are slower then H or He with the same kinetic energy. That means that over time the heat density becomes less along the expansion vector. This effect spreads the heat over a bigger volume and lowers the heat(energy) density, making that area cooler.

Your definition of metals is interesting.

That is the definition of metals Astronomers use. I just learned it from Dr. Becky. It was shocking at first but makes sense from an astronomical point of view. Still, I will never be comfortable calling carbon or neon a metal.

The primary way molecular clouds cool is by emitting the energy of the heat as light, which can escape the cloud.

A hot cloud of gas means there's lots of atoms and electrons zipping around, bumping into each other. If one of those bumps gives enough energy to an atom's electron, that electron gets excited to a higher energy level. When that excited electron jumps back down to a lower one, it emits the energy difference as light. Hydrogen and Helium are very simple atoms, meaning they only have 1-2 electrons to excite, few energy levels to excite to, and the minimum energy you need to make that first jump is higher. Conversely, "metals" have many more electrons and much more complex energy level structures with smaller, easier to accomplish jumps. Molecules, such as H2O, CO2, etc even more so. Therefore metals and molecules are much more likely to be able to take the energy of thermal collisions and convert it into outgoing light.

Take a look at these spectra for a visual: https://images.nagwa.com/figures/628134219594/1.svg

Each vertical line corresponds to one possible electron transition in the corresponding atom. You can imagine collisions as randomly firing along these spectra, and only when they hit one of the lines does light get emitted. So you can see, for H and He you're going have a way harder time getting hits. (And, in real life your shots aren't uniformly spread either, but skewed towards the red side at the temperatures molecular clouds are typically at. For hydrogen, you'd only have a single target to try to hit, vs the forest of lines that nitrogen has for example)

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In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium.

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Welcome to astonomy, where everything that isn't H/He is a metal and what is currently going on in a galaxy 50 million light years away doesn't matter.

https://en.m.wikipedia.org/wiki/Metallic_hydrogen

Yay!! I know I know its theoretical but I bet you didn't even know this!!

Basically, any element that formed later than the big bang is a metal

Coming from a chemistry background this will take some getting used to.

Chemically speaking, a metal is a substance whose outermost electrons are only loosely attracted to the nucleus and form an “electron sea” when in the solid state. Which is why metals are better conductors than non-metals.

Our chemistry teacher would always bring up this definition in the first few weeks of class but it makes sense from an astronomical standpoint