I don't understand why it takes at least three atoms to form a molecule. In a cloud of hydrogen atoms, what is the third one doing in the production of h2?
The other response discussing conservation of energy and momentum is basically correct. However, the third particle doesn't need to be an atom! Emitting a photon with the right energy and spin will also suffice. Typically the transition rates via photon are extremely low, but they can be intentionally triggered with other photons. This yields the incredibly useful tool called photo-association to create molecules from cold atoms.
There was a discussion a few days ago which had excellent comments and links explaining why you need at least 3 atoms to form a molecule.
Layman's guess. It probably has to do with conservation of energy and momentum. To form a molecule, you have to change the energy state of the electrons in both atoms, and in order to do so you have to respect the conservation laws, so I guess it takes a third atom to absorb/provide the required amounts of energy or momentum.
I think this is correct, and that a good illustration is planetary orbits - 2 bodies alone can never collide if they didn’t start out in intersecting trajectories. They need a third to perturb their movements or they will just keep orbiting the same way forever.
Won’t the orbits decay due to tidal forces in some cases?
In reality, yes - for purpose of illustration, I was thinking of idealized planets acting as point masses.
Tidal forces are much too weak to capture planets. In practice, yoy always need three bodies.
I don't know the answer but I could imagine that it works like a kind of catalyst.
Yeah, I was confused too, there’s plenty of molecules made up of two atoms. Sounds like the guess from other commenters about conservation of energy and momentum are most likely to be correct!