Why isn't most hydrogen in the universe molecular (diatomic), instead of atomic (monoatomic)?

Similar questions have been asked before; but, why?

Is the monoatomic hydrogen left over from the Big Bang? And hasn't had the opportunity to collide with other hydrogen atoms yet?

Or are hydrogen molecules that do form split up after absorbing ultraviolet radiation or X-rays? Which occurs more frequently than molecule-forming random collisions?

What is the 'activation energy' needed to form diatomic hydrogen from atomic? Is it too much for the randomly-colliding hydrogen atoms to form molecules in the depths of space?

I mean, if the loose protons and electrons shot out by stars can (usually) find each other to form atoms, which is why neutral atomic hydrogen is more common between the stars and planets than ionized hydrogen, why can't the lonely atoms find each other to form $${\rm H_2}$$?

• Could you be so kind and link the similar questions you are mentioning, please? Apr 8 at 7:46
• Related, but different question: astronomy.stackexchange.com/questions/38829/… Apr 8 at 7:48
• Proton and electron attract each other. Not sure it is at play, but it is. Conversely H is stable and neutral. Apr 8 at 11:58

2 Answers

Yes, the atomic hydrogen is probably mostly left over from the Big Bang. [Edited to add: Not sure how much that is true and how much present-day atomic hydrogen is the result of recombination.] And, yes, $${\rm H}_{2}$$ does get dissociated by high-energy photons -- and also by cosmic rays, which can penetrate dense, dusty clouds that block most of the high-energy photons.

The real issue, as I understand it, is that's actually very difficult to make $${\rm H}_{2}$$ by colliding two H atoms in the gas phase. This is because when two atoms collide, the resulting (temporary) molecule almost always has too much energy to remain stable and will very rapidly dissociate back into the two atoms -- unless you can somehow get rid of the excess energy before it does so.

Now, you can do this if the reaction actually involved producing extra components (like a more typical molecular reaction -- e.g., $$AB + C \rightarrow AC + B$$), since then the excess energy can be put into the kinetic energy of the products (while still preserving momentum). But in a reaction like $${\rm H} + {\rm H} \rightarrow {\rm H}_{2}$$, there's no second body to allow that.

You could also do this if the "extra component" is some third body that happens to hit the other two at the same time (without becoming part of the molecule), since it can rebound with excess kinetic energy. But while this can work in dense gasses (e.g., in a laboratory on Earth), it will happen much too rarely in interstellar or intergalactic space.

You can also do this if the temporary molecule can emit a photon carrying away the excess energy. However, such transitions are strongly forbidden, especially in the case of a symmetric molecule like $${\rm H}_{2}$$, which means that on average they will take too long to occur.

Finally, you can get rid of the energy if the reaction actually takes place on a surface that acts as a catalyst by absorbing the excess energy. This is thought to be the primary way $${\rm H}_2$$ is formed in the interstellar medium: two H atoms that are adsorbed onto a dust grain combine on the grain's surface.

I mean, if the loose protons and electrons shot out by stars can (usually) find each other to form atoms, which is why neutral atomic hydrogen is more common between the stars and planets than ionized hydrogen, why can't the lonely atoms find each other to form $$H_{2}$$?

In addition to the fundamental problem outlined above, it's also easier (as Alchemista pointed out) for protons and electrons to "find each other" because they are attracted by their opposite electrical charges, something which doesn't happen in the case of neutral atoms.

This is one of those questions that is easy to state but complicated to answer - and this won’t at all be a complete answer, but mostly a quick outline of some important factors to consider and terms you might search for in order to learn more.

The question of why the interstellar medium (ISM) has the structure it does is a long-standing one, and one that a graduate-level course on the ISM would spend a fair amount of time on.

Briefly, our current model is that of a three-phase ISM, with some regions cold and neutral (molecular clouds with H2 and other molecules), some regions warm (mix of neutral H and ionized H), and some regions very hot (mostly ionized, i.e. protons and electrons rather than H). These regions must have roughly the same gas pressure as each other, or a higher-pressure region would expand into any adjacent lower-pressure region. This pressure balance means that hotter regions must be less dense, and cooler regions more dense (cf. the ideal gas law in the form that includes density). And on long timescales they aren’t stable, but change dynamically due to star formation and supernova explosions.

The conditions in a given part of the galaxy are set by a complicated balance between various heating and cooling mechanisms.

So this is a start, but overall, the short answer is that all of those phases exist, with a strong dependence on the particular local conditions.