Proton Decoupling & H & He?

[Morgo]

TL;DR Summary

Why does proton decoupling thermal barrier not apply to Hydrogen and Helium?

Why does proton decouplng apply to all the other elements but not hydrogen and helium?

I've read the pre proton decoupling account of universe formation, which had hydrogen and helium available pretty much Day 1. But proton decoupling- essentially the 3000oC barrier when there is too much heat/energy for protons/electrons to be trapped by a neutron, means that atomic matter cannot form until the universe's temperature drops below that temperature barrier.

So why doesn't this factor which prevents all other elements forming, not previously apply to hydrogen/helium?

 

[Nugatory]

I've read the pre proton decoupling account…..

You will get more and more helpful answers if you can tell us where you read this. Otherwise we don’t know whether you’ve encountered an incomplete/misleading explanation.
And there’s a fair chance that someone here will be familiar with that particular source.

 

[Mordred]

I believe you meant 3000 degrees Kelvin and not Celsius. This is the temperature where hydrogen can form with stability at 75 percent ratio. 25 percent ratio can occur at roughly 6000 kelvin. This can be shown using the Saha equations.

Protons however drop out of thermal equilibrium at a higher temperature This would occur when the expansion rate exceeds the proton reaction rate given by its cross section.
The source may have had a statement to the effect of "decouples from thermal equilibrium" which is a different process than decoupling that is involved in say an NMR (spin spin and J couplings) which I know little of those processes.
Others can help there as that's one area I've never studied.

The early universe processes however I'm well versed in.
Knowing the source would be extremely helpful in knowing what type of decoupling process is being described.

 

[mfb]

You seem to mix at least two completely different processes in your question.

~3000 C is the temperature range where electrons and protons combined to form neutral hydrogen after 400,000 years. At that time nuclear reactions had long stopped. The only elements present were hydrogen, helium, and traces of lithium.

Nucleosynthesis happened much earlier, in the first minutes. The conditions allowed the formation of helium and lithium, but there is no fusion process that would produce heavier elements in any relevant amount with the conditions of the very early universe.

Protons and electrons were never captured by neutrons.