Really how rare are hydrogen atoms?

Question

Hasok Chang, a chemist, but perhaps more a philosopher recently published the book Is Water H2O? . Chang used the question and an extensive web of arguments ultimately as a platform to support a pluralistic approach to scientific endeavor. I don't know that I buy it all, but in a latter chapter he made some statements regarding 'fundamental' components of chemical reactions, and one in particular that interested me and brings me to the point of my question.

Hydrogen gas as I understand it naturally occurs as a hydrogen molecule - two hydrogen atoms stuck together as a diatomic molecule. If ionized, the electron is stripped from the atoms and you have hydrogen ions or rather protons. But what about single hydrogen atoms? Are these non-existent or just a rare thing? Is there some way I can arrange to have a container of single hydrogen atoms and prevent their grouping into hydrogen molecules without ionizing them?

For what answers that follow - do these apply in general to the stable nature of all diatomic gases?

Answer 1

While usually short-lived, isolated hydrogen atoms (also called hydrogen radicals) are well studied and chemically important species. Bond formation in simple species is always an exothermic reaction (usually highly so) with zero activation energy, which means bond formation is thermodynamically very favourable and has no kinetic barrier stopping it from happening. Simply put, the isolated atoms really want to react with each other, and they'll attract each other at a distance, so if at some moment you create a container with pure monoatomic hydrogen, the hydrogen atoms will react as soon as possible to regenerate dihydrogen molecules (hydrogen gas).

The best you can do is expose the dihydrogen molecules to energetic conditions, so that a fair amount of bonds are being constantly broken at a rate comparable to which they're formed. A simple way to do this is to strongly heat the hydrogen gas; enough thermal energy will cause the bonds to break, though the dihydrogen bond is unusually strong ($\rm{436\ kJ\ mol^{-1}}$) so even at several thousand degrees kelvin there will be only a small portion of free hydrogen atoms in the gas (see Mithoron's comment). A more interesting option is to shine a strong ultraviolet light into the gas, as the dihydrogen molecules can separate into their constituent atoms upon absorption of photons with energy around $\rm{4.52\ eV}$, which corresponds to a photon wavelength of $\rm{274\ nm}$.

Any substance can have its bonds broken through the above two methods (among others), so the same applies to all other diatomic molecules. The stronger the bond between the atoms, the more uncommon is it to find the isolated atoms. For substances with very weak bonds, such as molecular dibromine or diiodine, mere exposure to visible light is enough to trigger formation of a fair amount of isolated atoms, something which is exploited in several reactions.

Keeping in mind that isolated atoms are very reactive, they are rather abundant in space. The average density of matter in the Universe is very, very low (about 1 atom per 5 cubic metres of space!), so isolated atoms can go a long time before finding anything to react with. This makes them extremely important for understanding astrochemistry.

Answer 2

Hydrogen atoms are the dominant species in the atmosphere beyond 2000 km.

Monoatomic oxygen concentration exceeds diatomic about 100km.

See Atmospheric Structure

See also Atomic Hydrogen Welding for a technique to produce atomic hydrogen as well as THE DISSOCIATION OF HYDROGEN INTO ATOMS.

At 3200K, hydrogen is about 50 percent dissociated in to atoms, at atmospheric pressure. Lower pressure greatly favors dissociation, so in the extreme upper atmosphere, monoatomic hydrogen is favorable.