Robert Clark wrote:
Atomic hydrogen or monatomic (or monoatomic) hydrogen fuel isn't a
type of nuclear engine. The terminology just means the hydrogen occurs
as single atoms rather than the usual H2 seen in hydrogen gas. The
specific impulse (ISP) of this fuel could be as high as 1500 s. The big
problem with this fuel is keeping it stable against it's recombining
into H2 when stored in its monatomic form. This still hasn't been
solved:

The recombination of H atoms into H2 is exothermic at temeratures belor
those of the surface of a star. Inhibiting the recombination will
require complicating the system, adding weight and reducing the GLOW
advantage of utilizing hydrogen in the first place.

Use of Atomic Fuels for Rocket-Powered Launch Vehicles Analyzed.
http://www.grc.nasa.gov/WWW/RT1998/5...laszewski.html

This has the look of a 'back-of-the-envelope' calculation that ignores
a *lot* of factors such as the weight of the cryogenic carrier fluid
and its container. For example, the lightest cryogenic fluid that can
be used is liquid helium, but this will be *superfluid* at the
temperatures involved, and will likely provide a very feeble barrier to
recombination of H atoms. Also, the weight of the helium is almost
negligible compared to the weight of the container needed to maintain
its liquid state, even in large amounts. LHe is also VERY expensive.

Current research is focused in storing it as individual atoms within
cryogenic liquid helium.
However, arcjet engines or arcjet torches that use hydrogen as the
fuel can produce monatomic hydrogen by high electrical power. High
power or temperature is needed to break the hydrogen bond of H2, at a
bond energy of about 104 kcal/mol.
Are there molecules with bonds of hydrogen at much lower bond
energies?

Try this table:
http://srdata.nist.gov/cccbdb/hf0k.asp

Methane looks *real* good.

I thought of molecules that specifically had the type of bonds known
in biochemistry as "hydrogen bonds". These are, in most cases, weaker
than covalent bonds. However, the only cases I've seen had an H atom
attached to another atom by a hydrogen bond but that H atom was also
attached to a second atom by a covalent bond. "Hydrogen bonds" always
have the H between two other atoms but is it possible for both of these
bonds to be of the "hydrogen bond" type?

No. "Hydrogen bonding" is peculiar to hydrogen (because of the small
size of the hydrogen atom *once it has already bonded chemically* to a
more electronegative atom such as a halogen (fluorine family),
chalcogen (oxygen family), or pnictogen (nitrogen family). Unlike
other atoms, hydrogen does not have a 'hard nut' of a filled 1S
orbital.

According to this page hydrogen iodide, HI, with a bond energy of 71
kcal/mole can be decomposed into H and I by uv light at a wavelength of
266 nm:

Chem 32 Virtual Manual.
http://kalee.tock.com/chem32/the2/26s.html

Are there other gases where a single hydrogen atom can be dissociated
at a lower energy?

Google "hydride photodissociation"

Tom Davidson
Richmond, VA