MadSci Network: Chemistry

Re: Monoatomic Carbon Stability

Date: Thu Oct 1 23:43:45 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 906153803.Ch

Unfortunately, you have not provided a reference, so that I am not sure 
quite what you mean by producing monatomic hydrogen. There are three 
distinct possibilities. The first, and most genuinely monatomic, is to 
produce isolated hydrogen atoms in a matrix of frozen neon, or some such 
material. Certainly, if a technique has been mastered to produce hydrogen 
in this form, it should require only a few fairly straightforward changes 
in procedure to obtain individual carbon atoms separated in the same way.
These changes might not be trivial. For example, gaseous H2 is readily 
obtained at any temperature above about 20 K; to handle gaseous C2 you 
require a temperature of at least 5000 K!

The second possibility is that you are referring to "metallic hydrogen". 
This is a form of hydrogen in which the protons form a regular and fixed 
array while the electrons move freely through the whole sample. It has 
often been supposed that hydrogen might adopt such a form at extremely high 
pressures and/or low temperatures. There is no equivalent structure for 
carbon. A metallic form of carbon is not achievable.

But I strongly suspect that you mean neither of these things. So how would 
we tell whether a substance was monatomic "at near absolute zero 
temperatures"? You see, it is very easy with a gas. You just have to see 
whether the particles move around individually, or in pairs or larger 
clusters. But how do you tell with a solid? For solid hydrogen, it is very 
easy. The hydrogen atoms are quite clearly grouped in pairs in the crystal 
structure. Each hydrogen atom has one neighbour about 74 pm away, and no 
other neighbours within 200 pm. So you could fairly call any structure of 
solid hydrogen that did not show this sort of pairing "monatomic hydrogen". 
With carbon, the stable solid form at all temperatures is graphite. This is 
a beeswax sort of arrangement. Each carbon atom has three neighbours about 
145 pm away, and these neighbouring arrangements link up into huge 
hexagonal sheets, which then stack on top of one another. Nearest atoms in 
different sheets are about 370 pm apart. Although gaseous carbon is 
diatomic C2, solid carbon most certainly is not!

The structure of graphite

The other crystalline structure for solid carbon is the diamond structure, 
in which each carbon atom has four nearest neighbours, tetrahedrally 
disposed, about 155 pm away. The structure forms a linked three dimensional 
network. Again, there is no suggestion of pairing into C2.

The structure of diamond

So are either of these solid forms "monatomic" carbon? They are certainly 
not "diatomic". Both are infinite network structures -- graphite in planar 
sheets; diamond a 3-dimensional array. If they are not what you mean by 
"monatomic" carbon, what would you be looking for in the structure of a 
solid form of carbon that would qualify it as "monatomic"?

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