Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 906153803.Ch
Message:
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"?