Ask A Scientist Chemistry Archive

name         Michele T.
status       student
age          20s

Question -   How does carbon bond with itself and other atoms to produce a wide range of molecules?
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The scientists have evidence that many billions years ago most of the carbon atoms present at 
Earth now existed in the form of a gas, the simple combination between 1 carbon and 4 hydrogen, 
CH4, methane, together with water, ammonia and hydrogen. These
molecules were the main constituents of the atmosphere.  Different kinds of energy, going through 
atmosphere made these simple molecules to break into very reactive fractions. These fractions 
recombined forming different and complex molecules. That way
amino acids were formed and after proteins and the same way  by different processes all compounds 
of organic chemistry were formed.
Now to know the story of how do the carbon atoms bond one must study  the development of atomic 
theory and the different bonding theories. The first explanation of the nature of chemical bonds 
is due to W.Kossel and G.N.Lewis back in 1916. Following that theory at organic compounds  the 
carbon atoms are bonded by covalent bonds, that is a bond that results when atoms share electrons. 
That is a very simple explanation. I hope that satisfies you.
And thanks for asking NEWTON.

Mabel
(Dr. Mabel Rodrigues)
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It is not easy to isolate a single "reason" why carbon has its unique ability to bond with itself 
and other atoms. Part of the "reason" is the energetics of so-called electron promotion. Here is 
how the "reason" goes.

First consider boron, the element to the left of carbon in the periodic table. Its lowest 
electronic states are: (2s2, 2p);(2s1,2p2); (2s2,3s); (2s1,2p2); (2s2,3p1). These states have 
energy: 0, 28875, 40040, 47860; 48610 cm^-1(i.e. energies are in wave numbers).

Second consider carbon. Its lowest electronic states are: (2s2,2p2); (2s2,2p2); (2s2,2p2);
(2s1,2p3); (2s1,2p1,3s1). These states have energy: 0, 10190, 21650, 33735, 60350 respectively.

Third consider nitrogen the element to the right of carbon in the periodic table. Its lowest 
electronic states are: (2s2,2p3); (2s2,2p3); (2s2,2p3); (2s2,2p2,3s1);(2s2,2p2,3s1). These 
states have energy: 0, 19225, 28840, 83300, 86160 respectively.

Note two things: 1. Some electron configurations have the same apparent configurations. 
For example, in the case of carbon (2s2,2p3); (2s2,2p3) with energies 19225 and 28840 cm^-1. 
In cases where this occurs it is due to the electron spins being unpaired (lower energy) and 
electron spins paired (higher energy). In any case these configurations are close in energy
compared to the energy differences involving configurations having a higher principle quantum 
number, e.g. 2s vs. 3s.
    The empirical evidence is that carbon typically involves the bonding of 4 electrons. This 
	means that in some way there is a an energy "cost" of promoting an electron from the 
	(2s2,2p2)-----> (2s1,2p3) configuration that is "paid back" by the formation of 4 bonds vs.
	2 bonds that would be expected from the bonding of two unpaired electrons in the 'p' orbitals
	in the configuration (2s2,2p2). This is admitedly an "after the fact" argument, but that is 
	what happens. The same thing happens with boron promoting an electron (2s2, 2p)----->
	2s1,2p2), giving boron a combining number of 3. In the case of nitrogen the energy cost is 
	considerably larger, so it shows a "normal" combining number of 3.
    In addition, carbon has just the "right" electronegativity that it neither is too ready to 
	donate its bonding electrons, nor does it hold on to them too tightly. It is the balance of 
	the electron "promotion" and the electronegativity that provides the basis for the formation 
	of such a wide variety of compounds by bonding to itself readily.
	
Vince Calder
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