The Number of Bonds That Carbon Can Make and Its Ability to Make Chains and Rings
The existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities.
In the three examples shown above, the central atom (carbon) does not have any non-bonding valence electrons; consequently the configuration may be estimated from the number of bonding partners alone. For molecules of water and ammonia, however, the non-bonding electrons must be included in the calculation. In each case there are four regions of electron density associated with the valence shell so that a tetrahedral bond angle is expected. The measured bond angles of these compounds (H2O 104.5º & NH3 107.3º) show that they are closer to being tetrahedral than trigonal or linear. Of course, it is the configuration of atoms (not electrons) that defines the the shape of a molecule, and in this sense ammonia is said to be pyramidal (not tetrahedral). The compound boron trifluoride, BF3, does not have non-bonding valence electrons and the configuration of its atoms is trigonal. Nice treatments of VSEPR theory have been provided by Oxford and Purdue.
The carbon and the four hydrogen atoms form the vertices of a three-dimensional shape known as a tetrahedron, which has four triangular faces; because of this, methane is said to have a tetrahedral geometry. More generally, when a carbon atom is bonded to four other atoms, the molecule (or part of a molecule) will take on a tetrahedral shape similar to that of methane. This happens because the electron pairs that make up the bonds repel each other, and the shape that maximizes their distance from each other is a tetrahedron. Most macromolecules are not classified as hydrocarbons, because they contain other atoms in addition to carbon and hydrogen, such as nitrogen, oxygen, and phosphorus. However, carbon chains with attached hydrogens are a key structural component of most macromolecules (even if they are interspersed with other atoms), so understanding the properties of hydrocarbons is important to understanding the behavior of macromolecules. Carbon’s ability to form bonds with four other atoms goes back to its number and configuration of electrons (Asimov, I., 1962). Carbon has an atomic number of six (meaning six protons, and six electrons as well in a neutral atom), so the first two electrons fill the inner shell and the remaining four are left in the second shell, which is the valence (outermost) shell. To achieve stability, carbon must find four more electrons to fill its outer shell, giving a total of eight and satisfying the octet rule. Carbon atoms may thus form bonds to as many as four other atoms. For example, in methane (CH_4 start subscript, 4, end subscript), carbon forms covalent bonds with four hydrogen atoms. Each bond corresponds to a pair of shared electrons (one from carbon and one from hydrogen), giving carbon the eight electrons it needs for a full outer shell (Reece, J. B., Urry, L. A., 2011).
Other important sources of carbon are fossil fuels such as coal, petroleum and natural gas. This is because fossil fuels are themselves formed from the decaying remains of dead organisms.
Asimov, I. (1962). The world of carbon (new, rev. ed.). New York, NY: Collier Books.
Hybrid orbitals. (n.d.). Retrieved July 22, 2015 from UC Davis ChemWiki: http://chemwiki.ucdavis.edu/Organic_Chemistry/Fundamentals/Hybrid_Orbitals.
Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). The chemical context of life. In Campbell Biology (10th ed., pp. 28-43). San Francisco, CA: Pearson.