All elements are made of the same stuff; protons, neutrons and electrons – just organized differently.
Take carbon, for example. The nucleus of all carbon atoms contains 6 protons. Surrounding the nucleus are 6 electrons, contained in two shells. Within these shells are orbitals.
Carbon’s electron configuration is most simply written in shorthand: 1s² 2s² 2p². This says that carbon’s first two electrons reside in the “s-orbital” of the first shell. Its third and fourth electrons lie in the s-orbital of the second shell. The fifth and sixth electrons are second shell “p-orbital” electrons.
Why is Carbon Special?
Carbon is special because its atoms can bond to each other to a practically unlimited degree. This is possible because of carbon’s electron configuration. To understand how this is so, it is important to touch on orbital theory. Empirical data proves that atomic orbitals hybridize to form molecular orbitals.
It is important to point out a few ground rules in understanding the unique properties of carbon.
- Orbitals hold a maximum of two electrons.
- When all orbitals within a shell are filled, the shell is closed.
- Different orbital types—s, p, d, f and g—have unique spatial symmetry.
S-orbitals are spatially symmetrical – they’re spherical, or ball-shaped. P-orbitals are shaped like barbells, directed along an axis. So p-orbital electrons are subdivided into px-orbital, py-orbital and pz-orbital electrons.
Scientists realized in order to explain the geometry of molecular carbon bonds, it’s necessary to apply the concept of hybridization. They also recognized that closed shells do not participate in the hybridization, but unfilled shells do; even if some of the orbitals within the shell are filled.
Carbon’s first shell is closed. The second shell is not closed. All four 2s- and 2p-orbital electrons participate. In addition, the orbitals themselves combine to form new—molecular—orbitals, having a uniform but different geometry than any of the participant atomic orbitals.
Since there were one s- and three p- atomic orbitals involved in the hybridization, the molecular orbitals are called sp³ molecular orbitals. The geometry of these orbitals is tetrahedral – a tetrahedron is a three-dimensional shape that looks like a three-sided pyramid. It has four faces: Three sides, and a bottom.
The number of single bonds that each carbon atom can form is four. Linking to itself, carbon fills each of its new orbitals completely. Its tetrahedral bonds are unstrained, relatively short and strong.
How Special is Carbon?
The ability of carbon to bond to itself to make a host of compounds makes it ideal as a base for the complexities of the chemistry of life. Simple carbon chains can be lengthened and have functionality added to form useful polymers. Even in its elemental form, carbon can form sheets, tubes, complex geodesic structures and crystals. Yes, carbon is special because carbon bonds to itself.