Carbon shares its four electrons in four covalent bonds with other
electron-sharing atoms. Because these four bonding electron pairs
still are bound to the atoms from which they came, they hold the
atoms together in a molecule.
When carbon is bonded to four other atoms, the four electron pairs
around the central carbon atom repel one another, thereby causing
the four bound atoms to occupy the corners of a tetrahedron in sp3
hybridization. This gives a geometric shape to the molecule (above).
The ultimate in carbon tetrahedra is the crystal lattice of diamond,
shown below, in which the tetrahedral array of carbon atoms goes
on forever, and there are no hydrogen atoms present at all. Hydrocarbons
that have only single bonds between carbon atoms can be thought
of as fragments of this three-dimensional diamond lattice. Organic
compounds in general are in a reduced state, and therefore are storehouses
of energy for the reasons discussed in Chapter 12.