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.