The many organic compounds found in living organisms-proteins and amino acids, sugars, organic acids and bases, fats, and hormones-are possible only because of the many different kinds of atoms and groups of atoms that can be attached to carbon backbones. In the next chapter we shall see what several of these compounds are like.

Before we do, however, we must look at the carbon backbones themselves. This chapter is devoted to hydrocarbons-compounds of carbon and hydrogen. Many of the ideas that we will develop about hydrocarbons will carry over directly to more complicated compounds.

The special properties of carbon that make it appropriate for construction of living organisms arise from its central position in the periodic table.

In the previous chapter we mentioned that life occupies the midranges of size and temperature in the universe.

Carbon occupies the midposition in Row 2 of the periodic table, and has exactly half as many electrons as are needed to fill. its outer shell. It neither loses nor gains electrons in chemical reactions, so its compounds are not ionic. This is critical for its role, since the nondirectional electrostatic forces between ions are inadequate for building elaborate molecules (right).