Many of the second-shell atoms are special in that they are small enough to come close together and share more than one electron pair in double bonds. This, as we shall see later, is crucial in molecular architecture and the absorption of light.

Double bonds contribute structural rigidity to large molecules. Rings and chains of carbon atoms with delocalized electrons have electronic energy levels close enough together to absorb and trap visible light. Chlorophyll, the key molecule in photosynthesis, is a large molecule with many delocalized electrons.

The ability to trap light is important, because the most abundant source of energy available to life on any planet is radiation from that planet's star.

These second-row atoms are small enough to make double bonds, but the larger atoms that we shall see in the next two chapters can do this only with difficulty, or not at all.

For this reason alone, life based on third-shell or heavier elements is unlikely. The third-shell atoms are too big, and the first-shell atoms, H and He, are too simple by themselves.

Life appears to be a property of second-shell atoms.