4. Electron Sharing and      Covalent Bonds  
     Double and Triple Bonds

Two atoms must come closer together to share the second electron pair than they do to share the first. Singly bonded carbon atoms are 1.54 A apart no matter where they are found, but carbon atoms in a double bond are only 1.35 A apart. The limit of sharing is a triple bond with three electron pairs shared between carbon atoms only 1.21 A apart. This ability to make multiple bonds also is found in the secondshell nonmetals N and O, and as we shall see in the latter chapters of this book, double bonding is important in determining the geometry and energy-trapping properties of key biological molecules. Except for special circumstances, the larger third-shell atoms, which we shall discuss in Chapter 6, cannot get close enough to one another to make multiple bonds. This flaw, alone, would be enough to rule them out as candidates for a chemistry of life.

  Page 12 of 54 HomeGlossary