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.
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