LEWIS STRUCTURES AND POLYATOMIC
MOLECULES
Connectivity
Thinking about how atoms and electrons are arranged in a molecule gets harder when
there are more than two atoms. Often you need to decide how the atoms are arranged
before writing the Lewis structure.
Let's draw the structure for carbonic acid, H2CO3. First we need to know which atoms
are connected together. That will tell us which atoms are sharing electrons.
We could put one atom in the middle of the structure and arrange all the others around
it. That may work sometimes, but often it gives the central atom too many bonds. A
second-row atom can't have more than eight electrons in its valence shell. As a result,
a carbon can't have more than four bonds, because there is no room for more
electrons.
Figure IM4.1. A carbon atom with a problem: too many valence electrons.
We could try to attach all the atoms in a string. Long strings of atoms like this are not
very common. There may be several reasons why this sort of arrangement isn't very
stable. First, when we consider formal charge a little later, you will see that placing all
the atoms in a row may cause unnecessary charge separation. Second, some atoms
such as oxygen are not very stable when bonded to other atoms of the same kind.
Repulsion between lone pairs may contribute to this instability. Because carbon often
has no lone pairs, it is an exception and it can bond to itself.
Figure IM4.2. A long string of atoms may still have problems when you start adding
in the rest of the electrons.
Note that when we try to arrange the atoms in either way, we would probably avoid
putting a hydrogen in the middle. That's because hydrogen's octet is just two electrons,
