Chemistry Reference
In-Depth Information
Comparing the Lewis Dot Diagrams
of Carbon Dioxide and Sulfur Dioxide
OO
OS
O
Figure 4-5c
Why does the sulfur dioxide bend? Notice that the sulfur has two pairs
of unshared electrons, which, as do all electrons, repel other electrons of
like charge. These unshared electrons are thought to effectively occupy
more three-dimensional space than shared pairs of electrons, which forces
the molecule to bend as the electrons repel each other.
What if four atoms combine to form a molecule, as in the cases of BH
3
and NH
3
? Again, we don't want to assume that they would have the same
shape just because the formulas look so similar. We want to look at the
valence shells of each of the central atoms—that is, the boron and the ni-
trogen—and see if there will be any unpaired electrons exerting forces on
the shared pairs.
Comparing the Lewis Dot Diagrams of Boron and Nitrogen
B
N
Figure 4-5d
Notice that once the boron atom makes single bonds with each of the
hydrogen atoms, it won't have any unpaired electrons distorting the shape
of the molecule. When the shared pairs of electrons repel each other, the
molecule will take on the shape called trigonal planar. The nitrogen, on the
other hand, will still have a pair of unshared electrons repelling the three
single bonds that it forms with the hydrogen atoms. This will push the shared
pairs of electrons away, forming a pyramidal molecule. This distinction is
