Chemistry Reference
In-Depth Information
Comparing the Lewis Dot Diagrams and Three-dimensional
Models of BH
3
and NH
3
Boron Trihydride
Ammonia
HB
H
H
H
N
H
H
Figure 4-5e
not very clear in a Lewis dot diagram, which shows a two-dimensional rep-
resentation of a molecule, but it will be more clear in a three dimensional
model, such as the “ball and stick” models that you probably have in your
chemistry laboratory. (See Figure 4-5e.)
There are many ways that five atoms can combine to form a molecule,
and thus several different shapes are possible. When four atoms of one type
surround one atom of another type, as in the example of CH
4
, a tetrahedral
molecule is formed. To understand the reason for this, you must think in
three dimensions again. Each of the four shared pairs of electrons (cova-
lent bonds) repel each other. In order to maximize the distance between
electrons, the molecule forms a tetrahedral shape. Once again, the Lewis
dot diagram fails to indicate this, but it is clear in a three-dimensional model
for the molecule, as Figure 4-5f illustrates.
A Two-dimensional and Three-dimensional Model for Methane (CH
4
)
H
HC
H
H
Figure 4-5f
