9
Sizes and Shapes of Molecules
A knowledge of molecular shapes and sizes is important to an understanding of
chemical reactions. The shape of a molecule (and the bond types it possesses)
has important implications for the manner in which it enters into chemical
reactions. The shape and size of molecules also influence their packing in the
crystalline state.
When atoms combine to produce molecules, they often do so in accord with
the octet rule. Your text undoubtedly contains a fairly detailed discussion of the
octet rule. In essence, it may be described as the tendency for an atom to lose,
gain, or share electrons in order to achieve an s*p
s configuration in the outermost shell. The simplest atoms (H, Li, Be, and so on) tend to achieve a Is
2
configuration, according to what might be called the duet rule.
In Chapter 8, we emphasize the loss and gain of electrons, leading to the
formation of electrically charged ions, such as Na
+ and Cl~. When electrons are
shared, a molecule is formed, and the atoms are connected by acovalent bond.
In this chapter we emphasize the approximate shapes, interatomic distances,
and bond energies of molecules and molecular ions that are held together by
covalent bonds.
COVALENT BOND ENERGIES
The strengths of the bonds that hold the atoms together in a molecule can be
determined in a variety of ways: for example, by direct calorimetric measure-