s8pe-20501-ca
11/17/05
11:23 AM
Page 140
Structure of Molecules
Molecules are very small. Even the largest molecules cannot be seen
by an optical microscope. But, a molecule’s shape is important. A
molecule’s shape has an effect on the properties of the substance.
For example, nylon is made from long molecules that look a lot
like chains. We call these long-chain molecules polymers. This shape
is what makes nylon flexible and stretchable. When you stretch a piece
of nylon, you’re actually stretching out the long-chain molecules.
The shapes of a substance’s molecules also can affect the substance’s boiling and melting points. For example, a substance with
longer, thinner molecules probably has a higher melting point than a
substance with shorter molecules. Longer molecules get tangled like
strands of spaghetti. It takes more energy to break them apart.
Making nylon is easy in
the laboratory. Atomic
units join together like
the links of a chain.
Long-chain molecules also may make a substance a good lubricant. A lubricant is any substance that reduces friction between two
surfaces. Motor oils contain long molecules made mostly of carbon
and hydrogen. These molecules easily slip and slide past one another.
There is little for them to get stuck on. The long molecules also can
slide easily into microscopic places in the surfaces.
Crystal Structure
Have you ever seen light reflected from a rock? You may have been
looking at a crystal. Crystals occur when atoms combine to form
repeating patterns.
Common table salt is one type of crystal. Salt contains the elements sodium and chlorine. You can think of a salt crystal as having
individual sodium and chlorine units. These units build up like
blocks. In the case of salt, each sodium unit attaches to one chlorine
unit. The pattern gets repeated. This is what gives salt its square look.
Crystals have many uses. They can bend light waves in certain
ways. Some crystals can change electrical energy into mechanical
energy. All these properties depend on the crystal structure.
The regular way in which
sodium and chlorine units
join together is evident
in the salt crystals in this
photo. Most salt crystals
in this photo are oddly
shaped because of
impurities.
140 Unit 2: The Structure of Matter
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