handbook of modern sensors

18.1 Materials 537
Fig. 18.3. The atomic building blocks for polymers.
chemists use only eight elements to create thousands of different plastics. These elements
are carbon (C), hydrogen (H), nitrogen (N), oxygen (O), fluorine (F), silicon
(Si), sulfur (S), and chlorine (Cl). Combining these elements in various ways produces
extremely large and complex molecules.
Each atom has a limited capacity (energy bonds) for joining with other atoms,
and every atom within a molecule must have all of its energy bonds satisfied if the
compound is to be stable. For example, hydrogen can bond only to one other atom,
whereas carbon or silicon must attach to four other atoms to satisfy its energy bonds.
Thus, H-H and H-F are stable molecules, whereas C-H and Si-Cl are not. Figure 18.3
shows all eight atoms and the corresponding energy bonds.
Adding more carbon atoms in a chain and more hydrogen atoms to each carbon
atom creates heavier molecules. For example, ethane gas (C 2 H 6 ) is heavier than
methane gas because it contains additional carbon and two hydrogen atoms. Its molecular
weight is 30. Then, the molecular weight can be increased in increments of 14
(1 carbon + 2 hydrogen), until the compound pentane (C 5 H 12 ) is reached. It is too
heavy to be gas and, indeed, it is liquid at room temperature. Further additions of
CH 2 groups makes progressively a heavier liquid until C 18 H 38 is reached. It is solid:
paraffin wax. If we continue and grow larger molecules, the wax becomes harder
and harder. At about C 100 H 202 , the material with a molecular weight of 1402 is tough
enough and is called a low-molecular-weight polyethylene, the simplest of all thermoplastics.
Continuing the addition of more CH 2 groups further increases the toughness
of the material until medium-molecular-weight (between 1000 and 5000 carbons) and