Plastics

There are more than 100 different chemical atoms, known as elements. They are represented by the chemist by the use of simple symbols such as "H" for hydrogen, "O" for oxygen, "C" for carbon, "N" for nitrogen, "Cl" for chlorine, and so on; these atoms have atomic weights of 1, 16, 12, 14, and 17 atomic units, respectively.

A chemical reaction between two or more atoms forms a molecule. Each molecule is characterized by its elemental constitution and its molecular weight. For example, when carbon is burned in oxygen, one atom of carbon (C) reacts with two atoms of oxygen (O₂; equivalent to one molecule of molecular oxygen) to form carbon dioxide (CO₂). The chemist represents this reaction by a chemical equation, i.e.,

Similarly, when four atoms of hydrogen (2H₂; equivalent to two molecules of molecular hydrogen) and two atoms of oxygen (O₂; equivalent to one molecule of oxygen) react to form two molecules of water (2H₂O), the chemist writes

Note that one molecule of oxygen combines with two molecules of hydrogen, and one atom of carbon combines with one molecule of hydrogen. This is because different elements have different combining capacities. Thus hydrogen forms one bond, oxygen two bonds, and carbon four bonds. These bonding capacities, or valences, are taken for granted when writing a chemical formula like H₂O.

In the case of methane, or CH₄, the carbon is bonded to four hydrogen atoms. But carbon can also form double bonds, as in ethylene (C₂H₄) where two CH₂molecules share a double bond. The chemist could also describe the ethylene molecule by the formula CH₂=CH₂, where the double bond is represented by an equal sign.

Plastic materials consist of many repeating groups of atoms or molecules (called monomers) in long chains, and hence are also known as polymers or macromolecules. Elements present in a polymer chain typically include oxygen, hydrogen, nitrogen, carbon, silicon, fluorine,

chlorine, or sulfur. The way the polymer chains are linked together and the lengths of the chains determine the mechanical and physical properties of the plastic.