The kind of molecule is determined by the exact type and number of atoms. For example, a water molecule is composed of three atoms: two of hydrogen and one of oxygen. The components of a water molecule are most simply expressed by the well-known chemical formula “H20” or by the structural diagram: H-O-H (see Figure 11). A water molecule is considered stable because the valences of each of its constituent atoms are satisfied: the two hydrogen atoms have formed one bond each, and the single oxygen atom has formed the two bonds it needs.

When dissimilar atoms join together (as with water), the resulting molecule is called a compound. There are two major types of compounds: organic and inorganic. Generally speaking, organic compounds contain carbon and inorganic compounds do not, though a few carbon-containing compounds (such as metallic cyanides, carbon dioxide, carbides, and carbonates) are studied as part of inorganic chemistry.

The specific way in which a molecule is formed depends on which type of compound it is. Inorganic compounds are formed when an atom gives up, or transfers, one of its orbiting electrons to a nearby atom. Because of the rules of valence, this electron transfer can help both the donor and the recipient attain greater stability. Because of the compact structure of the carbon atom, it is much less inclined to give up an electron. It will, however, share an electron with a nearby atom (such as hydrogen) to attain a more stable compound.

As previously stated, each atom has its own atomic weight. When atoms unite to form a molecule, the sum of these atomic weights is then known as the molecular weight. For example, a methane molecule (CH4) combines the atomic weight of one carbon atom (12) with the atomic weight of four hydrogen atoms (1 x 4), for a total molecular weight of 16. In addition to hydrogen, oxygen, and carbon, there are a handful of other atomic elements that form the basis for the majority of chemical raw materials used in the sealing industry. These include nitrogen (N), fluorine (F), silicon (Si), sulfur (S), and chlorine (Cl). The atomic weights and valences of each of these elements are listed in Table 1.

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