Chapter 5
The atoms of many elements can combine to form molecules. The atoms in most molecules are held together by strong attractive forces called chemical bonds. These bonds are formed via the interaction of the valence electrons of the combining atoms. The chemical and physical properties of the resulting molecules are often very different from those of their constituent elements.
In addition to the very strong forces within a molecule, there are weaker intermolecular forces between molecules. These intermolecular forces, although weaker than the intramolecular chemical bonds, are of considerable importance in understanding the physical properties of many substances. We shall discuss intermolecular forces in greater detail when we look at the different phases of matter later on. Processes that involve the breaking and forming of chemical bonds are generally considered chemical processes, while those that only involve interactions between molecules are generally considered physical processes.
In the formation of chemical bonds, many molecules contain atoms bonded according to the octet rule, which states that an atom tends to bond with other atoms until it has eight electrons in its outermost shell, thereby forming a stable electron configuration similar to that of the noble gas elements. Exceptions to this rule are as follows: hydrogen, which can have only two valence electrons (the configuration of He); lithium and beryllium, which bond to attain two and four valence electrons, respectively; boron, which bonds to attain six; and elements beyond the second row, such as phosphorus and sulfur, which can expand their octets to include more than eight electrons by incorporating d orbitals.
When classifying chemical bonds, it is helpful to introduce two distinct types: ionic bonds and covalent bonds. In ionic bonding, one or more electrons from an atom with a smaller ionization energy are transferred to an atom with a great electron affinity, and the resulting ions are held together by electrostatic forces. In covalent bonding, an electron pair is shared between two atoms. In many cases, the bond is partially covalent and partially ionic; we call such bonds polar covalent bonds.