Chemical Bond
A chemical bond is any force of attraction between two atoms strong enough to hold the atoms together for some period of time. At least five primary types of chemical bonds are known, ranging from very strong to very weak. They are covalent, ionic, metallic, and hydrogen bonds, and London forces.
In all cases, a chemical bond ultimately involves forces of attraction between the positively-charged nucleus of one atom and the negatively-charged electron of a second atom. Understanding the nature of chemical bonds has practical significance since the type of bonding found in a substance explains to a large extent the macroscopic properties of that substance.
An ionic bond is one in which one atom completely loses one or more electrons to a second atom. The first atom becomes a positively charged ion and the second, a negatively charged ion. The two ions are attracted to each other because of their opposite electrical charges.
In a covalent bond, two atoms share one or more pairs of electrons. For example, a hydrogen atom and a fluorine atom each donate a single electron to form a shared pair that constitutes a covalent bond between the two atoms. Both electrons in the shared pair orbit the nuclei of both atoms.
In most cases, covalent and ionic bonding occur in such a way as to satisfy the Law of Octaves. Essentially that law states that the most stable configuration for an atom is one in which the outer energy level of the atom contains eight electrons or, in the case of smaller atoms, two electrons.
Ionic and covalent bonds might appear to represent two distinct limits of electron exchange between atoms, one in which electrons are totally gained and lost (ionic bonding) and one in which electrons are shared (covalent bonding). In fact, most chemical bonds fall somewhere between these two extreme cases. In the hydrogen-fluorine example mentioned above, the fluorine nucleus is much larger than the hydrogen nucleus and, therefore, exerts a greater pull on the shared electron pair. The electrons spend more time in the vicinity of the fluorine nucleus and less time in the vicinity of the hydrogen nucleus. For this reason, the fluorine end of the bond is more negative than the hydrogen end, and the bond is said to be a polar covalent bond. A non-polar covalent bond is possible only between two atoms with equal attraction for electrons as, for example, between two atoms of the same element.
Metallic bonds are very different from ionic and covalent bonds in that they involve large numbers of atoms. The outer electrons of these atoms feel very little attraction to any one nucleus and are able, therefore, to move freely throughout the metal.
Hydrogen bonds are very weak forces of attraction between atoms with partial positive and negative charges. Hydrogen bonds are especially important in living organisms since they can be broken and reformed easily during biochemical changes.
London forces are the weakest of chemical bonds. They are forces of attraction between two uncharged molecules. The force appears to arise from the temporary shift of electrical charges within each molecule.
Resources
Books
Giddings, J. Calvin. Chemistry, Man, and Environmental Change: An Integrated Approach. San Francisco: Canfield Press, 1973.
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