A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons as in covalent bonds
chemical bonding, any of the interactions that account for the association of atoms into molecules, ions, crystals, and other stable species that make up the familiar substances of the everyday world. When atoms approach one another, their nuclei and electrons interact and tend to distribute themselves in space in such a way that the total energy is lower than it would be in any alternative arrangement. If the total energy of a group of atoms is lower than the sum of the energies of the component atoms, they then bond together and the energy lowering is the bonding energy.
The ideas that helped to establish the nature of chemical bonding came to fruition during the early 20th century, after the electron had been discovered and quantum mechanics had provided a language for the description of the behaviour of electrons in atoms. However, even though chemists need quantum mechanics to attain a detailed quantitative understanding of bond formation, much of their pragmatic understanding of bonds is expressed in simple intuitive models. These models treat bonds as primarily of two kinds—namely, ionic and covalent.
The formation of ionic bonds
Lewis formulation of an ionic bond
In Lewis terms, the formation of an ionic bond stems from the transfer of electrons from one atom to another. When such a transfer occurs, all the valence electrons on the more electropositive element (from one of the first three groups on the left in the periodic table) are removed to expose the core of the atom. The electrons so released are accepted into the empty orbitals of the valence shell of the more electronegative atom (typically from the groups immediately to the left of the noble gases); the valence shell is thereby filled. Thus, the formation of the ionic compound sodium chloride can be represented by the following process:
Covalent bonds
When none of the elements in a compound is a metal, no atoms in the compound have an ionization energy low enough for electron loss to be likely. In such a case, covalence prevails. As a general rule, covalent bonds are formed between elements lying toward the right in the periodic table (i.e., the nonmetals). Molecules of identical atoms, such as H2 and buckminsterfullerene (C60), are also held together by covalent bonds.
Lewis formulation of a covalent bond
In Lewis terms a covalent bond is a shared electron pair. The bond between a hydrogen atom and a chlorine atom in hydrogen chloride is formulated as follows:
First, an atom may complete its octet by sharing more than one pair of electrons with a bonded neighbour. Two shared pairs of electrons, represented by a double dash (=), form a double bond. Double bonds are found in numerous compounds, including carbon dioxide:
Three shared pairs of electrons are represented by a triple dash (≡) and form a triple bond. Triple bonds are found in, for example, carbon monoxide, nitrogen molecules, and acetylene, shown respectively as:
A double bond is stronger than a single bond, and a triple bond is stronger than a double bond. However, a double bond is not necessarily twice as strong as a single bond, nor is a triple bond necessarily three times as strong. Quadruple bonds, which contain four shared pairs of electrons, are rare but have been identified in some compounds in which two metal atoms are bonded directly together.
Resonance
There is sometimes an ambiguity in the location of double bonds. This ambiguity is illustrated by the Lewis structure for ozone (O3). The following are two possible structures:
In such cases, the actual Lewis structure is regarded as a blend of these contributions and is written:
The blending together of these structures is actually a quantum mechanical phenomenon called resonance, which will be considered in more detail below. At this stage, resonance can be regarded as a blending process that spreads double-bond character evenly over the atoms that participate in it. In ozone, for instance, each oxygen-oxygen bond is rendered equivalent by resonance, and each one has a mixture of single-bond and double-bond character (as i
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