Chemical bond
In chemistry, a chemical bond is a force between two atoms that is strong enough to see the two atoms that are exerting the force on each other as an entity. It can happen that the two atoms form a stable entity (a diatomic molecule), an example being two nitrogen atoms chemically bound to the stable molecule N2. It can also happen that the two atoms are part of a larger aggregate. For instance, the chemical bond C—O between a carbon (C) atom and an oxygen (O) atom in the molecule methanol H3C—OH is a strong and easily recognizable bond. Two atoms may also be bound in a crystal, such as sodium (Na) and chlorine (Cl) that appear in a 1-1 ratio in crystalline kitchen salt (NaCl).
Traditionally one distinguishes the following types of bonds:
- Covalent bonds. These are the bonds most commonly found in organic chemistry. They take place mainly between hydrogen, carbon, nitrogen, and oxygen. They lead to stable, recognizable, molecules that remain intact in the solid, liquid, and gaseous aggregation state. Covalent binding is caused by electron pairing, a phenomenon that requires quantum mechanics for a complete understanding.
- Ionic bonds. Here atom A loses an electron to its bonding partner B, so that A becomes the cation A+ and B the anion B−. Consecutively, the ions bind strongly through the Coulomb interaction. Systems of which the atoms are bound by ionic interactions are usually crystals, the example of kitchen salt (Na+—Cl−) was already mentioned. It requires advanced laboratory techniques to separate ionically bound molecules from crystals, because the crystals are very stable.
- Metallic bonds. A number of metal atoms can crystallize to form a metal, which is a solid recognized by high electric and thermal conductivity. The bonding is caused by delocalized electrons forming electronic bands. The mechanism is akin to the formation of molecular orbitals in molecules. An explanation of metallic bonding is offered by quantum mechanics. Metal molecules (M2, M3, etc.) are not easily prepared experimentally.
- Intermolecular (also known as Van der Waals) bonds. These are bonds between stable molecules, see the articles intermolecular forces and Van der Waals forces for more details. For many years it was believed that hydrogen bonding should be classified as a separate type of bond, but modern theoretical chemistry recognizes it as a special type of intermolecular bond.
It took several centuries before chemical bonding was fully understood, but at present it is generally accepted that quantum mechanical explanations based on Coulomb's electrostatic law[1] give satisfactory accounts of all kinds of bonds.[2] Gravitational forces, strong nuclear forces, even magnetic forces, do not play any significant role in chemical bonding.
Note
- ↑ That is, explanations derived from quantum mechanical energy operators containing Coulomb interactions only (plus electronic kinetic energies).
- ↑ As late as 1916 the famous American chemist Gilbert N. Lewis disagreed strongly with this statement. In his lecture given at the December meeting of the Sections of Physics and Chemistry of the American Association for the Advancement of Science, the American Physical Society, and the American Chemical Society (see Science Magazine pp. 297-302 (1917); DOI), he declared the following: "Therefore, unless we are willing, under the onslaught of quantum theories, to throw overboard all of the basic principles of physical science, we must conclude that the electron in the Bohr atom not only ceases to obey Coulomb's law, but exerts no influence whatsoever upon another charged particle at any distance. Yet it is on the basis of Coulomb's law that the equations of Bohr were derived."