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 mechanics gives a wholly satisfactory explanation of all kinds of bonds. The quantum mechanical explanations are based on Coulomb's electrostatic law. Gravitational forces, strong nuclear forces, even magnetic forces, do not play any significant role in chemical bonding.