bond

The formation of a covalent bond between two hydrogen atoms to form a hydrogen molecule (H₂), and between two hydrogen atoms and an oxygen atom to form a molecule of water (H₂O). The sharing means that each atom has a more stable arrangement of electrons (its outer electron shells are full).

In chemistry, the result of the forces of attraction that hold together atoms in an element or compound. The principal types of bonding are ionic, covalent, metallic, and intermolecular (such as hydrogen bonding).

The type of bond formed depends on the elements concerned and their electronic structure. In an ionic or electrovalent bond, common in inorganic compounds, the combining atoms gain or lose electrons to become ions; for example, sodium (Na) loses an electron to form a sodium ion (Na⁺) while chlorine (Cl) gains an electron to form a chloride ion (Cl⁻) in the ionic bond of sodium chloride (NaCl).

In a covalent bond, the atomic orbitals of two atoms overlap to form a molecular orbital containing two electrons, which are thus effectively shared between the two atoms. Covalent bonds are common in organic compounds, such as the four carbon-hydrogen bonds in methane (CH₄). In a dative covalent or coordinate bond, one of the combining atoms supplies both of the valence electrons in the bond.

A metallic bond joins metals in a crystal lattice, the atoms occupy lattice positions as positive ions, and valence electrons are shared between all the ions in an ‘electron gas’.

In a hydrogen bond, a hydrogen atom joined to an electronegative atom, such as nitrogen or oxygen, becomes partially positively charged, and is weakly attracted to another electronegative atom on a neighbouring molecule.

The strongest known noncovalent bond is the superbond that is formed between avidin, a protein found in egg white, and the growth factor biotin. It is almost impossible to separate the two molecules once bonded. The superbond is used in a number of biomedical research applications.