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diatomic molecule
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Gerhard Herzberg
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diatomic molecule

chemistry
Written by
Ken Stewart
Ken Stewart is a former educator with an honours degree in chemistry, physics, and mathematics. 
Ken Stewart
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diatomic molecule, any chemical compound that is made up of only two atoms.

The two atoms can be the same type of atom, such as oxygen (O2), where both atoms in the molecule are oxygen atoms; such molecules are known as homonuclear diatomic molecules. Other examples of homonuclear diatomic molecules include: hydrogen (H2), nitrogen (N2), fluorine (F2), chlorine (Cl2), iodine (I2), bromine (Br2), lithium (Li2), carbon (C2), and helium (He2). Diatomic molecules can also consist of two different atoms, such as sodium chloride (NaCl; also called table salt); such molecules are called heteronuclear diatomic molecules.

Both homonuclear and heteronuclear diatomic molecules are bonded to each other in a straight line; these bonds can be single, double, or triple bonds, referring to the number of interacting electron pairs between the two atoms. The bonds also can be ionic, covalent, or polar covalent. An example of bonding in diatomic molecules is carbon monoxide (CO), which contains a triple bond. A standard double bond first forms between a carbon atom and an oxygen atom, which completes the valence shell for the oxygen. To stabilize the double bond, a lone pair of electrons from the oxygen atom is shared by the carbon atom, forming a coordinate covalent bond (or dative bond).

Diagram showing four types of crystal bonding: ionic bonding with sodium and chlorine ions, covalent bonding with carbon atoms sharing electrons, metallic bonding with iron ions surrounded by free electrons, and molecular bonding with hydrogen molecules held by weak electrical attraction.
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chemical bonding: Molecular orbitals of period-2 diatomic molecules

Some diatomic molecules are formed only under extreme conditions. For example, helium dimers are formed at extremely low temperatures when van der Waals forces bring the two helium atoms together to form a diatomic molecule. The ability of helium dimers to form was predicted by American physicist John Clark Slater in 1928; in the early 1990s the existence of helium dimers was demonstrated in research led by American chemists George C. McBane and W. Ronald Gentry. 

Ken Stewart