chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, the double bond rule states that elements with a

principal quantum number In quantum mechanics, the principal quantum number (symbolized ''n'') is one of four quantum numbers assigned to each electron in an atom to describe that electron's state. Its values are natural numbers (from 1) making it a discrete variable. A ...

greater than 2 for their

valence electron In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair forms ...

s (

period 3 element A period 3 element is one of the chemical elements in the third row (or period) of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the ele ...

s and higher) tend not to form multiple bonds (e.g.

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

s and

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...

s). The double bonds, when they exist, are often weak due to poor

orbital overlap In chemical bonds, an orbital overlap is the concentration of orbitals on adjacent atoms in the same regions of space. Orbital overlap can lead to bond formation. Linus Pauling explained the importance of orbital overlap in the molecular bond ang ...

. Although such compounds are not intrinsically unstable, they instead tend to

polymerize In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many for ...

. An example is the rapid polymerization that occurs upon condensation of

disulfur Disulfur is the diatomic molecule with the formula S2. It is analogous to the dioxygen molecule but rarely occurs at room temperature. This violet gas is the dominant species in hot sulfur vapors. S2 is one of the minor components of the atmosph ...

, the heavy analogue of . Numerous violations to the rule exist.

Other meanings

Another unrelated double bond rule exists that relates to the enhanced reactivity of

sigma bonds In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of ...

attached to an atom adjacent to a double bond. In bromoalkenes, the C–Br bond is very stable, but in an

allyl In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, ...

bromide, this bond is very reactive. Likewise, bromobenzenes are generally inert, whereas

benzylic In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure . Benzyl features a benzene ring () attached to a methylene group () group. Nomenclature In IUPAC nomenclature, the prefix benzyl refers to a subst ...

bromides are reactive. The first to observe the phenomenon was

Conrad Laar Conrad Peter Laar (22 March 1853 – 11 February 1929) was a German chemist. He coined the expression tautomerism Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reac ...

in 1885. The name for the rule was coined by Otto Schmidt (1874–1943) in 1932.

References

{{Reflist Chemical bonding