Unpaired electron
Encyclopedia
In chemistry
, an unpaired electron is an electron that occupies an orbital
of an atom
singly, rather than as part of an electron pair
. As the formation of electron pairs is often energetically favourable, either in the form of a chemical bond
or as a lone pair
, unpaired electrons are relatively uncommon in chemistry, because an entity that carries an unpaired electron is usually rather reactive. In organic chemistry
they typically only occur briefly during a reaction on an entity called a radical
; however, they play an important role in explaining reaction pathways.
Radicals are uncommon in s- and p-block chemistry, since the unpaired electron occupies a valence
p orbital or an sp, sp2 or sp3 hybrid orbital
. These orbitals are strongly directional and therefore overlap to form strong covalent bonds, favouring dimerisation of radicals. Radicals can be stable if dimerisation would result in a weak bond or the unpaired electrons are stabilised by delocalisation. In contrast, radicals in d- and f-block chemistry are very common. The less directional, more diffuse d and f orbitals, in which unpaired electrons reside, overlap less effectively, form weaker bonds and thus dimerisation is generally disfavoured. These d and f orbitals also have comparatively smaller radial extension, disfavouring overlap to form dimers.
More stable entities with unpaired electrons do exist, e.g. the oxygen
-molecule has two unpaired electrons and the nitric oxide
molecule has one. According to Hund's rule, the spins of unpaired electrons are aligned parallel and this gives these molecules paramagnetic
properties.
The most stable examples of unpaired electrons are found on the atoms and ions of lanthanide
s. The incomplete f-shell of these entities does not interact very strongly with the environment they are in and this prevents them from being paired. The ion with the largest number of unpaired electrons is Gd3+
with seven unpaired electrons.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, an unpaired electron is an electron that occupies an orbital
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
of an atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
singly, rather than as part of an electron pair
Electron pair
In chemistry, an electron pair consists of two electrons that occupy the same orbital but have opposite spins.Because electrons are fermions, the Pauli exclusion principle forbids these particles from having exactly the same quantum numbers. Therefore the only way to occupy the same orbital, i.e....
. As the formation of electron pairs is often energetically favourable, either in the form of a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
or as a lone pair
Lone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
, unpaired electrons are relatively uncommon in chemistry, because an entity that carries an unpaired electron is usually rather reactive. In organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
they typically only occur briefly during a reaction on an entity called a radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
; however, they play an important role in explaining reaction pathways.
Radicals are uncommon in s- and p-block chemistry, since the unpaired electron occupies a valence
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
p orbital or an sp, sp2 or sp3 hybrid orbital
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...
. These orbitals are strongly directional and therefore overlap to form strong covalent bonds, favouring dimerisation of radicals. Radicals can be stable if dimerisation would result in a weak bond or the unpaired electrons are stabilised by delocalisation. In contrast, radicals in d- and f-block chemistry are very common. The less directional, more diffuse d and f orbitals, in which unpaired electrons reside, overlap less effectively, form weaker bonds and thus dimerisation is generally disfavoured. These d and f orbitals also have comparatively smaller radial extension, disfavouring overlap to form dimers.
More stable entities with unpaired electrons do exist, e.g. the oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
-molecule has two unpaired electrons and the nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
molecule has one. According to Hund's rule, the spins of unpaired electrons are aligned parallel and this gives these molecules paramagnetic
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
properties.
The most stable examples of unpaired electrons are found on the atoms and ions of lanthanide
Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
s. The incomplete f-shell of these entities does not interact very strongly with the environment they are in and this prevents them from being paired. The ion with the largest number of unpaired electrons is Gd3+
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...
with seven unpaired electrons.