Pair of valence electrons which are not shared with another atom in a covalent bond
Lone pairs (shown as pairs of dots) in the Lewis structure of hydroxide
In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bond[1] and is sometimes called an unshared pair or non-bonding pair. Lone pairs are found in the outermost electron shell of atoms. They can be identified by using a Lewis structure. Electron pairs are therefore considered lone pairs if two electrons are paired but are not used in chemical bonding. Thus, the number of electrons in lone pairs plus the number of electrons in bonds equals the number of valence electrons around an atom.
Lone pair is a concept used in valence shell electron pair repulsion theory (VSEPR theory) which explains the shapes of molecules. They are also referred to in the chemistry of Lewis acids and bases. However, not all non-bonding pairs of electrons are considered by chemists to be lone pairs. Examples are the transition metals where the non-bonding pairs do not influence molecular geometry and are said to be stereochemically inactive. In molecular orbital theory (fully delocalized canonical orbitals or localized in some form), the concept of a lone pair is less distinct, as the correspondence between an orbital and components of a Lewis structure is often not straightforward. Nevertheless, occupied non-bonding orbitals (or orbitals of mostly nonbonding character) are frequently identified as lone pairs.
Lone pairs in ammonia (A), water (B), and hydrogen chloride (C)
A single lone pair can be found with atoms in the nitrogen group, such as nitrogen in ammonia. Two lone pairs can be found with atoms in the chalcogen group, such as oxygen in water. The halogens can carry three lone pairs, such as in hydrogen chloride.
In VSEPR theory the electron pairs on the oxygen atom in water form the vertices of a tetrahedron with the lone pairs on two of the four vertices. The H–O–H bond angle is 104.5°, less than the 109° predicted for a tetrahedral angle, and this can be explained by a repulsive interaction between the lone pairs.[2][3][4]
Various computational criteria for the presence of lone pairs have been proposed. While electron density ρ(r) itself generally does not provide useful guidance in this regard, the Laplacian of the electron density is revealing, and one criterion for the location of the lone pair is where L(r) = –∇2ρ(r) is a local maximum. The minima of the electrostatic potential V(r) is another proposed criterion. Yet another considers the electron localization function (ELF).[5]
^IUPAC Gold Book definition: lone (electron) pair
^Fox, M.A.; Whitesell, J.K. (2004). Organic Chemistry. Jones and Bartlett Publishers. ISBN 978-0-7637-2197-8. Retrieved 5 May 2021.
^McMurry, J. (2000). Organic Chemistry 5th Ed. Ceneage Learning India Pvt Limited. ISBN 978-81-315-0039-2. Retrieved 5 May 2021.
^Lee, J.D. (1968). Concise Inorganic Chemistry. Student's paperback edition. Van Nostrand. Retrieved 5 May 2021.
^Kumar, Anmol; Gadre, Shridhar R.; Mohan, Neetha; Suresh, Cherumuttathu H. (2014-01-06). "Lone Pairs: An Electrostatic Viewpoint". The Journal of Physical Chemistry A. 118 (2): 526–532. Bibcode:2014JPCA..118..526K. doi:10.1021/jp4117003. ISSN 1089-5639. PMID 24372481.
a lonepair refers to a pair of valence electrons that are not shared with another atom in a covalent bond and is sometimes called an unshared pair or...
where lonepairs occupy positions that allow them to experience less repulsion. Lonepair–lonepair (lp–lp) repulsions are considered stronger than lone pair–bonding...
central atom by a lonepair of valence electrons leaves the general form of the electron arrangement unchanged with the lonepair now occupying one position...
diagrams that show the bonding between atoms of a molecule, as well as the lonepairs of electrons that may exist in the molecule. A Lewis structure can be...
compounds and functional groups that contain a basic nitrogen atom with a lonepair. Amines are formally derivatives of ammonia (NH3), wherein one or more...
Applied Chemistry (IUPAC) as the interaction of a reaction centre with a lonepair of electrons in an atom or the electrons present in a sigma or pi bond...
Yang. Color–flavor locking Superinsulator Lonepair Electron pair Cooper, Leon N. (1956). "Bound electron pairs in a degenerate Fermi gas". Physical Review...
a bonding pair of electrons. An equatorial lonepair is repelled by only two bonding pairs at 90°, whereas a hypothetical axial lonepair would be repelled...
conventionally represented as having alternating single and multiple bonds. Lonepairs, radicals or carbenium ions may be part of the system, which may be cyclic...
which the lonepair of electrons of a ring-nitrogen atom is not part of the aromatic system and extends in the plane of the ring. This lonepair is responsible...
"donor" atom or group (Dn), and another electronegative atom bearing a lonepair of electrons—the hydrogen bond acceptor (Ac). Such an interacting system...
the lonepairs on the nitrogen atoms are donated to the metal cation. The less well-characterised ways involve dinitrogen donating electron pairs from...
or they can occur as a lonepair of valence electrons. They also fill the core levels of an atom. Because the spins are paired, the magnetic moment of...
two hydrogens and two lonepairs, and the H2O geometry is simply described as bent without considering the nonbonding lonepairs.[citation needed] However...
this contraindication between ferromagnetism and ferroelectricity. In lone-pair-active multiferroics, the ferroelectric displacement is driven by the...
lonepair with the aryl substituent. The observed geometry reflects a compromise between two competing factors: 1) stabilization of the N lonepair in...
tetragonal distortion. Some molecules, such as XeF6 or IF− 6, have a lonepair that distorts the symmetry of the molecule from Oh to C3v. The specific...
molecule by the interaction of two pi bonds or between a pi bond and lonepair of electrons present on an adjacent atom. This change in electron arrangement...
orbital. The lonepair is in an sp2 orbital, projecting outward from the ring in the same plane as the σ bonds. As a result, the lonepair does not contribute...
electron pair. An example is boron trifluoride (BF3), whose boron atom has a vacant orbital that can form a covalent bond by sharing a lonepair of electrons...
cos−1(−1/3) ≈ 109.5°. However, the three hydrogen atoms are repelled by the electron lonepair in a way that the geometry is distorted to a trigonal pyramid (regular...
aromatic rings, the lonepair of electrons is not part of the aromatic system and extends in the plane of the ring. This lonepair is responsible for the...
presence of lonepairs. There are several variants of bending, where the most common is AX2E2 where two covalent bonds and two lonepairs of the central...
donating a pair of electrons to a metal centre. For example, in hexamminecobalt(III) chloride, each ammonia ligand donates its lonepair of electrons...
absorption. A feature of these auxochromes is the presence of at least one lonepair of electrons which can be viewed as extending the conjugated system by...
the general formula C−Y−C−X, where Y is a heteroatom with one or more lonepairs, and X is an electronegative atom or group. The magnitude of the anomeric...
oxygen atom and the two peripheral hydrogen atoms with oxygen having two lonepairs of electrons. Valence bond theory suggests that H 2O is sp3 hybridized...
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