The lone pair electrons refer to the bonding electrons used to form covalent bondsatomNon bonding electrons that are not used to form covalent bonds often exist in the outermost electron layer.These unbound valence electron pairs are called lone pairs.The so-called "solitary" is because it is not bonded, while the "right" is because two electrons with opposite spins will pair up.The lone pair electrons are not shared by molecules or ionsValence layerOfElectron pair。The existence and distribution of lone pair electrons in molecules affect molecularshape、dipole moment、Key length、Bond energyThe molecular influence on the composition of light atoms is particularly significant.Lewis baseAlkalinity of (Lewis), ligand passes throughCoordination atomAndCentrosomeOfBonding，Nucleophilic reactionThe occurrence of all of them is through lone pair electrons.[1]
Chinese name
Lone pair electron
Foreign name
lone-pair electron[2]
Nature
Unbound valence electron
Role
Make the bond angle of molecule smaller
Impact
Cause the change of some molecular configurations
Instance
There is a pair of lone electrons on the nitrogen atom of ammonia molecule
After atoms form molecules, in order to meet the octet structure, some atoms still have valence electrons that are not used for bonding. They are called non bonding electrons or non bonding electrons, or lone pair electrons, or unshared electrons.For example, the oxygen atom has 6 valence electrons, and the neutral oxygen atom with 2 bonds has 4 (2 pairs) unbound electrons.The N, S, P, X (halogen) in the molecule also has lone pair electrons.The existence of lone pair electrons is closely related to the physical and chemical properties of organic molecules, so we should pay attention to their existence state.
Some atoms with abnormal valence bonds in some molecules also carry formal charge.The electrons on a covalent bond are shared by the two bonding atoms, so each atom has one electron in form, regardless of its polarity.[3]
Impact
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An electron pair in which a molecule or ion does not share a valence shell.The existence and distribution of lone pair electrons in molecules affect the shape, dipole moment, bond length and bond energy of molecules, especially the molecular composition of light atoms.The alkalinity of Lewis base, the bond between ligand and centrosome through coordination atom, and the occurrence of nucleophilic reaction are all through lone pair electrons.[1]
Example
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For example, the nitrogen atom of ammonia molecule has a pair of lone electrons;There are two pairs of lone electrons on the oxygen atom of water molecule.Since the electron cloud of lone pair electrons is larger than that of bonding electron pairs in space, it has stronger repulsive effect on bonding electrons, resulting in the reduction of molecular bond angle.If methane has no lone pair electrons, the bond angle is 109.5 ℃, while the bond angles of ammonia and water molecules are 107 ° and 104.5 ° respectively.The description of molecular geometry does not include lone pair electrons, somethaneThe molecule is tetrahedron;Ammonia molecule is triangular cone and water molecule is curved.[1]
Relevant knowledge
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Valence shell electron pair repulsion theory(English: Valence Shell Electron Pair Replacement (VSEPR for short) is a chemical model used to predict the morphology of a single covalent molecule.The theory predicts the geometric configuration of molecules by calculating the valence shell electron number and coordination number of the central atom, and constructs a reasonableLewis structure To represent the positions of all bonds and lone pairs of electrons in the molecule.
Valence shell electron pair repulsion theoryIt is based on the fact that the geometric configuration of molecules or ions is mainly determined by the repulsion between the electron pairs related to the central atom.The pair of electrons can be either bonded or unbound (called lone pair electrons).Only the valence electrons of the central atom can have a meaningful impact on the shape of the molecule.There are three cases of repulsion between electron pairs in molecules: repulsion between lone pair electrons (lone lone repulsion);Repulsion between lone pair electrons and bonding electron pairs (lone forming repulsion);The repulsion between bonding electron pairs (formation repulsion).The molecules will try to avoid these repulsions to maintain stability.When repulsion cannot be avoided, the whole molecule tends to form the structure with the weakest repulsion (the way with the smallest difference from the ideal shape).The repulsion between lone pair electrons is considered to be greater than that between lone pair electrons and bonding electron pairs, and the latter is greater than that between bonding electron pairs.Therefore, the molecule is more inclined to the weakest Cheng Cheng rejection.In molecules with more ligands, the electron pairs cannot even maintain an angle of 90 °, so their electron pairs tend to be distributed on multiple planes.[1]
