What Is a Lone Pair?

Solitary paired electrons refer to non-bonded electrons that are not used to form covalent bonds in the outermost electron layer of the atom. These unbonded valence electron pairs are called lone pair electrons. The so-called "solitary" is because it is unbonded, and the "pair" is because two electrons with opposite spins will pair. A lone pair electron is an electron pair in which a molecule or ion does not share a valence layer. The existence and distribution of electrons in the molecule affect the shape, dipole moment, bond length, and bond energy of the molecule, and the effect on molecules composed of light atoms is particularly significant. The basicity of Lewis base (Lewis), the ligand through the bonding of the coordinating atom and the central body, the occurrence of nucleophilic reactions and so on all through lone pair of electrons. ^[1]

After the atoms form molecules, in order to satisfy the octahedral structure, some atoms have valence electrons that are not used for bonding.

Molecules or ions do not share valence electron pairs. The existence and distribution of electrons in the molecule affect the shape, dipole moment, bond length, and bond energy of the molecule, and the effect on molecules composed of light atoms is particularly significant. The basicity of Lewis base (Lewis), the ligand through the bonding of the coordinating atom and the central body, the occurrence of nucleophilic reactions and so on all through lone pair of electrons. ^[1]

For example, there are two lone pairs of electrons on the nitrogen atom of the ammonia molecule; two lone pairs of electrons on the oxygen atom of the water molecule. Because the electron cloud of the lone pair of electrons is larger than the bonding electron pair in space, it has a stronger repulsive effect on the bonding electrons, resulting in a decrease in the bond angle of the molecule. For example, methane has no lone pair of electrons, and the bond angle is 109.5 ° C, while the bond angles of ammonia and water molecules are 107 ° and 104.5 °, respectively. When describing the molecular geometry, lone electrons are not included, so

The valence layer electron pair mutual exclusion theory (English: ValenceShellElectronPairRepulsion, VSEPR for short) is a chemical model used to predict the morphology of a single covalent molecule. The theory predicts the geometric configuration of the molecule by calculating the number of valence electrons and coordination number of the central atom, and constructs a reasonable Lewis structure formula to represent the positions of all bonds and lone pair electrons in the molecule.

The basis of the valence layer electron pair mutual exclusion theory is that the geometric configuration of the molecule or ion is mainly determined by the repulsion between the electron pairs associated with the central atom. The electron pair can be either bonded or unbonded (called a lone pair of electrons). Only the valence electrons of the central atom can have a meaningful effect on the shape of the molecule. The three cases of repulsion between electron pairs in a molecule are: repulsion between lone pair electrons (solitary-solitary repulsion); repulsion between lone pair electrons and bonded electron pair (solitary-repulsion); Exclusion (into-into exclusion). Molecules will try their best to avoid these exclusions to maintain stability. When repulsion cannot be avoided, the entire molecule tends to form the weakest repulsion structure (the way with the smallest difference from the ideal shape). The repulsion between the lone pair of electrons is considered to be greater than the repulsion between the lone pair of electrons and the bonding electron pair, which in turn is greater than the repulsion between the bonding electron pair. Therefore, molecules are more inclined to the weakest formation-to-rejection. Among 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]