elektroniryhmää

An elektroniryhmä, or electron group, is a fundamental concept in chemistry used to describe regions around a central atom where electrons are likely to be found. This concept is central to understanding molecular geometry and predicting the shape of molecules.

Electron groups can be either bonding pairs of electrons or lone pairs of electrons. A bonding pair of electrons is involved in a covalent bond between two atoms, while a lone pair consists of valence electrons on an atom that are not involved in bonding. Each of these, whether a single bond, a double bond, a triple bond, or a lone pair, counts as one electron group.

The number of electron groups around a central atom dictates the electron geometry. According to VSEPR theory (Valence Shell Electron Pair Repulsion), electron groups repel each other and will arrange themselves as far apart as possible in three-dimensional space to minimize this repulsion. This arrangement then leads to specific electron geometries. For example, two electron groups will arrange themselves linearly, three will arrange trigonally planar, four will arrange tetrahedrally, five will arrange trigonal bipyramidally, and six will arrange octahedrally.

While electron geometry describes the arrangement of all electron groups, molecular geometry describes the arrangement of only the atoms. The presence of lone pairs can lead to differences between electron geometry and molecular geometry, as lone pairs occupy space but are not bonded to another atom. Understanding electron groups is therefore crucial for both predicting and explaining the shapes of molecules, which in turn influences their physical and chemical properties.