![]() ![]() ![]() The H–N–H bond angles in NH 3 are slightly smaller than the 109.5° angle in a regular tetrahedron ( (Figure)) because the lone pair takes up more space than the bonding pairs. Again, there are slight deviations from the ideal because lone pairs occupy larger regions of space than do bonding electrons. The ideal bond angles in a trigonal pyramid are based on the tetrahedral electron-pair geometry. In the ammonia molecule, the three hydrogen atoms attached to the central nitrogen are not arranged in a flat, trigonal planar molecular structure, but rather in a three-dimensional trigonal pyramid ( (Figure)) with the nitrogen atom at the apex and the three hydrogen atoms forming the base. (This is because a bonding is simultaneously pulled toward two different atoms.) VSEPR theory predicts these distortions by postulating that lone pairs take up more room than bonding pairs. (c) The actual bond angles deviate slightly from the idealized angles because the lone pair takes up a larger region of space than do the single bonds, causing the HNH angle to be slightly smaller than 109.5°.Īs seen in (Figure), small distortions from the ideal angles in (Figure) can result from differences in repulsion between various regions of electron density. (b) The trigonal pyramidal molecular geometry is determined from the electron-pair geometry. (a) The electron-pair geometry for the ammonia molecule is tetrahedral with one lone pair and three single bonds. One of these regions, however, is a lone pair, and this lone pair influences the molecular geometry of the molecule ( (Figure)). On the other hand, the ammonia molecule, NH 3, also has four electron pairs associated with the nitrogen atom, and thus has a tetrahedral electron-pair geometry. ![]() The electron-pair geometries will be the same as the molecular geometries when there are no lone electron pairs around the central atom, but they will be different when there are lone pairs present on the central atom.įor example, the methane molecule, CH 4, which is the major component of natural gas, has four bonding pairs of electrons around the central carbon atom the electron-pair geometry is tetrahedral, as is the molecular geometry( (Figure)). The geometry that includes only the placement of the atoms in the molecule is called the molecular geometry. We differentiate between these two situations by naming the geometry that includes all electron pairs the electron-pair geometry. Molecular geometry describes the location of the atoms, not the electrons. The electron-pair geometries shown in (Figure) describe all regions where electrons are located, bonds as well as lone pairs. It is important to note that electron-pair geometry around a central atom is not the same thing as its molecular geometry. The bond angle is 180° ( (Figure)).Įlectron-pair Geometry versus Molecular Geometry With two bonds and no lone pairs of electrons on the central atom, the bonds are as far apart as possible, and the electrostatic repulsion between these regions of high electron density is reduced to a minimum when they are on opposite sides of the central atom. The Lewis structure of BeF 2 ( (Figure)) shows only two electron pairs around the central beryllium atom. Other interactions, such as nuclear-nuclear repulsions and nuclear-electron attractions, are also involved in the final arrangement that atoms adopt in a particular molecular structure.Īs a simple example of VSEPR theory, let us predict the structure of a gaseous BeF 2 molecule. We should understand, however, that the theory only considers electron-pair repulsions. VSEPR theory predicts the arrangement of electron pairs around each central atom and, usually, the correct arrangement of atoms in a molecule. The electrostatic repulsion of these electrons is reduced when the various regions of high electron density assume positions as far from each other as possible. The electrons in the valence shell of a central atom form either bonding pairs of electrons, located primarily between bonded atoms, or lone pairs. The VSEPR model assumes that electron pairs in the valence shell of a central atom will adopt an arrangement that minimizes repulsions between these electron pairs by maximizing the distance between them. Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular structure, including approximate bond angles around a central atom, of a molecule from an examination of the number of bonds and lone electron pairs in its Lewis structure. ![]()
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