Molecular structure describes the location of the atoms, not the electrons. The electron-pair geometries shown in Figure 3 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 structure. Electron-pair Geometry versus Molecular Structure The basic electron-pair geometries predicted by VSEPR theory maximize the space around any region of electron density (bonds or lone pairs). Two regions of electron density around a central atom in a molecule form a linear geometry three regions form a trigonal planar geometry four regions form a tetrahedral geometry five regions form a trigonal bipyramidal geometry and six regions form an octahedral geometry. The BeF 2 molecule adopts a linear structure in which the two bonds are as far apart as possible, on opposite sides of the Be atom.ДЇigure 3 illustrates this and other electron-pair geometries that minimize the repulsions among regions of high electron density (bonds and/or lone pairs).

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 2) 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.ДЄs 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. Bond distances (lengths) and angles are shown for the formaldehyde molecule, H 2CO.

Bond distances are measured in Г…ngstroms (1 Г… = 10 вЂ“10 m) or picometers (1 pm = 10 вЂ“12 m, 100 pm = 1 Г…). A bond distance (or bond length) is the distance between the nuclei of two bonded atoms along the straight line joining the nuclei. A bond angle is the angle between any two bonds that include a common atom, usually measured in degrees. However, molecular structure is actually three-dimensional, and it is important to be able to describe molecular bonds in terms of their distances, angles, and relative arrangements in space ( Figure 1). Thus far, we have used two-dimensional Lewis structures to represent molecules.

Assess the polarity of a molecule based on its bonding and structure.

Explain the concepts of polar covalent bonds and molecular polarity.

Predict the structures of small molecules using valence shell electron pair repulsion (VSEPR) theory.

All rights reserved.By the end of this section, you will be able to: You can use your mouse to manipulate the molecule in the right hand 'Jmol' image.ДЄ VSEPR tutorial on the WWW VSEPR tutorial on the WWW, URL: Д¬opyright 1996-2015 Prof Mark Winter. Tetrahedral for the four shape-determining electron pairs There are four bonded groups, therefore there are no lone pairs. The coordination geometry of carbon is consequently tetrahedral. This corresponds to four shape-determining electron pairs. The calculation for methane shows that the carbon atom is associated with 8 electrons in the Пѓ framework.

Ch4 molecular geometry