![]() The electron-pair geometries will be the same as the molecular structures 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. The structure that includes only the placement of the atoms in the molecule is called the molecular structure. We differentiate between these two situations by naming the geometry that includes all electron pairs the electron-pair geometry. Molecular structure describes the location of the atoms, not the electrons. The Boron atom has only three pairs of valence shell electrons in BCl 3. 9: (a) H 2 O has four regions of electron density around the central atom, so it has a tetrahedral electron-pair geometry. ![]() The three Cl atoms form an equilateral triangle. Thus, the electron-pair geometry is tetrahedral and the molecular structure is bent with an angle slightly less than 109.5°. The electron-pair geometries describe all regions where electrons are located, bonds as well as lone pairs. The geometry of BCl 3 is also given in Figure 7.2: it is trigonal planar, with all four atoms lying in the same plane, and all Cl B Cl bond angles equal to 120o. It is important to note that electron-pair geometry around a central atom is not the same thing as its molecular structure. 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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