Shapes of Molecules and Ions VSEPR Theory

Once we have counted the number of electron groups surrounding the central atom, we can determine its specific shape from the number of atoms bonded to the central atom. 

In the electron-dot formula of CO2, there are two electron groups (two double bonds) attached to the central atom. According to VSEPR theory, minimal repulsion occurs when two electron groups are on opposite sides of the central C atom. This gives the CO2 molecule a linear electron-group geometry and a linear shape with a bond angle of 180°. 

Central Atoms with Three Electron Groups 

Central Atoms with Four Electron Groups 

In a molecule of methane, CH4, the central C atom is bonded to four H atoms. From the electron-dot formula, you may think that CH4 is planar with 90° bond angles. However, the best geometry for minimum repulsion is tetrahedral, which places the four electron gronps at the comers of a tetrahedron, giving bond angles of 109°. When there are four atoms attached to four electron groups, the shape of the molecule or polyatomic ion is tetrahedral. 

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Table 1.4 Shapes of Molecules and Ions from VSEPR Theory...

VSEPR theory (10) Valence shell electron pair repulsion theory, which predicts the three-dimensional shape of molecules and ions based on the arrangement of electron pairs (nonbonding pairs and bonds) about the central atom. 

TABLE 1.3 SHAPES OF MOLECULES AND IONS FROM VSEPR THEORY ... 

Finally, in addition to simply representing a pair of shared electrons, a chemical bond has structural implications as well. Because electrons are negatively charged, when there are several distinct bonds, they will tend to be physically separated from each other. This idea is the basis for a method to predict the geometry of molecules called the Valence Shell Electron Pair Repulsion (VSEPR) theory. Using this theory, the general shape of molecules and ions can be predicted. 

The electron pairs around a central atom will become oriented in space to get as far away from one another as possible. Thus, two pairs will be oriented with one pair on each opposite side of the central atom. Three pairs will form a triangle around the central atom, and four pairs will be located at the comers of a regular tetrahedron with the central atom in the center (see i Eigure 4.5 and > Eigure 4.6). The VSEPR theory can be used to predict shapes of molecules and ions with five or more pairs on the central atom, but we will not go beyond four pairs in this book. 

The shapes of molecules and ions can be predicted by the valence shell electron pair repulsion theory (VSEPR). If the Lewis structure is drawn for a molecule or a polyatomic ion, the shape of this molecule or ion can be predicted using this theory. 

VSEPR theory can also be used to explain the shapes of molecules or ions that contain a double or triple bond. A double or triple bond has the same effect as a single bond because all the bonding pairs of electrons are located between the two atoms forming a covalent bond. A double or triple bond is therefore counted as one bonding pair (one electron domain) when predicting the shapes of molecules and ions. 

Chapter 10, Properties of Solids and Liquids, introduces electron-dot formulas for molecules and ions with single and multiple bonds as well as resonance structures. Electronegativity leads to a discussion of the polarity of bonds and molecules. Electron-dot formulas and VSEPR theory illustrate covalent bonding and the three-dimensional shapes of molecules and ions. The attractive forces between particles and their impact on states of matter and changes of state are described. Combining Ideas from Chapters 8, 9, and 10 follows as an interchapter problem set. 

Use VSEPR theory to predict the shapes of molecules and polyatomic ions. (Section 4.8)... 

The original VSEPR theory (Figure 4.56) is a classical theory that accounts for the shapes of most main group molecules. Lewis structures are simple representations of molecules and ions that emphasize the importance of electron pairs (represented by dots and/or crosses). VSEPR theory proposes that the favoured molecular shape is that which minimizes the repulsion between electron pairs in the valence shell of the central atom. 

The VSEPR model is based upon Lewis structures which assume that all valence electrons are paired and a chemical bond requires two electrons. The model is limited to simple compounds of the main group elements (s- and p-blocks) and some transition metal ions (those with d° and d configurations). It can only predict exact bond angles for molecules with no lone pairs. It is theoretically unsatisfactory since electrons do not behave as static point charges and it provides no information about the stability of molecules. However, VSEPR theory is a simple and powerful model that satisfactorily predicts and explains the shapes of a large number of molecules and ions from the s- and p-blocks. 

The valence-shell electron-pair repulsion (VSEPR) theory is a theory used to predict probable shapes of molecules and polyatomic ions based on the mutual repulsions of electron pairs found in the valence shell of the central atom in the structure. 

Two theories go hand in hand in a discussion of covalent bonding. The valence shell electron pair repulsion (VSEPR) theory helps us to understand and predict the spatial arrangement of atoms in a polyatomic molecule or ion. It does not, however, explain hoav bonding occurs, ] ist where it occurs and where unshared pairs of valence shell electrons are directed. The valence bond (VB) theory describes how the bonding takes place, in terms of overlapping atomic orbitals. In this theory, the atomic orbitals discussed in Chapter 5 are often mixed, or hybridized, to form new orbitals with different spatial orientations. Used together, these two simple ideas enable us to understand the bonding, molecular shapes, and properties of a wide variety of polyatomic molecules and ions. 

The shapes of several simple molecules and ions as predicted by VSEPR theory are shown in Table 1.3. In this table we have also included the hybridization state of the central atom. 

The VSEPR theory has its roots in the observation prior to 1940 that isoelectronic molecules or polyatomic ions usually adopt the same shape. Thus BF3, B03 C03, COF2 and NO3 are ail isoelectronic, and they all have planar triangular structures. As developed in more recent years, the VSEPR theory rationalises molecular shapes in terms of repulsions between electron pairs, bonding and nonbonding. It is assumed that the reader is familiar with the rudiments of the theory excellent expositions are to be found in most inorganic texts. 

An advantage of VSEPR is its foundation upon Lewis electron-pair bond theory. No mention need be made of orbitals and overlap. If you can write down a Lewis structure for the molecule or polyatomic ion in question, with all valence electrons accounted for in bonding or nonbonding pairs, there should be no difficulty in arriving at the VSEPR prediction of its likely shape. Even when there may be some ambiguity as to the most appropriate Lewis structure, the VSEPR approach leads to the same result. For example, the molecule HIO, could be rendered, in terms of Lewis theory as ... 

We have seen earlier (Chapter 4) that the valence shell electron pair repulsion theory (VSEPR theory) can be very usefully applied to explain the shapes of simple covalent molecules and polyatomic ions built around a central atom. 

Finally, in VSEPR theory, double and triple bonds are treated in the same way as single bonds. And polyatomic ions are treated similarly to molecules. (Remember to consider all of the electron pairs present in any ion or molecule.) Thus, Lewis structures and Figure 5.4 can be used together to predict the shapes of polyatomic ions as well as molecules with double or triple bonds. 

Although molecular orbital theory is in many ways the most powerful of the bonding models, it is also the most complex, so we continue to use the other models when they do an adequate job of explaining or predicting the properties of a molecule. For example, if you need to predict the three-dimensional shape of an AB molecule on an exam, you should draw its Lewis structure and apply the VSEPR model. Don t try to draw its molecular orbital diagram. On the other hand, if you need to determine the bond order of a diatomic molecule or ion, you should draw a molecular orbital diagram. In general chemistry, it is best to use the simplest theory that can answer a particular question. 

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