Learning Objectives Predict the structures of small molecules using valence shell electron pair repulsion (VSEPR) theory. The Lewis electron-pair approach can be used to predict the number and types of bonds between the atoms in a substance, and it indicates which atoms have lone pairs of electrons. This approach gives no information about the actual arrangement of atoms in space, however. We continue our discussion of structure and bonding by introducing the valence-shell electron-pair repulsion (VSEPR) model (pronounced “vesper”), which can be used to predict the shapes of many molecules and polyatomic ions. Keep in mind, however, that the VSEPR model, like any model, is a limited representation of reality; the model provides no information about bond lengths or the presence of multiple bonds. The VSEPR Model The VSEPR model can predict the structure of nearly any molecule or polyatomic ion in which the central atom is a nonmetal, as well as the structures of many molecules and polyatomic ions with a central metal atom. The premise of the VSEPR theory is that electron pairs located in bonds and lone pairs repel each other and will therefore adopt the geometry that places electron pairs as far apart from each other as possible. This theory is very simplistic and does not account for the subtleties of orbital interactions that influence molecular shapes; however, the simple VSEPR counting procedure accurately predicts the three-dimensional structures of a large number of compounds, which cannot be predicted using the Lewis electron-pair approach. We can use the VSEPR model to predict the geometry of most polyatomic molecules and ions by focusing only on the number of electron pairs around the central atom , ignoring all other valence electrons present. According to this model, valence electrons in the Lewis structure form electron groups (regions of electron density), which may consist of a single bond, a double bond, a triple bond, a lone pair of electrons, or even a single unpaired electron, which in the VSEPR model is counted as a lone pair. Because electrons repel each other electrostatically, the most stable arrangement of electron groups (i.e., the one with the lowest energy) is the one that minimizes repulsions. Groups are positioned around the central atom in a way that produces the molecular structure with the lowest energy, as illustrated in Figure \(\PageIndex{1}\). It is important to note that electron group geometry around a central atom is not the same thing as its molecular structure. Electron group geometries describe all regions where electrons are located, bonds as well as lone pairs. Molecular structure describes the location of the atoms alone, not including the lone pair electrons. We differentiate between these two situations by naming the geometry that includes all electron pairs the electron group geometry . The structure that includes only the placement of the atoms in the molecule is called the molecular structure (or molecular shape) . The electron group 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. Predicting Electron Group Geometry and Molecular Structure The following procedure uses VSEPR theory to determine electron group geometry and molecular structures (molecular shape): Draw the Lewis structure of the molecule or polyatomic ion. Count the number of electron groups or regions of electron density (lone pairs and bonds) around the central atom. A single, double, or triple bond counts as one electron group. Determine the electron group geometry by placing the groups as far apart as possible. Determine the molecular structure (looking at the bonded groups only). Table \(\PageIndex{1}\) summarizes the shapes of molecules based on the number of electron groups and surrounding atoms. Number of Electron Groups on Central Atom Number of Bonding Groups Number of Lone Pairs Electron Group Geometry Molecular Structure 2 2 0 linear linear 3 3 0 trigonal planar trigonal planar 3 2 1 trigonal planar bent 120° 4 4 0 tetrahedral tetrahedral 4 3 1 tetrahedral trigonal pyramidal 4 2 2 tetrahedral bent 109° Two Electron Groups Any molecule with only two atoms is linear . A molecule whose central atom contains only two electron groups orients those two groups as far apart from each other as possible, which is 180° apart. When the two electron groups are 180° apart, the atoms attached to those electron groups are also 180° apart, so the overall molecular structure is linear. Examples include BeH 2 and CO 2 : Three Electron Groups A molecule with three electron groups orients the three groups as far apart as possible. They adopt the positions of an equilateral triangle, 120° apart and in a plane. The shape of such molecules is trigonal planar . An example is BF 3 : Some substances have a trigonal planar electron group distribution but have atoms bonded to only two of the three electron groups. An example is GeF 2 : From an electron group geometry perspective, GeF 2 has a trigonal planar shape, but its real shape is dictated by the positions of the atoms. This molecular structure is called bent 120° or angular. Four Electron Groups A molecule with four electron groups about the central atom orients the four groups in the direction of a tetrahedron with bond angles of approximately 109.5°. If there are four atoms attached to these electron groups, then the molecular structure is also tetrahedral . Methane (CH 4 ) is an example. This diagram of CH 4 illustrates the standard convention of displaying a three-dimensional molecule on a two-dimensional surface. The straight lines are in the plane of the page, the solid wedged line is coming out of the plane toward the reader, and the dashed wedged line is going out of the plane away from the reader. NH 3 is an example of a molecule whose central atom has four electron groups, but only three of them are bonded to surrounding atoms. Although the electron groups are oriented in the shape of a tetrahedron , from a molecular geometry perspective, the shape of NH 3 is trigonal pyramidal . H 2 O is an example of a molecule whose central atom has four electron groups, but only two of them are bonded to surrounding atoms. Although the electron groups are oriented in the shape of a tetrahedron, the shape of the molecule is bent 109° or angular. A molecule with four electron groups about the central atom, but only one electron group bonded to another atom, is linear because there are only two atoms in the molecule. Shapes of Molecules with Double or Triple Bonds Double or triple bonds count as a single electron group. The Lewis electron dot diagram of formaldehyde (CH 2 O) is shown in Figure \(\PageIndex{9}\). The central C atom has three electron groups around it because the double bond counts as one electron group. The three electron groups repel each other to adopt a trigonal planar shape . (The lone electron pairs on the O atom are omitted for clarity.) The molecule will not be a perfect equilateral triangle because the C–O double bond is different from the two C–H bonds, but both planar and triangular describe the appropriate approximate shape of this molecule. Example \(\PageIndex{1}\) What is the approximate shape of each molecule? PCl 3 NOF Solution The first step is to draw the Lewis structure of the molecule. For \(\ce{PCl3}\), the electron dot diagram is as follows: The lone electron pairs on the Cl atoms are omitted for clarity. The P atom has four electron groups with three of them bonded to surrounding atoms, so the molecular shape is trigonal pyramidal. The electron dot diagram for \(\ce{NOF}\) is as follows: The N atom has three electron groups on it, two of which are bonded to other atoms. The molecular shape is bent. Exercise \(\PageIndex{1}\) What is the approximate molecular shape of \(\ce{CH2Cl2}\)? Answer Tetrahedral Exercise \(\PageIndex{2}\) Ethylene (\(\ce{C2H4}\)) has two central atoms. Determine the geometry around each central atom and the shape of the overall molecule. (Hint: hydrogen is a terminal atom.) Answer Trigonal planar about both central C atoms. Molecules With Multiple Central Atoms The VSEPR model can be used to predict the structure of somewhat more complex molecules with more than one central atom by using VSEPR as described above for each central atom individually. We will demonstrate with methyl isocyanate (CH 3 –N=C=O), a volatile and highly toxic molecule that is used to produce the pesticide Sevin. Start by looking at the electron groups around the first carbon atom at the left, which is connected to three H atoms and one N atom by single bonds. There are four groups or electrons or four bonds around the carbon. We can therefore predict the CH 3 –N portion of the molecule to be roughly tetrahedral , similar to methane: The nitrogen atom is connected to one carbon by a single bond and to the other carbon by a double bond, producing a total of three bonds, C–N=C. For nitrogen to have an octet of electrons, it must also have a lone pair: Because multiple bonds are not shown in the VSEPR model, the nitrogen is effectively surrounded by three electron groups. Thus according to the VSEPR model, the C–N=C fragment should be bent with an angle ~120°. The carbon in the –N=C=O fragment is doubly bonded to both nitrogen and oxygen, which in the VSEPR model gives carbon a total of two electron pairs. The N=C=O angle should therefore be 180°, or linear. The three fragments combine to give the following structure: Certain patterns are seen in the structures of moderately complex molecules. For example, carbon atoms with four bonds (such as the carbon on the left in methyl isocyanate) are generally tetrahedral. Similarly, the carbon atom on the right has two double bonds that are similar to those in CO 2 , so its geometry, like that of CO 2 , is linear. Recognizing similarities to simpler molecules will help you predict the molecular geometries of more complex molecules. Example \(\PageIndex{3}\) Use the VSEPR model to predict the molecular geometry of propyne (H 3 C–C≡CH), a gas with some anesthetic properties. Given: chemical compound Asked for: molecular geometry Strategy: Count the number of electron groups around each carbon, recognizing that in the VSEPR model, a multiple bond counts as a single group. Use Figure \(\PageIndex{3}\) to determine the molecular geometry around each carbon atom and then deduce the structure of the molecule as a whole. Solution: Because the carbon atom on the left is bonded to four other atoms, we know that it is approximately tetrahedral. The next two carbon atoms share a triple bond, and each has an additional single bond. Because a multiple bond is counted as a single bond in the VSEPR model, each carbon atom behaves as if it had two electron groups. This means that both of these carbons are linear, with C–C≡C and C≡C–H angles of 180°. Exercise \(\PageIndex{3}\) Predict the geometry of allene (H 2 C=C=CH 2 ), a compound with narcotic properties that is used to make more complex organic molecules. Answer The terminal carbon atoms are trigonal planar, the central carbon is linear, and the C–C–C angle is 180°.