48 Predicting Molecular Shapes: VSEPR Model (M9Q1)

Introduction

This section explores how we predict the molecular and electron-pair shapes of molecules using the VSEPR (Valence Shell Electron Pair Repulsion) theory. We will first go over what VSEPR theory is and how it defines an electron-pair geometry and a molecular geometry. Then we will go over the steps for determining the electron-pair and molecular geometries for any molecule. An added feature of this section will be the addition of three dimensional interactive models for a number of molecules. All colored three dimensional models will be hyperlinked to an interactive model for the molecule. These interactive models are part of the WebMO lab that you will have in the future. If you would like to look at these models at your leisure you can do so on the UW Madison Chemistry Website. The section below provides a more detailed description of these topics, worked examples, practice problems and a glossary of important terms.

Learning Objectives for Predicting Molecular Shapes: VSEPR Model

 

| Key Concepts and Summary | Glossary | End of Section Exercises |

Lewis Structures with Wedge-Dash Notation

The Lewis structure gives us meaningful information about the bonds between atoms, but Lewis structures do not depict how the molecule exists in three-dimensions. Depicted on the left in Figure 1 is the Lewis structure for methane (CH4) showing the central carbon atom singly bonded to four hydrogen atoms:

Two structures for methane are shown. In the structure at the left, a carbon atom has four single bonds, one to each of four hydrogen atoms. These hydrogen atoms are arranged on top, bottom, left and right of the carbon atom. In the image at the right, the carbon atom is in the center and a hydrogen atom is arranged on top. Then the other three hydrogen atoms are arranged along the bottom. One is in line with the paper and is just a single bond. One hydrogen atom is coming out of the paper and is shown as a wedge. One hydrogen atom is going into the page and is shown as a dashed line. These hydrogen atoms are at the four points of a tetrahedron.
Figure 1. Methane (CH4) depicted by a Lewis structure (left) and dash and wedge notation (right).

The Lewis structure is misleading because it depicts methane as a planar ‘plus-shaped’ molecule, yet we know that methane is a non-planar, tetrahedral molecule. Representing three dimensions on 2D paper (or computer screen) is challenging. Chemists use dash and wedge notation to draw 3D molecules. Bonds drawn as simple lines are located in the plane of the page or screen. Bonds drawn with a solid triangle, or wedge, represent bonded atoms coming out of the page/screen towards the observer; bonds drawn with dashed triangle, or dash, represent bonded atoms going into the page/screen away from the observer. The correct Lewis structure with dash and wedge notation for CH4 is depicted on the right in Figure 1. It communicates the structure of methane more clearly than a traditional Lewis structure without dash and wedge notation

Notice that on the structure with dash and wedge notation, one carbon-hydrogen bond points straight up from the carbon and is in the plane of the paper (indicated by the simple line). The second carbon-hydrogen bond in the plane of the paper is specifically drawn at ~109° from that first bond. We’ll talk below about why that angle is important to communicate the correct shape of methane. These three atoms define an H-C-H plane that is located in the same plane as the paper/screen. Heading counter-clockwise on the H-C-H plane, the next bond is the solid wedge that represents the carbon-hydrogen bond coming out of the H-C-H plane towards the observer. Notice that it is not pointing straight down, but it is slightly to the right of the center. The fourth carbon-hydrogen bond is going back behind the H-C-H plane away from the observer. Notice that it is not pointing straight to the left, but it is slightly below the horizontal position.

The dash and wedge notation is shown again below (left) adjacent to a space-filling model of CH4 (center) and the ball and stick model (right). All three images represent the same bond placements in space (Figure 2).

Three representations of methane are shown. In the one at the left, there is a structure labeled “Dash and Wedge”. In this structure, the carbon atom is in the middle and the 4 hydrogen atoms are arranged in a tetrahedral geometry around the carbon. There are single bonds between the carbon and each hydrogen. The middle structure is labeled “Space filling” and it shows a big black ball, understood to be carbon, with 4 white balls stuck on the outside, understood to be hydrogen. The third structure is labeled “ball and stick” and shows a screenshot of the geometry as seen on Web M O. There is a black sphere in the middle, then the bonds are represented as sticks and there are white balls at the end of each of 4 sticks, for the 4 hydrogen atoms.
Figure 2. Three representations of methane (CH4). Click on the image to be taken to an interactive model for methane.

In both the space-filling and ball and stick models, the white spheres represent hydrogen atoms, and the black sphere represents the carbon atom. Training your brain to convert between 2D drawings and 3D bond placement in space is challenging. We recommend manipulating atoms and bonds using both 3D model kits (which can be checked out from College Library) and also online tools. All ball and stick models in this textbook are linked to the WebMO program where you can freely rotate this molecule to better understand the bond positions in 3D space. Once in the program, click on the top, curved arrow () on the left-hand toolbar and drag atoms to rotate the molecule.

VSEPR Theory

To understand molecular behavior, it is important to be able to describe bonds in terms of their distances, angles, and relative arrangements in space (Figure 3). A bond angle is the angle between any two bonds that include a common atom, usually measured in degrees. A bond distance (or bond length) is the distance between the nuclei of two bonded atoms along the straight line joining the nuclei. Bond distances are typically measured in Ångstroms (1 Å = 10–10 m) or picometers (1 pm = 10–12 m, 100 pm = 1 Å).

Two structures of formaldehyde (C H subscript 2 O) are shown. In the one at the left, the carbon atom is in the middle and the 2 H atoms and the 1 O atom are arranged around the carbon atom in a trigonal planar geometry. There are single bonds between the carbon and the hydrogen atoms and then a double bond between the carbon and oxygen atom. The oxygen atom also has two lone pairs. The bond angles are labeled. The O-C-H bond angles are 122.4 degrees and the H-C-H bond angle is 115.4 degrees. In the structure on the right, we have a ball and stick model that shows the black carbon sphere in the middle attached to two white hydrogen spheres and double bonded to a red oxygen sphere. A chart to the right indicates that the C-H bond length is 1.110 Angstroms and the C double bond O bond length is 1.207 Angstroms.
Figure 3. Formaldehyde depicted by a Lewis structure (left), ball and stick model (center), and a comparison of bond lengths (right).

Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular geometry, 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. 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. 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 electrostatic repulsion of these electrons is reduced when the various regions of high electron density assume positions as far from each other as possible.

VSEPR theory predicts the arrangement of electron pairs around each central atom and, usually, the correct arrangement of atoms in a molecule using only electron pair repulsions. This neglects other important factors which contribute to electronic and molecular geometries such as orbital mixing, pi conjugation, hyperconjugation, dipole-dipole interactions, hydrogen-bonding, etc. Many of these other factors will be explored in this course and future chemistry courses.

As a simple example of VSEPR theory, let us predict the structure of a gaseous BeF2 molecule. The Lewis structure of BeF2 (Figure 4) shows only two electron pairs around the central beryllium atom. 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 F-Be-F bond angle is 180° (Figure 4) in the BeF2 molecule resulting in a linear geometry.

A Lewis structure is shown. A fluorine atom with three lone pairs of electrons is single bonded to a beryllium atom which is single bonded to a fluorine atom with three lone pairs of electrons. The angle of the bonds between the two fluorine atoms and the beryllium atom is labeled, “180 degrees.”
Figure 4. The BeF2 molecule adopts a linear structure in which the two bonds are as far apart as possible, on opposite sides of the Be atom.

Figure 5 illustrates this and other electron-pair geometries that minimize the repulsions among regions of high electron density (bonds and/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.

A table with four rows and six columns is shown. The header column contains the phrases, “Number of regions,” “Spatial arrangement,” “Wedge/dash Notation,” and “Electron pair Geometry.” The first row reads: “Two regions of high electron density ( bonds and/or unshared pairs )”, “Three regions of high electron density ( bonds and/or unshared pairs ),” “Four regions of high electron density ( bonds and/or unshared pairs ),” “Five regions of high electron density ( bonds and/or unshared pairs ),” and “Six regions of high electron density ( bonds and/or unshared pairs ).” The second row shows diagrams of orbitals. The first image shows two oval-shaped orbs with an arrow indicating an angle of 180 degrees. The second image shows three oval-shaped orbs with an arrow indicating an angle of 120 degrees. The third image shows four oval-shaped orbs with an arrow indicating an angle of 109.5 degrees. The fourth image shows five oval-shaped orbs with an arrow indicating an angle of 90 and 120 degrees. The fifth image shows six oval-shaped orbs with an arrow indicating an angle of 90 degrees. The third row contains Lewis structures. The first structure shows a beryllium atom single bonded to two hydrogen atoms. The second structure shows a boron atom single bonded to three hydrogen atoms. The third structure shows a carbon atom single bonded to four hydrogen atoms. The fourth structure shows a phosphorus atom single bonded to five fluorine atoms. The fifth structure shows a sulfur atom single bonded to six fluorine atoms. The fourth row contains the phrases “Linear; 180 degree angle,” Trigonal Planar; all angles 120 degrees,” “Tetrahedral; all angles 109.5 degrees,” “Trigonal bipyramidal; angles of 90 degrees and 120 degrees. An attached atom may be equatorial, ( in the plane of the triangle ), or axial, ( above the plane of the triangle ),” and “Octahedral; 90 degrees or 180 degrees.”
Figure 5. The basic electron-pair geometries predicted by VSEPR theory maximize the space around any region of electron density (bonds or lone pairs).

Electron-pair Geometry vs. Molecular Geometry

It is important to note that electron-pair geometry around a central atom is not the same thing as its molecular geometry. The electron-pair geometries shown in Figure 5 describe all regions where electrons are located, bonds as well as lone pairs. Molecular geometry describes the location of the atoms, not the lone pairs. Even though molecular geometry does not describe the location of the lone pairs, the lone pairs still influence the location of the atoms and this influence is still taken into account.

We differentiate between these two situations by naming the geometry that includes all electron pairs the electron-pair geometry. The structure that includes only the placement of the atoms in the molecule is called the molecular geometry. The electron-pair geometries will be the same as the molecular geometry 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.

For example, the methane molecule, CH4, which is the major component of natural gas, has four bonding pairs of electrons (i.e., four regions of electron density) around the central carbon atom; the electron-pair geometry is tetrahedral, as is the molecular geometry (Figure 6).

Four representations of methane are shown. In the one at the left, the molecular formula is written as C H subscript 4. Then there is a right facing arrow to the Lewis structure. In the Lewis structure, the carbon atom has four single bonds, one to each of four hydrogen atoms. These hydrogen atoms are arranged on top, bottom, left and right of the carbon atom. Then there is a right facing arrow to a structure labeled “Electron-pair geometry: tetrahedral”. In this structure, the carbon is in the center and there is a wedge and dash structure showing the 4 hydrogen atoms at the corners of the tetrahedron. Then there is a right facing arrow to a structure labeled “Molecular geometry: tetrahedral”. This structure is a ball and stick model that shows a screenshot of the geometry as seen on Web M O. There is a black sphere in the middle, then the bonds are represented as sticks and there are white balls at the end of each of 4 sticks, for the 4 hydrogen atoms.
Figure 6. Representations of methane (CH4). There are four regions of electron density around the central carbon atom. Since all four regions are bonding regions, the electron-pair geometry is the same as the molecular geometry.

The ammonia molecule, NH3, also has four electron pairs associated with the nitrogen atom (i.e., four regions of electron density), and thus has a tetrahedral electron-pair geometry. One of these regions, however, is a lone pair, which is not included in the molecular geometry, though it does influence the shape of the molecule (Figure 7).

Four representations of ammonia are shown. In the one at the left, the molecular formula is written as N H subscript 3. Then there is a right facing arrow to the Lewis structure. In the Lewis structure, the nitrogen atom has a single bond to each of three hydrogen atoms. There is also a lone pair on the nitrogen. These hydrogen atoms are arranged to the bottom, left and right of the nitrogen atom and the lone pair is on top. Then there is a right facing arrow to a structure labeled “Electron-pair geometry: tetrahedral”. In this structure, the nitrogen is in the center and there is a wedge and dash structure showing the 3 hydrogen atoms at the corners of the tetrahedron. The fourth corner of the tetrahedron is occupied by the lone pair. Then there is a right facing arrow to a structure labeled “Molecular geometry: trigonal pyramidal”. This structure is a ball and stick model that shows a screenshot of the geometry as seen on Web M O. There is a blue sphere in the middle for nitrogen, then the bonds are represented as sticks and there are white balls at the end of each of 3 sticks, for the 3 hydrogen atoms. The lone pair is not represented in this structure.
Figure 7. Representations of ammonia (NH3). There are four regions of electron density around the central nitrogen atom—three bonding regions (each a single bond) and one non-bonding region (lone pair).

In the ammonia molecule, the three hydrogen atoms attached to the central nitrogen are not arranged in a flat, trigonal planar molecular geometry, but rather in a three-dimensional trigonal pyramid (Figure 7) with the nitrogen atom at the apex and the three hydrogen atoms forming the base. The ideal bond angles in a trigonal pyramid are based on the tetrahedral electron pair geometry. Small bond angle distortions occur due to the lone electron pair on nitrogen. The H–N–H bond angles in NH3 are slightly smaller than the 109.5° angle in a regular tetrahedron.

Figure 8 illustrates the ideal molecular geometries, which are predicted based on the electron-pair geometries for various combinations of lone pairs and bonding pairs. Because wedge and dash structures are often confusing at first, we also provide a typical representation of a wedge and dash structure for each combination. While there are other ways that you could rotate the molecule in space and different placements of the wedges and dashes that would also be considered correct, these depictions have been chosen to be the most straightforward to draw and interpret. 

Figure 8.  VSEPR-predicted molecular and electron-pair geometries. The molecular geometries are identical to the electron-pair geometries when there are no lone pairs present. A, central atom; X, peripheral atom; E, unshared electron pair.

Example 1

Predicting Electron-pair Geometry and Molecular Geometry: Lone Pairs on the Central Atom
Predict the electron-pair geometry and molecular geometry of a water molecule.

Solution
The Lewis structure of H2O indicates that there are four regions of high electron density around the oxygen atom: two lone pairs and two chemical bonds:

Two representations of water are shown. In the Lewis structure on the left, the oxygen atom contains two lone pairs and then has a single bond to each of two hydrogen atoms. The label beneath reads “electron-pair geometry: tetrahedral”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a red sphere in the middle for oxygen and then two white spheres in a bent geometry for the hydrogen atoms. The lone pairs are not represented in this structure. The label beneath it reads “molecular geometry: bent”.
Figure 9. H2O has four regions of electron density around the central atom, so it has a tetrahedral electron-pair geometry. Two of the electron regions are lone pairs, so the molecular geometry is bent.

We predict that these four regions are arranged in a tetrahedral (Figure 8) fashion to minimize the electron repulsion, as shown in Figure 9. Thus, the electron-pair geometry is tetrahedral and the molecular geometry is bent with an angle slightly less than 109.5°. In fact, the bond angle is predicted to be 103.7°.

Check Your Learning
The hydronium ion, H3O+, forms when acids are dissolved in water. Predict the electron-pair geometry and molecular geometry of this cation.

Answer:

electron pair geometry: tetrahedral; molecular geometry: trigonal pyramidal

According to VSEPR theory, the terminal atom locations (Xs in Figure 8) are equivalent within the linear, trigonal planar, and tetrahedral electron-pair geometries. It does not matter which X is replaced with a lone pair because the molecules can be rotated to convert positions.

For trigonal bipyramidal electron-pair geometries, however, there are two distinct X positions, as shown in Figure 8: an axial position (if we hold a model of a trigonal bipyramid by the two axial positions, we have an axis around which we can rotate the model) and an equatorial position (three positions form an equator around the middle of the molecule). As shown in Figure 8, the axial position is surrounded by bond angles of 90°, whereas the equatorial position has more space available because of the 120° bond angles. In a trigonal bipyramidal electron-pair geometry, lone pairs or larger atoms or groups tend to occupy equatorial positions rather than axial positions because the equatorial positions are more spacious and minimize electron repulsion.

Theoretically, we can come up with three possible arrangements for the three bonds and two lone pairs for the ClF3 molecule (Figure 10). The stable structure is the one that puts the lone pairs in equatorial locations, giving a T-shaped molecular geometry.

Four sets of images are shown and labeled, “a,” “b,” “c,” and “d.” Each image is separated by a dashed vertical line. Image a shows a six-faced, bi-pyramidal structure where the central vertical axis is labeled, “Axial,” and the horizontal plane is labeled, “Equatorial.” Image b shows a pair of diagrams in the same shape as image a, but in these diagrams, the left has a chlorine atom in the center while the right has a chlorine atom in the center, two fluorine atoms on the upper and lower ends, and one fluorine in the left horizontal position. Image c shows a pair of diagrams in the same shape as image a, but in these diagrams, the left has a chlorine atom in the center while the right has a chlorine atom in the center and three fluorine atoms in each horizontal position. Image d shows a pair of diagrams in the same shape as image a, but in these diagrams, the left has a chlorine atom in the center while the right has a chlorine atom in the center, two fluorine atoms in the horizontal positions, and one in the axial bottom position.
Figure 10. (a) In a trigonal bipyramid, the two axial positions are located directly across from one another, whereas the three equatorial positions are located in a triangular arrangement. (b–d) The two lone pairs (red lines) in ClF3 have several possible arrangements, but the T-shaped molecular geometry (b) is the one actually observed, consistent with the larger lone pairs both occupying equatorial positions.
Two representations of trifluorochloride are shown. In the Lewis structure on the left, the chlorine atom is in the middle and has 2 lone pairs. There are three fluorine atoms arranged around the chlorine atom, one on top, one on bottom, and one to the right. There are single bonds between the Cl and F atoms. The lone pairs are shown on each fluorine atom to complete the octet. The label beneath the structure reads “electron-pair geometry: trigonal bipyramidal”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a dark green sphere in the middle for the chlorine atom. Then there are light green spheres to the top, right, and bottom of the chlorine atom for each of the fluorine atoms. The bond angles are shown. The bond angle between the F on top to the Cl to the F on bottom is 180.7 degrees. The bond angle between the F on top to the Cl to the F to the right is 87.1 degrees. The bond angle between the F on bottom to the Cl to the F on the right is 87.1 degrees.
Figure 11. Representations of chlorine trifluoride (ClF3).

When a central atom has two lone electron pairs and four bonding regions, we have an octahedral electron-pair geometry. The two lone pairs are on opposite sides of the octahedron (180° apart), giving a square planar molecular geometry that minimizes lone pair-lone pair repulsions (Figure 8).

Predicting Electron-pair Geometry and Molecular Geometry

The following procedure uses VSEPR theory to determine the electron pair geometries and the molecular geometries:

  1. Write the Lewis structure of the molecule or polyatomic ion.
  2. Count the number of regions of electron density (lone pairs and bonds) around the central atom. A single, double, or triple bond counts as one region of electron density.
  3. Identify the electron-pair geometry based on the number of regions of electron density: linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral (Figure 8).
  4. Use the number of lone pairs to determine the molecular geometry (Figure 8). If more than one arrangement of lone pairs and chemical bonds is possible, choose the one that will minimize repulsions, remembering that lone pairs occupy more space than multiple bonds, which occupy more space than single bonds. In trigonal bipyramidal arrangements, repulsion is minimized when every lone pair is in an equatorial position. In an octahedral arrangement with two lone pairs, repulsion is minimized when the lone pairs are on opposite sides of the central atom.

The following examples illustrate the use of VSEPR theory to predict the molecular geometry of molecules or ions that have no lone pairs of electrons. In this case, the molecular geometry is identical to the electron pair geometry.

Example 2

Predicting Electron-pair Geometry and Molecular Geometry: CO2 and BH3
Predict the electron-pair geometry and molecular geometry for each of the following:

(a) carbon dioxide, CO2

(b) boron trichloride, BH3

Solution
(a) We write the Lewis structure of CO2 as:

Three separate resonance structures for CO2 are displayed left to right. The left structure has the central carbon double bonding to each of the two periphery oxygen atoms. The formal charges for each of these atoms is zero. The central resonance structure has the central carbon triple bonding to the oxygen on the left, and single bonding to the oxygen on the right. The formal charges for these atoms are +1, 0, and -1 going left to right. The rightmost resonance structure has the central carbon triple bonding to the oxygen on the right and single bonding to the oxygen on the left.

This shows us two regions of high electron density around the carbon atom—each double bond counts as one region, and there are no lone pairs on the carbon atom. Using VSEPR theory, we predict that the two regions of electron density arrange themselves on opposite sides of the central atom with a bond angle of 180°. The electron-pair geometry and molecular geometry are identical, and CO2 molecules are linear.

(b) We write the Lewis structure of BH3 as:

Two representations of B H subscript 3 are shown. In the Lewis structure on the left, the boron atom is in the middle. There are three hydrogen atoms arranged around the boron atom in a trigonal planar geometry. There are single bonds between the B and H atoms. The label beneath the structure reads “electron-pair geometry: trigonal planar”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a light pink sphere in the middle for the boron atom. Then there are white spheres arranged in a trigonal planar geometry for the hydrogen atoms. The label beneath the structure reads “molecular geometry: trigonal planar”.

Thus we see that BH3 contains three bonds, and there are no lone pairs of electrons on boron. The arrangement of three regions of high electron density gives a trigonal planar electron-pair geometry. The B–H bonds lie in a plane with 120° angles between them. BH3 also has a trigonal planar molecular geometry.

Check Your Learning
Carbonate, CO32−, is a common polyatomic ion found in various materials from eggshells to antacids. What are the electron-pair geometry and molecular geometry of this polyatomic ion?

Answer:

The electron-pair geometry is trigonal planar and the molecular geometry is trigonal planar. Due to resonance, all three C–O bonds are identical and the bond angles are all 120o. Whether they are single, double, or an average of the two, each bond counts as one region of electron density.

Two representations of carbonate are shown. In the Lewis structure on the left, the carbon atom is in the middle surrounded by 3 oxygen atoms. There are single bonds to each of two oxygen atoms and then a double bond between the carbon and a third oxygen atom. The lone pairs are shown on the oxygen atoms to complete the octets. There are brackets around the entire structure and a superscript negative 2. The label reads “electron-pair geometry: trigonal planar”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a black sphere in the middle for the carbon atom. Then there are three red spheres arranged in a trigonal planar geometry around the carbon. The label beneath the structure reads “molecular geometry: trigonal planar”.
Figure 12. Representations of CO32-.

Example 3

Predicting Electron-pair Geometry and Molecular Geometry: Ammonium
Two of the top 50 chemicals produced in the United States, ammonium nitrate and ammonium sulfate, both used as fertilizers, contain the ammonium ion. Predict the electron-pair geometry and molecular geometry of the NH4+ cation.

Solution
We write the Lewis structure of NH4+ as:

A Lewis structure shows a nitrogen atom single bonded to four hydrogen atoms. The structure is surrounded by brackets with a superscripted positive sign.

We can see that NH4+ contains four bonds from the nitrogen atom to hydrogen atoms and no lone pairs. We expect the four regions of high electron density to arrange themselves so that they point to the corners of a tetrahedron with the central nitrogen atom in the middle (Figure 8). Therefore, the electron pair geometry of NH4+ is tetrahedral, and the molecular geometry is also tetrahedral.

Two representations of ammonium are shown. In the Lewis structure on the left, the nitrogen atom is in the middle surrounded by 4 hydrogen atoms. There are single bonds between the nitrogen and each hydrogen atom. The hydrogen atoms are arranged in a tetrahedral geometry. There are brackets around the entire structure and a superscript plus sign. The label reads “electron-pair geometry: tetrahedral”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a blue sphere in the middle for the nitrogen atom. Then there are four white spheres arranged in a tetrahedral around the nitrogen. The label beneath the structure reads “molecular geometry: tetrahedral”.
Figure 13. Representations of NH4+. Click on the image above to be taken to WebMO.

Check Your Learning
Identify a molecule with trigonal bipyramidal molecular geometry.

Answer:

Any molecule with five electron pairs around the central atoms including no lone pairs will be trigonal bipyramidal. PF5 is a common example.

The next several examples illustrate the effect of lone pairs of electrons on molecular geometry.

Example 4

Predicting Electron-pair Geometry and Molecular Geometry: SF4
Sulfur tetrafluoride, SF4, is extremely valuable for the preparation of fluorine-containing compounds used as herbicides (i.e., SF4 is used as a fluorinating agent). Predict the electron-pair geometry and molecular geometry of a SF4 molecule.

Solution
The Lewis structure of SF4 indicates five regions of electron density around the sulfur atom: one lone pair and four bonding pairs:

A Lewis diagram depicts a sulfur atom with one lone pair of electrons single bonded to four fluorine atoms, each with three lone pairs of electrons.

We expect these five regions to adopt a trigonal bipyramidal electron-pair geometry. In the typical fashion, the lone pair occupies one of the equatorial positions, which leads to a trigonal bipyramidal electron-pair geometry and the molecular geometry is that of a seesaw (Figure 8).

Four representations of sulfur tetrafluoride are shown. In the one at the left, the molecular formula is written as S F subscript 4. Then there is a right facing arrow to the Lewis structure. In the Lewis structure, the sulfur atom is in the middle with one lone pair to the left. The fluorine atoms are arranged on top, bottom and to the right. There are two fluorine atoms on the right of the sulfur, one has a wedge and one has a dash. These are all single bonds between sulfur and fluorine. There is also an image of the sulfur atom at the center of an octahedron, showing that there is a fluorine on top and bottom in the axial positions and then two in the equatorial positions around the center. The label beneath says “electron-pair geometry: trigonal bipyramidal”. Then there is a right facing arrow to a structure that is a ball and stick model that shows a screenshot of the geometry as seen on Web M O. There is a yellow sphere in the middle for sulfur, then the bonds are represented as sticks and there are light green balls at the end of each of 4 sticks, for the 4 fluorine atoms. The bond angle between the F on top the S and the F on bottom is labeled as 188.45 degrees. The label beneath reads “molecular geometry: seesaw”.
Figure 14. Representations of SF4.

Check Your Learning
Predict the electron pair geometry and molecular geometry for molecules of XeF2.

Answer:

The electron-pair geometry is trigonal bipyramidal. The molecular geometry is linear.

Example 5

Predicting Electron-pair Geometry and Molecular Geometry: XeF4
Of all the noble gases, xenon is the most reactive, frequently reacting with elements such as oxygen and fluorine. Predict the electron-pair geometry and molecular geometry of the XeF4 molecule.

Solution
The Lewis structure of XeF4 indicates six regions of high electron density around the xenon atom: two lone pairs and four bonds:

A Lewis structure depicts a xenon atom with two lone pairs of electrons that is single bonded to four fluorine atoms, each with three lone pairs of electrons.

These six regions adopt an octahedral arrangement (Figure 8), which is the electron-pair geometry. To minimize repulsions, the lone pairs should be on opposite sides of the central atom (Figure 15 and Figure 16). The five atoms are all in the same plane and have a square planar molecular geometry.

Two diagrams are shown and labeled, “a” and “b.” Diagram a shows a xenon atom in the center of an eight-sided octahedral shape. Diagram b shows the same image as diagram a, but this time there are fluorine atoms located at the four corners of the shape in the horizontal plane. They are connected to the xenon by single lines.
Figure 15. (a) XeF4 adopts an octahedral arrangement with two lone pairs (red lines) and four bonds in the electron-pair geometry. (b) The molecular geometry is square planar with the lone pairs directly across from one another.

 

Two representations of Xenon tetrafluoride are shown. In the Lewis structure on the left, the xenon atom is in the middle with two lone pairs. There is a single bond to each of 4 fluorine atoms arranged on top, bottom, left and right. The octets of the fluorine atoms have been filled with lone pairs. The label beneath reads “electron-pair geometry: octahedral”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a teal sphere in the middle for the xenon atom. Then there are four light green spheres arranged in a square planar geometry. The label beneath the structure reads “molecular geometry: square planar”.
Figure 16. Representations of XeF4.

Check Your Learning
In a certain molecule, the central atom has three lone pairs and two bonds. What will the electron pair geometry and molecular geometry be?

Answer:

electron pair geometry: trigonal bipyramidal; molecular geometry: linear

Molecular Geometry for Multi-center Molecules

When a molecule or polyatomic ion has only one central atom, the molecular geometry completely describes the shape of the molecule. Larger molecules do not have a single central atom, but are connected by a chain of interior atoms that each possess a “local” geometry. The way these local structures are oriented with respect to each other also influences the molecular shape, but such considerations are largely beyond the scope of this introductory discussion. For our purposes, we will only focus on determining the local structures.

Example 6

Predicting Structure in Multicenter Molecules
The Lewis structure for the simplest amino acid, glycine, H2NCH2CO2H, is shown here. Predict the local geometry for the nitrogen atom, the two carbon atoms, and the oxygen atom with a hydrogen atom attached:

A Lewis structure depicts a nitrogen atom with one lone pair of electrons that is single bonded to two hydrogen atoms and a carbon atom, which is, in turn, single bonded to two hydrogen atoms and another carbon atom. This carbon atom is double bonded to an oxygen atom with two lone pairs of electrons and single bonded to an oxygen that has two lone pairs of electrons and a single bond to a hydrogen atom.

Solution

A Lewis structure depicts a nitrogen atom with one lone pair of electrons that is single bonded to two hydrogen atoms and a carbon atom. The atoms described are drawn with bonds that indicate a three-dimensional, tetrahedral shape around the nitrogen atom. The carbon is, in turn, single bonded to two hydrogen atoms and another carbon atom, and again, a tetrahedral, three dimensional configuration is indicated by the types of bonds. This second carbon atom is double bonded to an oxygen atom and single bonded to an oxygen that has two lone pairs of electrons and a single bond to a hydrogen atom.

Consider each central atom independently. The electron-pair geometries:

  • nitrogen––four regions of electron density, tetrahedral
  • carbon (CH2)––four regions of electron density, tetrahedral
  • carbon (CO2)—three regions of electron density, trigonal planar
  • oxygen (OH)—four regions of electron density, tetrahedral

The local structures:

  • nitrogen––three bonds, one lone pair, trigonal pyramidal
  • carbon (CH2)—four bonds, no lone pairs, tetrahedral
  • carbon (CO2)—three bonds (double bond counts as one region of electron density), no lone pairs, trigonal planar
  • oxygen (OH)—two bonds, two lone pairs, bent (109°)

Check Your Learning
Another amino acid is alanine, which has the Lewis structure shown here. Predict the electron-pair geometry and local structure of the nitrogen atom, the three carbon atoms, and the oxygen atom with hydrogen attached:

A Lewis structure depicts a nitrogen atom with two lone pairs of electrons that is single bonded to two hydrogen atoms and a carbon atom, which is, in turn, single bonded to a hydrogen atom, a methyl group and another carbon atom. This carbon atom is single bonded to an oxygen atom with two lone pairs of electrons and single bonded to an oxygen that has two lone pairs of electrons and a single bond to a hydrogen atom.

Answer:

electron-pair geometries:

nitrogen—tetrahedral

carbon (CH)—tetrahedral

carbon (CH3)—tetrahedral

carbon (CO2)—trigonal planar

oxygen (OH)—tetrahedral

local structures:

nitrogen—trigonal pyramidal

carbon (CH)—tetrahedral

carbon (CH3)—tetrahedral

carbon (CO2)—trigonal planar

oxygen (OH)—bent

 

The molecular shape simulator lets you build various molecules and practice naming their electron-pair geometries and molecular geometries.

Example 7

Molecular Simulation
Using molecular shape simulator allows us to control whether bond angles and/or lone pairs are displayed by checking or unchecking the boxes under “Options” on the right. We can also use the “Name” checkboxes at bottom-left to display or hide the electron pair geometry (called “electron geometry” in the simulator) and/or molecular geometry (called “molecular shape” in the simulator).

Two representations of H C N are shown. In the Lewis structure on the left, the carbon atom is in the middle. There is a single bond to a hydrogen atom at the left and a triple bond to a nitrogen atom on the right. The nitrogen atom has 1 lone pair. The label beneath the structure reads “electron-pair geometry: linear”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a black sphere in the middle for the carbon atom. Then there is a white sphere to the left for the hydrogen atom and a blue sphere to the right for the nitrogen atom. The label beneath the structure reads “molecular geometry: linear”.

Figure 17. Representations of HCN. Click on the image above to be taken to the WebMO representation of HCN.

Build the molecule HCN in the simulator based on the following Lewis structure:

Click on each bond type or lone pair at right to add that group to the central atom. Once you have the complete molecule, rotate it to examine the predicted molecular geometry. What molecular geometry is this?

Solution
The molecular geometry is linear.

Check Your Learning
Build a more complex molecule in the simulator. Identify the electron-group geometry, molecular geometry, and bond angles. Then try to find a chemical formula that would match the structure you have drawn.

Answer:

Answers will vary. For example, an atom with four single bonds, a double bond, and a lone pair has an octahedral electron-group geometry and a square pyramidal molecular structure. XeOF4 is a molecule that adopts this structure.

Key Concepts and Summary

VSEPR theory predicts the three-dimensional arrangement of atoms in a molecule. It states that valence electrons will assume an electron-pair geometry that minimizes repulsions between areas of high electron density (bonds and/or lone pairs). Molecular geometry, which refers only to the placement of atoms in a molecule and not the electrons, is equivalent to electron-pair geometry only when there are no lone electron pairs around the central atom. When lone electron pairs exist, they affect the molecular geometry, even if their location is not directly specified in the molecular geometry name.

Being able to convert between 2D and 3D representations of molecules in your head is important and takes practice. Remember to use the resources mentioned in this section to practice manipulating these structures.

Glossary

axial position
location in a trigonal bipyramidal geometry in which there is another atom at a 180° angle and the equatorial positions are at a 90° angle
bond angle
angle between any two covalent bonds that share a common atom
bond distance
(also, bond length) distance between the nuclei of two bonded atoms
electron-pair geometry
arrangement around a central atom of all regions of electron density (bonds, lone pairs, or unpaired electrons)
equatorial position
one of the three positions in a trigonal bipyramidal geometry with 120° angles between them; the axial positions are located at a 90° angle
linear
shape in which two outside groups are placed on opposite sides of a central atom
molecular geometry
structure that includes only the placement of the atoms in the molecule
octahedral
shape in which six outside groups are placed around a central atom such that a three-dimensional shape is generated with four groups forming a square and the other two forming the apex of two pyramids, one above and one below the square plane
tetrahedral
shape in which four outside groups are placed around a central atom such that a three-dimensional shape is generated with four corners and 109.5° angles between each pair and the central atom
trigonal bipyramidal
shape in which five outside groups are placed around a central atom such that three form a flat triangle with 120° angles between each pair and the central atom, and the other two form the apex of two pyramids, one above and one below the triangular plane
trigonal planar
shape in which three outside groups are placed in a flat triangle around a central atom with 120° angles between each pair and the central atom
valence shell electron-pair repulsion theory (VSEPR)
theory used to predict the bond angles in a molecule based on positioning regions of high electron density as far apart as possible to minimize electrostatic repulsion

Chemistry End of Section Exercises

  1. Explain why the HOH molecule is bent, whereas the HBeH molecule is linear.
  2. What feature of a Lewis structure can be used to tell if a molecule’s (or ion’s) electron-pair geometry and molecular geometry will be identical?
  3. Explain the difference between electron-pair geometry and molecular geometry.
  4. Predict the electron pair geometry and the molecular geometry of each of the following molecules or ions:
    1. SF6
    2. PCl5
    3. BeH2
    4. CH3+
  5. What are the electron-pair geometry and the molecular geometry of each of the following molecules or ions?
    1. ClF5
    2. ClO2−
    3. TeCl42−
    4. PCl3
    5. SeF4
    6. PH2−
  6. Identify the electron pair geometry and the molecular geometry of each of the following molecules:
    1. ClNO (N is the central atom)
    2. CS2
    3. Cl2CO (C is the central atom)
    4. Cl2SO (S is the central atom)
    5. SO2F2 (S is the central atom)
    6. XeO2F2 (Xe is the central atom)
    7. ClOF2+ (Cl is the central atom)
  7. Describe the molecular geometry around the indicated atom or atoms:
    1. the sulfur atom in sulfuric acid, H2SO4 [(HO)2SO2]
    2. the chlorine atom in chloric acid, HClO3 [HOClO2]
    3. the oxygen atom in hydrogen peroxide, HOOH
    4. the nitrogen atom in nitric acid, HNO3 [HONO2]
    5. the oxygen atom in the OH group in nitric acid, HNO3 [HONO2]
    6. the central oxygen atom in the ozone molecule, O3
    7. each of the carbon atoms in propyne, CH3CCH
    8. the carbon atom in Freon, CCl2F2
    9. each of the carbon atoms in allene, H2CCCH2
  8. Draw the Lewis electron dot structures for these molecules, including resonance structures where appropriate:
    1. CS32−
    2. CS2
    3. CS
    4. predict the molecular shapes for CS32− and CS2 and explain how you arrived at your predictions
  9. What is the molecular geometry of the stable form of FNO2? (N is the central atom.)

Answers to Chemistry End of Section Exercises

  1. The placement of the two sets of unpaired electrons in water forces the bonds to assume a tetrahedral arrangement, and the resulting HOH molecule is bent. The HBeH molecule (in which Be has only two electrons to bond with the two electrons from the hydrogens) must have the electron pairs as far from one another as possible and is therefore linear.
  2. Absence of electron pairs on the central atom.
  3. Space must be provided for each pair of electrons whether they are in a bond or are present as lone pairs. Electron-pair geometry considers the placement of all electrons. Molecular geometry considers only the bonding-pair geometry.
  4. (a) Both the electron geometry and the molecular geometry are octahedral.
    (b) Both the electron geometry and the molecular geometry are trigonal bipyramid.
    (c) Both the electron geometry and the molecular geometry are linear.
    (d) Both the electron geometry and the molecular geometry are trigonal planar.
  5. (a) electron-pair geometry: octahedral, molecular geometry: square pyramidal;
    (b) electron-pair geometry: tetrahedral, molecular geometry: bent;
    (c) electron-pair geometry: octahedral, molecular geometry: square planar;
    (d) electron-pair geometry: tetrahedral, molecular geometry: trigonal pyramidal;
    (e) electron-pair geometry: trigonal bypyramidal, molecular geometry: seesaw;
    (f) electron-pair geometry: tetrahedral, molecular geometry: bent
  6. (a) electron-pair geometry: trigonal planar, molecular geometry: bent;
    (b) electron-pair geometry: linear, molecular geometry: linear;
    (c) electron-pair geometry: trigonal planar, molecular geometry: trigonal planar;
    (d) electron-pair geometry: tetrahedral, molecular geometry: trigonal pyramidal;
    (e) electron-pair geometry: tetrahedral, molecular geometry: tetrahedral;
    (f) electron-pair geometry: trigonal bipyramidal, molecular geometry: seesaw;
    (g) electron-pair geometry: tetrahedral, molecular geometry: trigonal pyramidal
  7. (a) tetrahedral; (b) trigonal pyramidal; (c) bent; (d) trigonal planar; (e) bent; (f) bent; (g) CH3CCH tetrahedral, CH3CCH linear; (h) tetrahedral; (i) H2CCCH2 linear; H2CCCH2 trigonal planar
  8. (a)The figure shows three Lewis structures that are each surrounded by brackets and have a superscripted 2 negative sign. They are written with a double-headed arrow in between each diagram. The first of this trio has a carbon atom single bonded to two sulfur atoms, each of which has thee lone pairs of electrons, and double bonded to a third sulfur atom with two lone pairs of electrons. The second and third diagrams have the same atoms present, but each time the double bond moves between a different carbon and sulfur pair. The lone electron pairs also shift to correspond with the bond changes.;
    (b)The Lewis structure shows a carbon atom double bonded to two sulfur atoms, each of which has two lone pairs of electrons.;
    (c)This diagram shows a carbon with one lone electron pair triple bonded to a sulfur with one lone electron pair.;
    (d) CS32− includes three regions of electron density (all are bonds with no lone pairs); the shape is trigonal planar; CS2 has only two regions of electron density (all bonds with no lone pairs); the shape is linear
  9. trigonal planar

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

UW-Madison Chemistry 103/104 Resource Book Copyright © by crlandis is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book