Molecular Geometry

VSEPR

At this point we are ready to explore the three dimensional structure of simple molecular (covalent) compounds and polyatomic ions. We will use a model called the Valence Shell Electron-Pair Repulsion (VSEPR) model that is based on the repulsive behavior of electron-pairs. This model is fairly powerful in its predictive capacity. To use the model we will have to memorize a collection of information.

The table below contains several columns. We already have a concept of bonding pair of electrons and non-bonding pairs of electrons. Bonding pairs of electrons are those electrons shared by the central atom and any atom to which it is bonded. Non-bonding pairs of electrons are those pairs of electrons on an individual atom that are not shared with another atom. In the table below the term bonding groups/domains (second from the left column) is used in the column for the bonding pair of electrons. Groups is a more generic term. Group is used when a central atom has two terminal atoms bonded by single bonds and a terminal atom bonded with two pairs of electrons (a double bond). In this case there are three groups of electrons around the central atom and the molecualr geometry of the molecule is defined accordingly. The term electron-pair geometry is the name of the geometry of the electron-pair/groups/domains on the central atom, whether they are bonding or non-bonding. Molecular geometry is the name of the geometry used to describe the shape of a molecule. The electron-pair geometry provides a guide to the bond angles of between a terminal-central-terminal atom in a compound. The molecular geometry is the shape of the molecule. So when asked to describe the shape of a molecule we must respond with a molecular geometry. If asked for the electron-pair geometry on the central atom we must respond with the electron-pair geometry. Notice that there are several examples with the same electron-pair geometry, but different molecular geometries. You should note that to determine the shape (molecular geometry) of a molecule you must write the Lewis structure and determine the number of bonding groups of electrons and the number of non-bonding pairs of electrons on the central atom, then use the associated name for that shape.

The table below summarizes the molecular and electron-pair geometries for different combinations of bonding groups and nonbonding pairs of electrons on the central atom.

Note: for bent molecular geometry when the electron-pair geometry is trigonal planar the bond angle is slightly less than 120 degrees, around 118 degrees. For trigonal pyramidal geometry the bond angle is slightly less than 109.5 degrees, around 107 degrees. For bent molecular geometry when the electron-pair geometry is tetrahedral the bond angle is around 105 degrees.

Lets consider the Lewis structure for CCl₄. We can draw the Lewis structure on a sheet of paper. The most convenient way is shown here.

Notice that there are two kinds of electron groups in this structure. Bonding electrons, which are shared by a pair of atoms and nonbonding electrons, which belong to a particular atom but do not participate in bonding. In CCl₄ the central carbon atom has four bonding groups of electrons. Each chlorine atom has three nonbonding pairs of electrons.

The arrangement of the atoms is correct in my structure. That is the carbon is the central atom and the four chlorine atoms are terminal. But this drawing does not tell us about the shape of the molecule. Lets look at what I mean. Here we have a ball and stick model of CCl₄.

This is a nice representation of a two dimensional, flat structure. The Cl-C-Cl bond angles appear to be 90 degrees. However, the actual bond angles in this molecule are 109.5 degrees. What does this do to our geometry?

Lets rotate this molecule to see what has happened. We see the actual molecular geometry is not flat, but is tetrahedral. This ball and stick model does not adequately represent why the molecule has to have this 3-dimensional arrangement. The shape we see is the only possible shape for a central carbon atom with four bonds. This geometry is a direct result of the repulsion experienced by the four groups of bonding electrons.

The shape of this molecule is a result of the electrons in the four bonds positioning themselves so as to minimize the repulsive effects. This was seen in the 'balloon' example we used in class. When four balloons of the same size are tied together the natural arrangement is as a tetrahedron. With bond angles of 109.5 degrees. As indicated in Table I, any compound containing a central atom with four bonding groups (pairs) of electrons around it will exhibit this particular geometry.

By recognizing that electrons repel each other it is possible to arrive at geometries which result from valence electrons taking up positions as far as possible from each other. The position of the atoms is dictated by the position of the bonding groups of electrons.

These ideas have been explored and have resulted in a theory for molecular geometry known as Valence Shell Electron-Pair Repulsion Theory.

VSEPR focuses on the positions taken by the groups of electrons on the central atom of a simple molecule. The positions can be predicted by imagining that all groups of electrons, whether they are bonding pairs of electrons (single bonds), nonbonding pairs or groups of electrons (multiple bonds) move as far apart as possible.

Arriving at the geometry of a molecules requires writing a correct Lewis structure, determining the number of bonding groups and nonbonding groups on the central atom of the molecule and then recalling, from memory, the correct geometry based on the numbers of bonding and nonbonnding groups of electrons.

Lone pairs of electrons are assumed to have a greater repulsive effect than bonding pairs. Because of the nonbonding pairs of electrons are spread over a larger volume of space compared to bonding electrons. Because nonbonding electrons are spread over more space they repel other electrons from a greater region of space. So it is more favorable, energetically, for nonbonding pairs of electrons to be as far away as possible from each other in space. So LP-LP repulsions are > LP-BP repulsions > BP-BP repulsions.

This can be used to explain the change in bond angles observed in going from methane to ammonia to water.

To predict the geometry of a molecule a reasonable Lewis structure must be written for the molecule. The number of bonding and nonbonding pairs of electrons on the central atom are then determined. Let's progress, systematically, through the five basic electron-pair geometries and detail the variations in molecular geometries that can occur.

Two Electron Pairs (Linear)

The basic geometry for a molecule containing a central atom with two pairs of electrons is linear. BeF₂ is an example. Another example of a linear compound is CO₂. However, its Lewis structure contains two double bonds. We need to recognize that multiple bonds should be treated as a group of electron pairs when arriving at the molecular geometry.

Three Electron Pairs (Trigonal Planar)

The basic geometry for a molecule containing a central atom with three pairs of electrons is trigonal planar. BF₃ is an example. If we replace a bonding pair with a lone pair, as in SO₂, the geometry is described as bent or angular.

Four Electron Pairs (Tetrahedral)

The basic geometry for a molecule containing a central atom with four pairs of electrons is tetrahedral. An example of this geometry is CH₄. As we replace bonding pairs with nonbonding pairs the molecular geometry become trigonal pyramidal (three bonding and one nonbonding), bent or angular (two bonding and two nonbonding) and linear (one bonding and three nonbonding). Notice that compounds with the same number of terminal atoms, BF₃ and NF₃, do not necessarily have the same geometry. In this case BF₃ has three bonding pairs and no nonbonding pairs with a geometry of trigonal planar, while NF₃ has three bonding pairs and one nonbonding pair with a geometry of trigonal pyramidal. Also note that SO₂ and H₂O have a similar descriptor for their respective geometry. Although each molecule can be described as having a bent geometry the respective bond angles are different. For SO₂ the O-S-O angle is near 120 degrees, actually slightly less than 120, about 118 degrees, for H₂O the H-O-H angle is near 105 degrees.

Five Electron Pairs (Trigonal Bipyramidal)

The basic geometry for a molecule containing a central atom with five pairs of electrons is trigonal bipyramidal. An example of this geometry is PCl₅. As we replace bonding pairs with nonbonding pairs the molecular geometry changes to seesaw (four bonding and one nonbonding), T-shaped (three bonding and two nonbonding) and linear (two bonding and three nonbonding). This is an interesting system because of the two different types of terminal atoms in the structure, axial and equitorial. The equitorial terminal atoms are those in the trigonal plane. The axial atoms are those above and below the trigonal plane. When the first bonding pair of electrons is replaced with a nonbonding pair that occurs in the trigonal plane. the reason for this is due to the smaller replusions between the lone pair and the bonding pairs of electrons. If the lone pair replaced an axial atom the repulsions would be greater. So as the bonding pairs of electrons are replaced with nonbonding pairs the equitorial atoms are replaced. So as we move from trigonal bipyramidal to linear the nonbonding pairs of electrons occupy the equitorial plane, not the axial positions.

Six Electron Pairs (Octahedral)

The basic geometry for a molecule containing a central atom with six pairs of electrons is octahedral. An example of this geometry is SF₆. As we replace bonding pairs with nonbonding pairs the molecular geometry changes to square pyramidal(five bonding and one nonbonding) to square planar (four bonding and two nonbonding). There are no other combinations of bonding groups and nonbonding pairs of electrons when the electron-pair geometry is octahedral. The replacement of the first bonding group can occur in any position and always produces a square pyramidal molecular geometry. However the seond bonding group replaced is always opposite the first producing the square planar molecular geometry.

Here are some examples of the 3-dimensional structure in simple compounds. (Note: you will need the MDL ChemScape Chime Plugin to view these files.)

Here are some examples of the 3-dimensional structure for more complex compounds. (Note: you will need the MDL ChemScape Chime Plugin to view these files.)

Here is the Purdue site we used in class.

Here are some VSEPR animations that do not require a plug-in to view.