The Valence Shell Electron Pair Repulsion (VSEPR) Theory is used to predict the shapes/geometries of molecules, based on the number of valence electron pairs around the central atom.
Sidgwick and Powell initially proposed this theory in 1940. The theory claims that the shapes or geometries of molecules or ions depend on the number of electron pairs around the central atom. These electron pairs can be lone pairs as well as bond pairs. However, it is actually the bond pairs that define the arrangement of atoms, and therefore the shape of the molecule. Meanwhile, the lone pairs only influence the shape.
A lone (non-bonding) pair refers to a pair of valence electrons that are not shared with any other atom. On the other hand, a bond pair refers to a pair of electrons in a bond.
Again in 1957, Nyholm and Gillespie suggested that the arrangement of atoms in a molecule or ion is largely determined by the repulsive interactions between the electron pairs present in the valence shell.
Postulates of VSEPR Theory Simplified
Like any other theory, VSEPR theory has its set of postulates:
1. Electron pairs arrange themselves as far apart as possible due to repulsion.
2. A lone pair (L.P) occupies more space than a bond pair (B.P).
3. The order of repulsion between the electron pairs is: L.P – L.P > L.P – B.P > B.P – B.P.
4. Both Lone pairs and Bond pairs contribute in determining the geometry.
5. Double and Triple bonds occupy more space, and cause more repulsion but are considered as single bonds in determining the geometry.
6. Shapes of molecules depend upon the number of bond pairs and lone pairs, and this information is provided in a VSEPR table/chart.
The detailed postulates of VSEPR theory are provided at the end of the article.
The VSEPR theory helps us to predict structures of various molecules. Some common VSEPR shapes are: linear, trigonal planar, bent, tetrahedral, trigonal pyramidal, trigonal bipyramidal, and octahedral.
The geometries are exactly what they sound like. For example, the NH3 molecule assumes a trigonal pyramidal geometry. The shape is a pyramid with a triangular base. At the vertices of the base lie the 3 hydrogen atoms and the top of the pyramid is the nitrogen atom with its lone pair.
Determining Molecular Geometries and Bond Angles by VSEPR Theory
1. The first step in determining the shape of molecules is to identify the central atom from the molecule. It is usually the atom that occurs once in the molecular formula. For example, in the case of NCl3, nitrogen is the central atom.
2. Once the central atom has been identified, the number of electrons present in its valence shell is considered. Nitrogen is a group 5A element, and so possesses 5 valence electrons.
Additionally, considering the formal charge on the central atom, an electron is added for each negative charge, and an electron is subtracted for each positive charge, if applicable.
3. The electrons are allotted to the number of bonds formed. One electrons is required from the central atom to form a bond pair. In the example provided, since 3 chlorine atoms are bonded with the central nitrogen atom, 3 electrons are required of the 5 valence electrons of N, to form 3 bond pairs.
Moreover, once all the σ-bonds have been assigned, the π-bonds should be considered depending on the valency of the bonded atoms.
4. The remaining electrons form lone pair(s), if any. 2 electrons are needed to form a single lone pair. Since 3 electrons of nitrogen are used to form single covalent bonds, 2 electrons remained of the total 5. These two electrons will form one lone pair of electrons.
5. Since the molecule NCl3 contains 3 bond pairs and one lone pair, its geometry will be trigonal pyramidal. Moreover, each bond angle will be 107⁰.
There are a total of 4 electron pairs. Therefore, the ideal geometry should be tetrahedral (109.5⁰). However, one bond pair is replaced by a lone pair, which offers more repulsion comparatively. This extra repulsion from the lone pair causes the bond pairs to move closer together. As a consequence, the bond angle is reduced from 109.5⁰ to 107⁰ and the shape deviates from tetrahedral to trigonal pyramidal.
ABxEy Classification: An Application of VSEPR Theory
According to VSEPR theory, the geometry of a molecule depends on the number of sigma bonds and lone pairs. In order to make a VSEPR chart easier to understand, the molecules are assigned the ABE types. In the ABxEy classification, “A” represents the central polyvalent atom, “x” is the number of other atoms (B) attached to the central atom (bond pairs), and “y” is the number of lone pairs (E) possessed by the central atom.
The sum (x + y) can be called the occupancy of an atom. It implies that the atom “A” is surrounded by (x + y) electron pairs. As an example, the occupancy of PCl3 is four, having 3 bond pairs and a lone pair. The occupancy is provided so that it is independent of the presence of multiple bonds, and takes into account the σ-bonds and lone pairs only.
Hybridization is oftentimes not included because it should be kept in mind that VSEPR theory is applied without any reference to hybridization.
For the given formulae, in the absence of any lone pair(s), the geometries are linear (AB2), trigonal planar (AB3), tetrahedral (AB4), trigonal bipyramidal (AB5), and octahedral (AB6). These are further divided into subgroups containing an increasing number of lone pairs at the expense of bond pairs.
Even though the positions of lone pairs E are specified, the actual geometry of molecules is determined by the positions of atoms A and B only.
Bond Angle: 180⁰.
Molecular Geometry: Linear.
Example: Carbon dioxide, CO2.
Bond Angle: 120⁰.
Molecular Geometry: Trigonal Planar.
Example: Sulfur trioxide, SO3.
Bond Angle: 119⁰.
Molecular Geometry: V-shaped or Bent.
Example: Sulfur dioxide, SO2.
Bond Angle: 109.5⁰.
Molecular Geometry: Tetrahedral.
Example: Methane, CH4.
Alternatively, for transition metals atoms/ions with a d8 system,
Bond Angle: 90⁰.
Molecular Geometry: Square Planar.
Example: Tetracyanonickelate(II) ion, Ni(CN)42-.
Bond Angle: 107⁰.
Molecular Geometry: Trigonal Pyramidal.
Example: Ammonia, NH3.
Bond Angle: 104.5⁰.
Molecular Geometry: V-shaped or Bent.
Example: Water, H2O.
Bond Angle(s): 90⁰ and 120⁰.
Molecular Geometry: Trigonal Bipyramidal.
Example: Phosphorus pentachloride, PCl5.
Bond Angle: 90⁰.
Molecular Geometry: Octahedral.
Example: Sulfur hexafluoride, SF6.
Comparison with Electron Geometries
Molecular geometry is very much related to electron geometries. At times, they can be the same but differ in the presence of lone pair(s). This is because the electron geometry focuses on the number of electron pairs whereas molecular geometry classifies the electron pairs into two types, bond pairs and lone pairs.
The increased repulsion provided by the lone pairs in molecular geometry is the cause of deviation from the electron geometry. A detailed comparison between the two is provided here.
Limitations of the VSEPR Theory
There do exist some molecules whose geometries cannot be accurately predicted with the VSEPR theory. One such molecule is XeF6 – the simplest molecular orbital treatment predicts octahedral shape. On the contrary, the VSEPR model considers seven electron pairs in the valence shell, and predicts its structure accordingly.
The lone pair must have a definite geometric position and occupy more space than the bond pairs. The three known neutral fluoride molecules with a coordination number of 7 are IF7, OsF7, and ReF7, all having a distorted pentagonal bipyramidal structure. Because of more repulsion from the presence of a lone pair, this geometry had to be ruled out.
The structure of XeF6 was difficult to determine, and is known as a distorted octahedron with the lone pair extending out through either a face, or an edge of the octahedron. However, the exception lies in the fact that the lone pair seems to be occupying lesser space than the bond pairs.
Further exceptions are the isoelectronic anions IF6– and BrF6–, that show lower symmetry than octahedral, and octahedral symmetry, respectively, despite having 7 valence electron pairs. For these structures in which the lone pair is stereochemically inactive, it is considered that the pair resides in an s-orbital.
Detailed Postulates of VSEPR Theory
1. Since the electrons are negatively charged, they repel each other. Because of this repulsion, the electron pairs present around the central atom are arranged as far apart as possible in space. Hence, there is minimum repulsion between the electron pairs.
2. The lone pair of electrons occupies more space (angular volume) as compared to the bond pair. This is because the lone pair is under the attractive influence of a single nucleus only whereas the bond pair is constrained by two nuclei.
3. Although the geometry depends on the relative arrangement of atoms, both the bond pairs and the lone pairs contribute in determining the geometry of the molecule.
4. Owing to the lone pairs occupying more space, repulsion between two lone pairs is the greatest. This is followed by repulsion between a lone pair and a bond pair, and then finally by the least repulsion between two bond pairs. The magnitude of repulsion is given by:
Lone Pair – Lone Pair > Lone Pair – Bond Pair > Bond Pair – Bond Pair
The presence of more repulsion from the lone pairs causes a decrease in the otherwise ideal bond angles. As a consequence, the geometry of the molecule is altered.
5. Moreover, in geometries having a difference between the axial and equatorial positions, the latter are favored by the lone pairs, on account of the extent of repulsion between electron pairs.
6. The two and three bond pairs of a double and triple bond, respectively, possess a higher charge density and occupy more space. Yet when determining the geometry, they are considered single bonds.
7. In addition to nature of electron pairs, positions and multiple bonds, another factor that can alter the regular geometry, and hence the bond angles, is the degree of electronegativity of the bonded atoms. However, there are two aspects to this.
If the electronegativity of the side atoms is increased, the electron density is shifted towards the ends of the molecule and lesser repulsion occurs between the bond pairs. Consequently the bond angle decreases. The bond angle in a NCl3 molecule is 107.1°, whereas that in a NF3 molecule is 101.9°.
On the other hand, if the electronegativity of the central atom is increased, there occurs more repulsion between the bond pairs and the bond angle increases. As a proof, the bond angle of PH3 is 93.5° while that of NH3 is 107.3°.
8. If the central atom is present in the third row or below in the periodic table, its lone pair occupies a stereochemically inactive s-orbital and bonds through p-orbitals. The bond angles are near 90° if the substituent electronegativity is ≤ ~ 2.5.
What are the postulates of VSEPR theory?
The postulates of VSEPR theory are summarized as follows:
1. Due to repulsion between the electron pairs, they are arranged as far apart as possible.
2. The lone pairs occupy more space than the bond pairs. So the order of repulsion is:
Lone Pair – Lone Pair > Lone Pair – Bond Pair > Bond Pair – Bond Pair
3. The geometry is decided by the number of bond pairs and lone pairs in the valence shell.
4. Multiple bonds occupy more space but are treated as single bonds in determining the geometry.
What is the basis of VSEPR theory?
The basis of VSEPR theory is the repulsion between electron pairs, which causes them to assume positions where there is least repulsion between them.
Why is VSEPR theory important?
VSEPR theory can be used to predict the shapes and bond angles of various molecules with high accuracy.
How do you draw VSEPR shapes?
The VSEPR shapes are drawn based on the number of bond pairs and lone pairs in the valence shell. 2 bond pairs result in a linear geometry, 3 bond pairs in a trigonal planar, and 4 bond pairs in a tetrahedral one. A VSEPR table can provide help with the shapes and bond angles.
How accurate is the VSEPR model?
Although VSEPR theory can be used to predict the geometries of molecules, it has its limitations with transition metals and isoelectronic species having 7 electron pairs in the valence shell.
How does VSEPR determine molecular geometry?
Since electrons are negatively charged, they have repulsive forces acting between them. The electron pairs present in the valence shell repel each other and occupy positions where repulsion between them is minimum. These positions collectively define the overall geometry of molecules.
How does VSEPR affect the shape of molecules?
The shape of molecules depends on the relative arrangement of atoms. It is the repulsion between electron pairs that causes them to arrange themselves as far apart from each other as possible. This minimizes the repulsive interactions between them.
What is VSEPR geometry of SO2?
The VSEPR geometry of SO2 is bent or V-shaped. This is explained as:
Sulfur, the central atom, has 6 valence electrons. There are two σ-bonds with the two oxygen atoms, utilizing two of sulfur’s electrons. Moreover, there are further two π-bonds with the oxygen atoms, requiring another two of sulfur’s electrons. The remaining two electrons form a lone pair. The double bonds are treated as single bonds so we have 2 bond pairs and one lone pair for the central sulfur atom. This results in the bent, or V-shaped geometry.
What is the VSEPR theory and the shape of molecules?
VSEPR is an acronym for the Valence Shell Electron Repulsion, and the theory claims that the shapes of molecules depend on the repulsion between the electron pairs present in the valence shell of the central atom. The shape essentially depends on the number of lone pairs and bond pairs present in the valence shell.
What is the structure of water in the VSEPR theory?
The geometry of a water molecule is bent, according to VSEPR theory. The central oxygen atom is bonded to two hydrogen atoms (2 bond pairs), and the remaining 4 valence electrons form 2 lone pairs. This results in a bent geometry with a bond angle of 104.5⁰.
How does the VSEPR theory classify molecules?
An application of the VSEPR theory is to classify molecules according to the ABE types. “A” refers to the central atom, “B” represents the bond pairs and the number of bond pairs is given in a subscript next to it, and “E” refers to the lone pairs and its number is given likewise.
Each of these types are associated with certain geometries, provided in a VSEPR table.