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CIE A-Level Chemistry Study Notes

3.5.1 VSEPR Theory: Understanding Molecular Shapes and Bond Angles

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a cornerstone of molecular chemistry, offering a comprehensive way to predict and explain molecular shapes and bond angles.

Fundamentals of VSEPR Theory

VSEPR theory, developed in the 1950s, is based on the idea that electron pairs in the valence shell of an atom repel each other. This repulsion shapes the three-dimensional arrangement of atoms in a molecule.

Principles of VSEPR Theory

  • Electron Pairs Repulsion: Electron pairs (bonding and lone pairs) repel each other. This repulsion is the key factor in determining the shape of a molecule.
  • Arrangement of Electron Pairs: Electron pairs arrange in a way that minimises repulsion, which leads to the molecule's shape.
  • Bond Angles: The angles between the bonds in a molecule depend on the arrangement of electron pairs around the central atom.
A diagram showing Valence Shell Electron Pair Repulsion (VSEPR)- Bond angles.

Image courtesy of aboabdelah

Applying VSEPR Theory to Molecules

VSEPR theory helps predict the shapes of various molecules, from simple diatomic molecules to more complex polyatomic forms.

BF₃ (Boron Trifluoride)

  • Geometry: Trigonal Planar
  • Bond Angle: 120°
  • Explanation: Boron, having three bonding pairs and no lone pairs, forms a trigonal planar shape for minimising repulsion.
Boron trifluoride120° bond angle

Image courtesy of Topblogtenz

CO₂ (Carbon Dioxide)

  • Geometry: Linear
  • Bond Angle: 180°
  • Explanation: Carbon with two double bonds and no lone pairs leads to a linear shape for CO₂.
Carbon Dioxide 180° bond angle

Image courtesy of UCLA – Chemistry and Biochemistry

CH₄ (Methane)

  • Geometry: Tetrahedral
  • Bond Angle: 109.5°
  • Explanation: Carbon in methane, surrounded by four bonding pairs and no lone pairs, adopts a tetrahedral shape.
CH₄ (Methane) 109.5° bond angle

Image courtesy of Benjah-bmm27 Vector: Jynto

NH₃ (Ammonia)

  • Geometry: Trigonal Pyramidal
  • Bond Angle: ~107°
  • Explanation: Nitrogen in NH₃ with three bonding pairs and one lone pair forms a trigonal pyramidal shape, with a slightly reduced bond angle.
NH₃ (Ammonia)107° bond angle

Image courtesy of DoSiDo

H₂O (Water)

  • Geometry: Bent
  • Bond Angle: ~104.5°
  • Explanation: Oxygen in water has two bonding pairs and two lone pairs, resulting in a bent shape with a reduced bond angle.
H₂O (Water) 104.5° bond angle

Image courtesy of Dan Craggs

SF₆ (Sulfur Hexafluoride)

  • Geometry: Octahedral
  • Bond Angle: 90°
  • Explanation: Sulfur in SF₆ with six bonding pairs and no lone pairs forms an octahedral shape.
SF₆ (Sulfur Hexafluoride) 90°bond angle

Image courtesy of Benjah-bmm27

PF₅ (Phosphorus Pentafluoride)

  • Geometry: Trigonal Bipyramidal
  • Bond Angle: 90° and 120°
  • Explanation: Phosphorus in PF₅, having five bonding pairs and no lone pairs, creates a trigonal bipyramidal shape.
PF₅ (Phosphorus Pentafluoride) bond angles

Image courtesy of Topblogtenz

Detailed Exploration of Molecular Shapes

In VSEPR theory, the shape of a molecule is influenced by the number of bonding and lone pairs around the central atom. Here's a deeper look at some common molecular shapes:

Linear Molecules

  • Examples: BeCl₂, CO₂
  • Characteristics: Molecules with two bonding pairs or double bonds tend to be linear.
  • Bond Angles: Typically 180°.

Trigonal Planar Molecules

  • Examples: BF₃, SO₃
  • Characteristics: These have three bonding pairs and no lone pairs.
  • Bond Angles: About 120°.

Tetrahedral Molecules

  • Examples: CH₄, SiCl₄
  • Characteristics: Four bonding pairs lead to this shape.
  • Bond Angles: Around 109.5°.

Trigonal Pyramidal Molecules

  • Examples: NH₃, PCl₃
  • Characteristics: Three bonding pairs and one lone pair create this shape.
  • Bond Angles: Slightly less than 109.5°.

Bent Molecules

  • Examples: H₂O, SO₂
  • Characteristics: Two bonding pairs and one or two lone pairs lead to a bent shape.
  • Bond Angles: Less than 120° for two lone pairs, about 120° for one lone pair.

Factors Influencing Molecular Shape

Other factors play a role in determining the shape of molecules:

Lone Pairs

  • Impact on Shape: They occupy more space than bonding pairs, leading to smaller bond angles.
  • Examples: The bond angles in H₂O are less than in CH₄ due to lone pairs on oxygen.

Double and Triple Bonds

  • Impact on Shape: They are treated as single electron pairs in VSEPR theory but have more repulsion than single bonds.
  • Examples: The linearity of CO₂ is due to its double bonds.

Electronegativity

  • Impact on Shape: Differences in electronegativity can distort the idealised shapes predicted by VSEPR.
  • Examples: NH₃'s shape is slightly altered due to nitrogen's higher electronegativity.

Limitations of VSEPR Theory

While VSEPR is a powerful tool, it has its limitations:

  • Size Prediction: It does not predict the relative sizes of molecules.
  • Chemical Bond Nature: VSEPR does not explain the nature of chemical bonds.
  • Exact Repulsion Reasons: The theory does not provide a detailed explanation of why electron pairs repel each other.

VSEPR theory is a fundamental aspect of A-level Chemistry, offering essential insights into the shapes and structures of molecules. Its simplicity and effectiveness make it an indispensable tool for students to understand molecular geometry.

FAQ

VSEPR theory can indeed predict the shapes of ions, including polyatomic ions like sulfate (SO₄²⁻). The approach is similar to that used for neutral molecules: considering the repulsion between electron pairs (both bonding and lone pairs) around the central atom. In the case of sulfate, the central sulfur atom is surrounded by four oxygen atoms with double bonds and no lone pairs. According to VSEPR theory, these four regions of electron density (the double bonds) will repel each other equally and arrange themselves as far apart as possible. This arrangement leads to a tetrahedral geometry for the sulfate ion. Each S-O bond is equivalent, and the bond angles are approximately 109.5°, typical of a tetrahedral structure. This example demonstrates how VSEPR theory extends beyond simple molecules to complex ions, using the same principles of electron pair repulsion to predict molecular geometry.

VSEPR theory accounts for multiple bonds (double or triple bonds) in a molecule by considering them as a single electron pair region. Despite being treated as one electron pair for shape prediction, multiple bonds exert a greater repulsive force than single bonds. This increased repulsion can affect the molecule's shape, especially when combined with single bonds or lone pairs. For instance, in carbon dioxide (CO₂), the two double bonds on the central carbon atom are treated as two regions of electron density. These double bonds repel each other more strongly than single bonds would, maintaining a linear shape with a bond angle of 180°. Similarly, in acetylene (C₂H₂), the triple bond between the two carbon atoms is considered as one electron pair region, but its higher electron density ensures a linear configuration. Thus, while multiple bonds are treated as single electron pair regions in VSEPR theory, their higher electron density and repulsion significantly influence the overall shape and bond angles in the molecule.

Hybridization is a concept that integrates well with VSEPR theory to explain molecular shapes. It involves the mixing of atomic orbitals to form new hybrid orbitals that can accommodate bonding and lone pairs of electrons. In ethene (C₂H₄), the carbon atoms undergo sp² hybridization. Each carbon atom in C₂H₄ uses three sp² hybrid orbitals for forming sigma bonds with two hydrogen atoms and one carbon atom. The unhybridized p orbital on each carbon overlaps to form a pi bond, contributing to the double bond between the carbon atoms. According to VSEPR theory, the three regions of electron density (two C-H single bonds and one C-C double bond) around each carbon atom repel each other, arranging in a trigonal planar geometry. This leads to a bond angle of approximately 120° around each carbon atom. Hybridization, therefore, complements VSEPR theory by explaining the formation of specific molecular shapes through the rearrangement and merging of atomic orbitals, as evidenced in the planar structure of ethene.

The bond angle in phosphine (PH₃) is less than that in ammonia (NH₃), a phenomenon that VSEPR theory helps to explain. In NH₃, the bond angle is approximately 107°, whereas in PH₃, it's around 93.5°. This difference is attributed to the size of the central atom and the influence of lone pairs. Nitrogen in NH₃ is smaller than phosphorus in PH₃. The smaller size of nitrogen allows the lone pair to exert a stronger repulsion on the bonding pairs, causing them to spread out more, which leads to a larger bond angle. Conversely, the larger central atom in PH₃ means that the lone pair's influence is less pronounced, leading to a smaller bond angle. Additionally, the bonding pairs in PH₃ are less tightly held due to phosphorus’s lower electronegativity compared to nitrogen, resulting in less repulsion among the bonding pairs. Therefore, the geometry of PH₃ is more compressed than NH₃, illustrating how VSEPR theory considers factors like the size of the central atom and electronegativity in determining molecular shapes and bond angles.

VSEPR theory explains the bond angles in water (H₂O) and oxygen difluoride (OF₂) through the concept of electron pair repulsion. Both molecules have a bent shape due to the two lone pairs on the oxygen atom. In H₂O, the bond angle is approximately 104.5°, while in OF₂, it's slightly larger, around 103°. This difference arises from the nature of the atoms bonded to oxygen. In water, the hydrogen atoms are less electronegative compared to fluorine in OF₂. Less electronegative atoms like hydrogen exert less repulsion on the bonding pairs, leading to a smaller bond angle. In contrast, fluorine, being highly electronegative, exerts a stronger repulsion on the bonding pairs, slightly increasing the bond angle. Furthermore, the larger size of fluorine atoms compared to hydrogen contributes to the difference in bond angles. The VSEPR theory thus accounts for variations in molecular geometries based on the types of atoms involved and their respective electronegativities, as well as the size and number of lone and bonding electron pairs.

Practice Questions

Explain why the shape of the ammonia (NH₃) molecule is trigonal pyramidal and not tetrahedral. Discuss the bond angles in the context of VSEPR theory.

The shape of the ammonia (NH₃) molecule is trigonal pyramidal due to the presence of one lone pair and three bonding pairs of electrons around the nitrogen atom. According to VSEPR theory, electron pairs repel each other, and the lone pair exerts a greater repulsive force compared to bonding pairs. This repulsion distorts the shape from a perfect tetrahedral, pushing the bonding pairs closer together. Consequently, the bond angles in NH₃ are around 107°, slightly less than the 109.5° in a perfect tetrahedral structure like CH₄. The reduction in bond angle is due to the lone pair's stronger repulsion, which compresses the bonding pairs closer than in a tetrahedral arrangement.

Using VSEPR theory, predict the shape of sulfur hexafluoride (SF₆) and explain the factors influencing its geometry.

Sulfur hexafluoride (SF₆) has an octahedral geometry, as predicted by VSEPR theory. This shape is determined by the presence of six bonding pairs of electrons around the central sulfur atom and the absence of lone pairs. In an octahedral structure, the electron pairs are positioned at 90° angles relative to each other. This arrangement minimises repulsion, as it allows the electron pairs to be as far apart as possible. The symmetrical distribution of the six fluorine atoms around the sulfur atom ensures equal repulsion between all electron pairs, stabilising the molecule in an octahedral configuration. The absence of lone pairs on the sulfur atom further supports this geometry, leading to uniform bond angles of 90°.

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