VSEPR Theory

Definition

VSEPR (Valence Shell Electron Pair Repulsion) theory is a model used to predict the geometry of molecules based on the arrangement of electron pairs around a central atom. It explains how the placement of bonding and non-bonding electron pairs determines the shape of a molecule, which is crucial for understanding its chemical properties and reactivity.

5 Must Know Facts For Your Next Test

1. VSEPR theory predicts that electron pairs around a central atom will arrange themselves in space to minimize repulsion and maximize stability.
2. The number of bonding and non-bonding electron pairs around a central atom determines the molecule's overall geometry, such as linear, trigonal planar, tetrahedral, and more.
3. Hybridization of atomic orbitals is a key concept in VSEPR theory, as it explains the formation of equivalent bonds and the resulting molecular shapes.
4. VSEPR theory is particularly useful for predicting the geometry of simple molecules, such as methane (CH$_4$) and ethane (C$_2$H$_6$).
5. The theory can also be applied to more complex molecules, including those with lone pairs of electrons, to determine their three-dimensional structure.

Review Questions

• Explain how the VSEPR theory is used to determine the structure of methane (CH$_4$)
  • According to the VSEPR theory, the central carbon atom in methane has four bonding electron pairs, which arrange themselves in a tetrahedral geometry to minimize repulsion. The four hydrogen atoms are then positioned at the corners of the tetrahedron, resulting in the characteristic tetrahedral structure of the methane molecule. This arrangement maximizes the distance between the electron pairs, leading to the stable and symmetric shape of the molecule.
• Describe how the VSEPR theory can be used to predict the structure of ethane (C$_2$H$_6$)
  • The VSEPR theory can be applied to the ethane molecule by considering the arrangement of the electron pairs around each carbon atom. Each carbon atom has four bonding electron pairs, which adopt a tetrahedral geometry. The two carbon atoms are then connected by a single bond, and the remaining hydrogen atoms are positioned at the remaining corners of the tetrahedra. This results in the characteristic staggered conformation of the ethane molecule, where the carbon-carbon bond and the surrounding hydrogen atoms are arranged to minimize repulsion between the electron pairs.
• Analyze how the VSEPR theory can be used to explain the hybridization and geometry of molecules containing atoms with lone pairs, such as water (H$_2$O) and ammonia (NH$_3$)
  • The VSEPR theory takes into account both bonding and non-bonding (lone) electron pairs when determining the molecular geometry. In the case of water (H$_2$O), the central oxygen atom has two bonding electron pairs and two non-bonding electron pairs. The four electron pairs arrange themselves in a tetrahedral geometry, but the presence of the two lone pairs causes the bond angles to deviate from the ideal 109.5 degrees, resulting in the characteristic bent or angular structure of the water molecule. Similarly, in ammonia (NH$_3$), the central nitrogen atom has three bonding electron pairs and one non-bonding electron pair, leading to a trigonal pyramidal geometry with the lone pair occupying more space and causing the bond angles to be less than the ideal 109.5 degrees.

Related terms

Electron Pair Geometry: The three-dimensional arrangement of all electron pairs, both bonding and non-bonding, around a central atom in a molecule.

Molecular Geometry:

The three-dimensional arrangement of the atoms in a molecule, which is determined by the electron pair geometry and the number of bonding pairs versus non-bonding pairs.

Hybridization:

The process of mixing atomic orbitals to form new, equivalent hybrid orbitals that can be used to describe the bonding in molecules.