VESPR (Valence Shell Electron Pair Repulsion) theory is a model that helps to predict the shape of molecules based on the number of valence electrons and the repulsion between electron pairs. It is an essential concept in chemistry that helps to understand the chemical and physical properties of molecules. This study guide will cover the basic concepts and principles of VESPR theory.
Valence Electrons: Valence electrons are the outermost electrons of an atom, which participate in chemical bonding. The number of valence electrons determines the chemical properties of the atom.
Electron Pair: Two electrons that occupy the same orbital are called an electron pair.
Electron Pair Repulsion: Electrons repel each other due to their negative charge, which causes the electron pairs to move away from each other to minimize the repulsion.
Steric Number: The steric number is the sum of the number of atoms bonded to a central atom and the number of lone pairs on the central atom.
Molecular Geometry: The molecular geometry is the three-dimensional arrangement of atoms in a molecule.
Bond Angle: The bond angle is the angle between two adjacent bonds in a molecule.
The VESPR theory is based on the idea that the electron pairs around a central atom will be arranged in a way that minimizes their repulsion. This leads to the formation of specific molecular geometries.
The VESPR theory is based on the following principles:
The repulsion between electron pairs is proportional to the number of electron pairs.
Electron pairs will arrange themselves around a central atom in a way that maximizes the distance between them.
Lone pairs occupy more space than bonded pairs and have a greater repulsion effect.
Multiple bonds have a greater repulsion effect than single bonds.
The following steps are used to determine the molecular geometry of a molecule using VESPR theory:
Draw the Lewis structure of the molecule.
Determine the steric number of the central atom.
Determine the electron pair geometry by counting the number of electron pairs around the central atom, including both bonded and lone pairs.
Determine the molecular geometry by considering only the bonded pairs.
Determine the bond angle based on the molecular geometry.
The following are the common molecular geometries based on the steric number of the central atom:
Steric Number 2: Linear Geometry, Bond Angle 180°.
Steric Number 3: Trigonal Planar Geometry, Bond Angle 120°; Bent Geometry, Bond Angle <120°.
Steric Number 4: Tetrahedral Geometry, Bond Angle 109.5°; Trigonal Pyramidal Geometry, Bond Angle <109.5°; Bent Geometry, Bond Angle <109.5°.
Steric Number 5: Trigonal Bipyramidal Geometry, Bond Angle 90° or 120°; Seesaw Geometry, Bond Angle <90° or <120°; T-Shaped Geometry, Bond Angle <90°; Linear Geometry, Bond Angle 180°.
Steric Number 6: Octahedral Geometry, Bond Angle 90°; Square Pyramidal Geometry, Bond Angle <90°; Square Planar Geometry, Bond Angle 90°.
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