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VSEPR Theory: Predicting Molecular Structures based on Electron-pair Repulsion, Study notes of Chemistry

The VSEPR (Valence Shell Electron-pair Repulsion) theory is a fundamental concept in chemistry used to predict the three-dimensional structures of molecules and ions. By minimizing the repulsive forces between electron pairs, we can determine the molecular shape around a central atom. a detailed explanation of the VSEPR theory, including its principles, steps for application, and examples of molecules with and without lone electron pairs, as well as molecules with multiple central atoms and multiple bonds.

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SCH4U: Structure and Properties of Matter
2.5 VSEPR Theory
The three-dimensional structure of a substance refers to the arrangement in
three dimensions of atoms that the substance comprises. Three-dimensional
structure often helps determine how the pure substance will behave in chemical
reactions. It also influences interactions with other substances, especially in
biological systems, where reactions have to be efficient and highly specific.
The VSEPR Theory
We can predict the approximate structure of a molecule or ionic compound using
the valence shell electron-pair repulsion (VSEPR) theory. The main idea
behind the VSEPR theory is that you can determine the structure around an atom
by minimizing the repulsive force between electron pairs.
According to VSEPR theory, bonded and lone pair electrons position themselves
as far apart as possible in a molecule to minimize the repulsive forces between
them, a concept called electron-pair repulsion.
The theory is based on the electrical repulsion of bonded and unbonded electron
pairs in a molecule or polyatomic ion and explains the atomic orientations in
molecules and ions.
VSEPR Theory
Only the valence shell electrons of the central atom(s) are important for
molecular shape.
Valence shell electrons are paired or will be paired in a molecule or
polyatomic ion.
Bonded pairs of electrons and lone pairs of electrons are treated
approximately equally.
Valence shell electron pairs repel each other electrostatically.
The molecular shape is determined by the positions of the electron pairs
when they are a maximum distance apart (with the lowest repulsion possible).
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2.5 VSEPR Theory

The three-dimensional structure of a substance refers to the arrangement in three dimensions of atoms that the substance comprises. Three-dimensional structure often helps determine how the pure substance will behave in chemical reactions. It also influences interactions with other substances, especially in biological systems, where reactions have to be efficient and highly specific.

The VSEPR Theory

We can predict the approximate structure of a molecule or ionic compound using the valence shell electron-pair repulsion (VSEPR) theory. The main idea behind the VSEPR theory is that you can determine the structure around an atom by minimizing the repulsive force between electron pairs.

According to VSEPR theory, bonded and lone pair electrons position themselves as far apart as possible in a molecule to minimize the repulsive forces between them, a concept called electron-pair repulsion.

The theory is based on the electrical repulsion of bonded and unbonded electron pairs in a molecule or polyatomic ion and explains the atomic orientations in molecules and ions.

VSEPR Theory

 Only the valence shell electrons of the central atom(s) are important for molecular shape.  Valence shell electrons are paired or will be paired in a molecule or polyatomic ion.  Bonded pairs of electrons and lone pairs of electrons are treated approximately equally.  Valence shell electron pairs repel each other electrostatically.  The molecular shape is determined by the positions of the electron pairs when they are a maximum distance apart (with the lowest repulsion possible).

The following arrangements occur if all of the electrons are bonding pairs

Sample Problem 2: The Shape of a Polyatomic Ion Predict the structure of the ammonium ion, NH 4 +, using the step-by-step VSEPR approach.

Sample Problem 3: The Shape of a Molecule with Lone Electron Pairs Predict the structure of phosphine, PH 3 , using the step-by-step VSEPR approach.

The VSEPR Theory and Molecules with More Than One Central Atom

Structures of Molecules with Two Central Atoms

Sample Problem 1: Molecular Shape with No Single Central Atom Predict the structure of methylamine, CH 3 NH 2 , using the step-by-step VSEPR approach.

VSEPR Theory and Multiple Bonds

Molecules or ionic compounds with double and triple bonds have different chemical properties than those with single bonds. In a double or triple bond, there is more than 1 bonding pair of electrons. When using the VSEPR theory to determine structure, you must place all of the multiple bonding electrons together.

Sample Problem 1: Predicting the Shape of a Molecule with Multiple Bonds Predict the structure of carbon dioxide, CO 2 , using the step-by-step VSEPR approach.

The VSEPR Theory: How Well Does It Work?

The VSEPR theory works well for most molecules and ionic substances that contain non-metallic elements. However, the theory fails in a few instances. For example, phosphine, PH 3 , has the same Lewis structure as ammonia, NH 3 : trigonal pyramidal. From the VSEPR theory, you would expect phosphine to have the same bond angles as NH 3 : 107°. However, the bond angles of phosphine are actually 94°. There are ways to explain this difference, but you have to add more rules to the theory. This example illustrates that simple theories are likely to have exceptions. The amazing thing about the VSEPR theory is that it correctly predicts the structures of so many pure substances.

Worksheet 2.5: VSEPR Theory

p. 216, Q 2, 3, 5, 6, 7, 9