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VSEPR Theory
The valence-shell electron-pair repulsion (VSEPR) model is often used in chemistry to predict the three dimensional arrangement, or the geometry, of molecules. This model predicts the shape of a molecule by taking into account the repulsion between electron pairs. This handout will discuss how to use the VSEPR model to predict electron and molecular geometry. Here are some definitions for terms that will be used throughout this handout:
Electron Domain – The region in which electrons are most likely to be found (bonding and nonbonding). A lone pair, single, double, or triple bond represents one region of an electron domain. H 2 O has four domains: 2 single bonds and 2 nonbonding lone pairs. Electron Domain may also be referred to as the steric number.
Electron domain geometry - The arrangement of electron domains surrounding the central atom of a molecule or ion.
Molecular geometry - The arrangement of the atoms in a molecule (The nonbonding domains are not included in the description).
Bond angles (BA) - The angle between two adjacent bonds in the same atom. The bond angles are affected by all electron domains, but they only describe the angle between bonding electrons.
Lewis structure - A 2-dimensional drawing that shows the bonding of a molecule’s atoms as well as lone pairs of electrons that may exist in the molecule.
Nonbonding Pairs Bonding Pairs
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Octet Rule – Atoms will gain, lose, or share electrons to have a full outer shell consisting of 8 electrons. When drawing Lewis structures or molecules, each atom should have an octet.
Formal Charge – The formal charge can be used to determine the dominant Lewis structure if there is more than one possible configuration. The formal charge of each atom in the molecule should ideally be 0.
Drawing a Lewis Structure
Before the VSEPR theory can be applied, the Lewis structure of the compound must first be drawn. To draw a Lewis structure, follow these steps:
- Determine the total number of valence electrons (𝑣𝑣𝑣𝑣−) by adding the valence electrons from each atom.
- Choose the central atom. The central atom is typically the least electronegative atom in the molecule and is usually written first in the molecular formula.
- Arrange the atoms with bonding electrons between all adjacent atoms.
- Fill in 𝑣𝑣𝑣𝑣−^ for outer atoms, so they have octets. There are a few exceptions to the octet rule: a. Molecules and polyatomic ions with an odd number of valence electrons will not have a full octet. i.e. ClO 2 , NO, and NO 2 b. Some molecules will have less than an octet if there are not enough valence electrons or if the formal charges favor the structure without the full octet. This is most often seen with boron and beryllium compounds. Hydrogen will also not have an octet; it can only have two electrons. c. Hypervalent molecules are formed only on central atoms from period 3 and below on the periodic table. Due to their larger size and the presence of d- orbitals, they can have more than 8 electrons. i.e. PF 5 , SF 4
- If necessary, place any extra 𝑣𝑣𝑣𝑣−^ on the central atom.
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VSEPR Theory Shapes
To predict the molecular geometry, follow these steps:
- Draw the Lewis structure.
- Count the electron domains, and determine whether they are bonding or non-bonding pairs.
- Determine the electron domain geometry, molecular geometry, and bond angles. The chart below shows 3-dimensional representations of Lewis structures given the number of bonding and nonbonding pairs within the molecule.
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Example:
Predict the electron domain geometry, molecular geometry, and bond angles of carbon dioxide, CO 2.
1.) Draw the Lewis structure.
2.) Count the electron domains, and determine whether they are bonding or non-bonding pairs. CO 2 has 2 electron domains that consist of bonding pairs.
3.) Determine molecular geometry and bonding angles from table. CO 2 has 2 electron domains, resulting in a linear electron domain geometry. Both electron domains are bonding pairs, so CO 2 has a linear molecular geometry with a bond angle of 180°.
Practice Problems: VSEPR Theory
Predict the electron domain geometry, molecular geometry, and bond angles of the following molecules after drawing valid Lewis structures. Note: some molecules may have more than one central atom.
1.) ClF 4 -^ 2.) NH 3 3.) C 2 H 2 4.) SF 4 5.) CH 3 COOH
