VSEPR Theory
VSEPR TheoryThe valence-shell electron-pair repulsion (VSEPR) model is often used in chemistry to predictthe three dimensional arrangement, or the geometry, of molecules. This model predicts theshape of a molecule by taking into account the repulsion between electron pairs. This handoutwill discuss how to use the VSEPR model to predict electron and molecular geometry. Here aresome definitions for terms that will be used throughout this handout:Electron Domain β The region in which electrons are most likely to be found (bondingand nonbonding). A lone pair, single, double, or triple bond represents one region of anelectron domain. H2O has four domains: 2 single bonds and 2 nonbonding lone pairs.Electron Domain may also be referred to as the steric number.Nonbonding PairsBonding PairsElectron domain geometry - The arrangement of electron domains surrounding thecentral atom of a molecule or ion.Molecular geometry - The arrangement of the atoms in a molecule (The nonbondingdomains are not included in the description).Bond angles (BA) - The angle between two adjacent bonds in the same atom. The bondangles are affected by all electron domains, but they only describe the angle betweenbonding electrons.Lewis structure - A 2-dimensional drawing that shows the bonding of a moleculeβsatoms as well as lone pairs of electrons that may exist in the molecule.Provided byThe Academic Center for Excellence1VSEPR TheoryApril 2019
Octet Rule β Atoms will gain, lose, or share electrons to have a full outer shell consistingof 8 electrons. When drawing Lewis structures or molecules, each atom should have anoctet.Formal Charge β The formal charge can be used to determine the dominant Lewisstructure if there is more than one possible configuration. The formal charge of eachatom in the molecule should ideally be 0.Drawing a Lewis StructureBefore the VSEPR theory can be applied, the Lewis structure of the compound must first bedrawn. To draw a Lewis structure, follow these steps:1. Determine the total number of valence electrons (π£π£π£π£ ) by adding the valence electronsfrom each atom.2. Choose the central atom. The central atom is typically the least electronegative atom inthe molecule and is usually written first in the molecular formula.3. Arrange the atoms with bonding electrons between all adjacent atoms.4. Fill in π£π£π£π£ for outer atoms, so they have octets. There are a few exceptions to the octetrule:a. Molecules and polyatomic ions with an odd number of valence electrons will nothave a full octet. i.e. ClO2, NO, and NO2b. Some molecules will have less than an octet if there are not enough valenceelectrons or if the formal charges favor the structure without the full octet. Thisis most often seen with boron and beryllium compounds. Hydrogen will also nothave an octet; it can only have two electrons.c. Hypervalent molecules are formed only on central atoms from period 3 andbelow on the periodic table. Due to their larger size and the presence of dorbitals, they can have more than 8 electrons. i.e. PF5, SF45. If necessary, place any extra π£π£π£π£ on the central atom.Provided byThe Academic Center for Excellence2VSEPR TheoryApril 2019
6. If the central atom does not have an octet, use outer lone pairs to form double or triplebonds.Example:Draw the Lewis structure of CO21. Determine the number of valence electrons.1[4 π£π£π£π£ ] (1 πΆπΆπΆπΆπΆπΆπΆπΆπΆπΆπΆπΆ) 2[6 π£π£π£π£ ] (2 πππππππππ£π£πΆπΆ) 16 π£π£π£π£ 2. Choose the central atom. Carbon is the least electronegative atom, so it will be in thecenter.3. Arrange the atoms with bonding electrons.4. Starting with the outer atoms, fill in valence electrons until the total number of π£π£π£π£ isreached.5. There are no extra electrons to place on the central atom.6. Carbon does not have an octet. Use outer lone pairs to form double bonds.Provided byThe Academic Center for Excellence3VSEPR TheoryApril 2019
VSEPR Theory ShapesTo predict the molecular geometry, follow these steps:1. Draw the Lewis structure.2. Count the electron domains, and determine whether they are bonding or non-bondingpairs.3. Determine the electron domain geometry, molecular geometry, and bond angles. Thechart below shows 3-dimensional representations of Lewis structures given the numberof bonding and nonbonding pairs within the molecule.Provided byThe Academic Center for Excellence4VSEPR TheoryApril 2019
Example:Predict the electron domain geometry, molecular geometry, and bond angles of carbondioxide, CO2.1.) Draw the Lewis structure.2.) Count the electron domains, and determine whether they are bonding or non-bondingpairs. CO2 has 2 electron domains that consist of bonding pairs.3.) Determine molecular geometry and bonding angles from table.CO2 has 2 electron domains, resulting in a linear electron domain geometry. Bothelectron domains are bonding pairs, so CO2 has a linear molecular geometry with a bondangle of 180 .Practice Problems: VSEPR TheoryPredict the electron domain geometry, molecular geometry, and bond angles of the followingmolecules after drawing valid Lewis structures. Note: some molecules may have more than onecentral atom.1.) ClF4-2.) NH34.) SF45.) CH3COOHProvided byThe Academic Center for Excellence3.) C2H25VSEPR TheoryApril 2019
Solutions1. Electron Domain Geometry: OctahedralMolecular Geometry: Square PlanarBond Angle: 90 2. Electron Domain Geometry: TetrahedralMolecular Geometry: Trigonal PyramidalBond Angle: 109.5 3. Electron Domain Geometry: LinearMolecular Geometry: LinearBond Angle: 180 4. Electron Domain Geometry: Trigonal BipyramidalMolecular Geometry: SawhorseBond Angle: 90 and 120 .5. C1: Electron Domain Geometry: TetrahedralMolecular Geometry: TetrahedralBond Angle: 109.5 C2: Electron Domain Geometry: Trigonal PlanarMolecular Geometry: Trigonal PlanarBond Angle: 120 O3: Electron Domain Geometry: TetrahedralMolecular Geometry: BentBond Angle: 109.5 Provided byThe Academic Center for Excellence6VSEPR TheoryApril 2019
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: 1. Determine the total number of valence electrons ( ) by adding the valence electrons from each atom. 2. Choose the central atom.
CHEM 1A: VSEPR Theory Now that we have an understanding of covalent bonding and how atoms share electrons to form molecules and polyatomic ions, we will use Lewis dot structures to predict electronic and molecular geometries.
UNIT 4 β CHEMICAL BONDING AND MOLECULAR STRUCTURE Concept -VSEPR & VBT KNOWLEDGE One Mark Questions Q1. On the basis of VSEPR theory predict the shape of Hydronium (H 3O ) ion (L-1) Ans. p
This molecular shape corresponds to the VSEPR notation for this ion, AX3E . Practice Problem Use VSEPR theory to predict the molecular shape for each of the following: (a) HCN (b) SO 2
Lewis structures show the two-dimensional distribution of atoms and electrons. The molecular geometry, or three-dimensional shape of a molecule or polyatomic ion, can be determined using valence-shell electron-pair repulsion (abbreviated VSEPR and pronounced βVES-perβ) theory, in
Experiment 14 - Lewis Dot Structures & Geometry by VSEPR Theory I. PURPOSE In this exercise we will learn and practice the determination of Lewis dot structures of covalently bonded molecules (and ions) as a tool for predicting or explaining the number of bonds present, the number and location of bonding and non-bonding electrons
The Valence-Shell Electron Pair Repulsion Model The valence-shell electron pair repulsion (VSEPR) model predicts the shapes of molecules and ions by assuming that the . Determine how many electrons pairs are around the central atom. Count a multiple bond as one pair. 3. Arrange the electrons pairs are shown in Table 8.8.
Experiment Overview The purpose of this activity is to construct six models of VSEPR molecular geometrics from paper using origami folding techniques. Pre-Lab Questions (Use a separate sheet of paper to answer the questions.) 1. Write the Lewis electron-dot symbol for each of the following atoms: hydrogen, boron, nitrogen, silicon, sulfur, and bromine.
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