Lesson 8.6 The Valence-Shell Electron-Pair Repulsion
(VSEPR) Model
Suggested Reading

Zumdahl Chapter 8 Section 8.13
Essential Question

What are the three-dimensional structures of molecules?
Learning Objective

Predict the shape and bond angles of molecules using the valence shell
electron pair repulsion (VSEPR) model.
Introduction
Molecular geometry is the general shape of a molecule, as determined by
the relative positions of the atomic nuclei. The valence shell electron pair
repulsion (VSEPR) model allows you to predict the molecular geometry of a
molecule or ion from Lewis structures. Keep in mind that VSEPR does not
explain chemical bonding, but it does help us to understand molecules after
bonds have formed.
The VSEPR Model
The valence shell electron pair repulsion (VSEPR) model predicts the
shapes of molecules by arranging valence-shell electron pairs (both
bonding and non-bonding) so that the electron pairs keep as far away from
one another as possible thereby minimizing electron-pair repulsions.
Watch this video about VSEPR to review the five basic electronic
geometries (Great video, don't skip!):
YouTube Video
https://www.youtube.com/watch?v=keHS-CASZfc
The basic electronic and molecular geometries for molecules with 2, 3, 4, 5,
and 6 regions of electron density around a central atom are shown below.
Please note that the number of regions of electron density are sometimes
referred to as steric number, as shown below.
Molecular versus Electronic Geometry
In the chart above there is more than one geometry for each steric number
greater than two. The different geometries are determined by the number of
lone pairs around the central atom. The electronic geometry is the shape the
molecule takes when both the bonding and nonbonding electrons are
considered. The molecular geometry is is the electronic geometry minus the
lone pairs, because the lone pairs are "invisible". The molecular geometry is
what is actually observed.
For example, consider ammonia (NH3)
Ammonia has a steric number of 4, so there are four regions of electron
density around the central atom; three bonding pairs and one lone pair.
The electronic geometry is tetrahedral. However, if we subtract the lone pair,
the molecular geometry is trigonal pyramidal. When predicting the molecular
geometry, be careful not to ignore the lone pair. In this example, ignoring
the lone pair would result in a trigonal planar molecular geometry, which is
not correct!
Notice that the bond angles of the molecular geometry are slightly less than
that of the tetrahedral molecular geometry. This reflects that fact that lone
pairs are more repulsive than bonded pairs.
The following chart may help you to better visualize the differences between
molecular and electronic geometry for each steric number.
Predicting Molecular Geometry Using the VSEPR
Model
1.
2.
3.
4.
Write the Lewis structure.
Determine how many electron pairs are around the central atom. Count
a multiple bond as one one pair. If resonance occurs, use one
resonance formula to determine this number.
Determine the electronic geometry that corresponds to the number of
electron pair, and arrange the electron pair accordingly.
Obtain the molecular geometry by "subtracting" the lone pairs.
Example: Predicting Molecular Geometries
Predict the geometry of the following molecules or ions,
using VSEPR: a) BeCl2 b) NO2Solution
a)
BeCl
2
Step 1: Write the Lewis structure (Recall that Be can have less than an octet while Cl always obeys the octet rule).
Step 2: Determine the number of electron pairs around the
central atom.
2
Step 3: Determine the electron geometry.
Linear
Step 4: Subtract the lone pairs to determine the molecular
geometry.
There are no lone pairs, so the molecular geometry is also
linear.
b) NO
2
Step 1: Write the Lewis structure.
Step 2: Using only one resonance structure, determine the
number of electron pairs around the central atom counting
the multiple bond as one pair.
3
Step 3: Determine the electron geometry.
Trigonal planar
Step 4: Subtract the lone pairs to determine the molecular
geometry.
Bent
HOMEWORK: Practice exercises 9.1 & 9.2, book questions page 386 questions
91-98 **you do not have to include bond angles in your drawings.