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# 11.4: The Geometrical Arrangement of Electrons and Molecular Shape

Difficulty Level: At Grade Created by: CK-12

## Student Behavioral Objectives

• The students will determine whether bonds are polar or non-polar.
• The students will determine whether simple molecules are polar or non-polar.

## Timing, Standards, Activities

Timing and California Standards
Lesson Number of 60 min periods CA Standards
The Geometrical Arrangement of Electrons and Molecular Shape 1.0 2f

### Activities for Lesson 4

Laboratory Activities

1. None

Demonstrations

1. None

Worksheets

1. Molecular Geometry Worksheet

1.Chelates

## Answers for The Geometrical Arrangement of Electrons and Molecular Shape (L4) Review Questions

• Sample answers to these questions are available upon request. Please send an email to teachers-requests@ck12.org to request sample answers.

## Multimedia Resources for Chapter 11

This website provides a lesson on Chemical Bonding.

This website provides a Polar Bears and Penguins Bonding Activity.

This website provides examples of polar and non-polar molecules.

This website provides a tutorial on drawing resonance structures.

This website provides an animation showing ionic and covalent bonding.

This website provides an introduction to drawing Lewis electron dot symbols.

The learner.org website allows users to view the Annenberg series of chemistry videos. You are required to register before you can watch the videos, but there is no charge to register. The video called “Chemical Bonds” explains the differences between ionic and covalent bonds using models and examples from nature.

This animation explores the differences between ionic and covalent bonding.

The website below provides a guide to drawing Lewis structures.

This website reviews the rules for naming binary covalent compounds and includes a quiz to test your understanding of these rules.

For an introduction to polar bonds see these two videos. The first video defines polar bonds. The second one consists of lecture and animation about polar bonds and polar molecules.

This animation reviews the differences in ionic, covalent, and polar covalent bonds.

The following websites provide more information on electron promotion and orbital hybridization.

This video is a ChemStudy film called “Shapes and Polarities of Molecules.”

An animation showing the molecular shapes that are generated by sharing various numbers of electron pairs around the central atom (includes shapes when some pairs of electrons are non-shared pairs).

The learner.org website allows users to view the Annenberg series of chemistry videos. You are required to register before you can watch the videos, but there is no charge to register. The video called “Molecular Architecture” is related to this lesson.

This website reviews how to predict molecular structure by using the VSEPR theory.

## Teacher's Resource Page for Molecular Models and Shapes

Lab Notes:

The molecular model kits must contain the 5-hole ball (trigonal bipyramidal) and the 6-hole ball (octahedral). Model kits containing these are available from Arbor Scientific.

You will need 1 kit for each student or pair of students.

If you wish to have complete sets for many years, you will need to supervise students closely. Small parts are easily lost. You may wish to check the kits in and out or assign the task to a lab assistant.

Formula Electron-Dot Formula
$PF_3$
$SF_6$
$ICl_3$
$HBr$
$H_2Se$

## Molecular Models and Shapes Laboratory

Background:

The shapes exhibited by molecules are often very difficult for beginning chemistry students to visualize, especially since most students' training in geometry is limited to plane geometry. To understand the geometric shapes exhibited by molecules, a course in solid geometry would be more useful. In this experiment, you will encounter some unfamiliar geometrical arrangements that will help you appreciate the complexity of molecular geometry.

The basic derivation and explanation of molecular shapes arises from the Valence Shell Electron Pair Repulsion theory, usually known by its abbreviation, VSEPR. This theory considers the environment of the most central atom in a molecule, and imagines first how the valence electron pairs of that central atom must be arranged in three-dimensional place around the atom to minimize repulsion among the electron pairs. The general principle is: for a given number of pairs of electrons, the pairs will be oriented in three-dimensional space to be as far away from each other as possible. For example, if a central atom were to have only two pairs of electrons around it, the electron pairs would be expected to be $180^\circ$ from each other.

The VSEPR theory then also considers which pairs of electrons are bonding pairs (have atoms attached) and which are non-bonding pairs (lone pairs). The overall shape of the molecule as a whole is determined by how many pairs of electrons are around the central atom (electronic geometry), and how many of these pairs are bonding pairs (molecular geometry). A simple example that clearly makes this distinction concerns the case in which the central atom has four valence electron pairs. Consider the Lewis structures of the following four molecules: hydrogen chloride, $HCl$; water, $H_2O$; ammonia, $NH_3$; and methane, $CH_4$. The central atom of each of these molecules is surrounded by four pairs of valence electrons. According to the VSEPR theory, these four pairs of electrons will be oriented in three-dimensional space to be as far away from each other as possible. The four pairs of electrons point to the corners of the geometrical figure known as a tetrahedron. The four pairs of electrons are said to be tetrahedrally oriented, and are separated by angles of approximately $109.5^\circ$. In all four cases, the electronic geometry is tetrahedral but the overall molecular geometry is tetrahedral for only one of the four molecules.

In the case of $HCl$, even though there are four pairs of electrons around the chlorine atom, three of them are not shared - there is no atom bonded by them. These spaces are empty. Since there are only two atoms joined by a bond (a pair of electrons), the molecular geometry will be linear. For $H_2O$, there are two shared pairs of electrons and two unshared pairs. The molecular geometry of this molecule will be angular (or bent). In $NH_3$, three of the electrons are shared and one is unshared. This molecular geometry will be pyramidal. In the final molecule, $CH_4$, all four pairs of electrons are shared and so the molecular geometry matches the electronic geometry and is tetrahedral.

Pre-Lab Questions

Draw Lewis structures and predict bond angles for the following molecules.

$PF_3$

$SF_6$

$ICl_3$

$HBr$

$H_2Se$

Purpose:

Models built according to the predictions of the VSEPR theory illustrate the reular patterns of molecular shapes.

Apparatus and Materials:

• 1 molecular model kit for each lab group
• 1 protractor for each lab group

Procedure:

Build each of the 19 models indicated in the following table of geometries. Measure all bond angles in your models. Sketch each of your models including bonding and non-bonding electron pairs and bond angles. See if you can find real molecules that would be expected to have each shape. (There are 5 models for which no real molecule is known.) You may need to sort through your textbook to find the examples.

Table of Electronic Geometries
Pairs on Central Atom Electronic Geometry Bonding Pairs Molecular Geometry Formula Type
1. 2 linear 2 linear $AB_2$
2. 3 trigonal planar 1 linear $AB$*
3. 3 trigonal planar 2 bent $AB_2$*
4. 3 trigonal planar 3 trigonal planar $AB_3$
5. 4 tetrahedral 1 linear $AB$
6. 4 tetrahedral 2 bent $AB_2$
7. 4 tetrahedral 3 pyramidal $AB_3$
8. 4 tetrahedral 4 tetrahedral $AB_4$
9. 5 trigonal bipyramidal 1 linear $AB$
10. 5 trigonal bipyramidal 2 linear $AB_2$
11. 5 trigonal bipyramidal 3 T-shape $AB_3$
12. 5 trigonal bipyramidal 4 distorted tetrahedron $AB_4$
13. 5 trigonal bipyramidal 5 trigonal bipyramidal $AB_5$
14. 6 octahedral 1 linear $AB$*
15. 6 octahedral 2 linear $AB_2$*
16. 6 octahedral 3 T-shape $AB_3$*
17. 6 octahedral 4 square planar $AB_4$
18. 6 octahedral 5 square pyramid $AB_5$
19. 6 octahedral 6 octahedral $AB_6$

*Indicates no molecules known.

Aug 18, 2012

Aug 13, 2014