EXPERIMENT 8

Molecular Models

Chemistry 120

Introduction

Models are often used by the chemist to visualize molecular structures. Molecular geometries can frequently influence chemical and physical properties, thus it is important to begin to recognize the way that atoms bond together and how they orient themselves in a molecule. In this experiment, you will build a variety of molecules using molecular models in order to become familiar with some of the more common geometries.

Lewis Structures

In 1916, G. N. Lewis developed a theory in which he proposed that atoms would bond together to share electrons so that all representative atoms (with the exception of H and He) would be surrounded by 8 valence electrons. Chemists still use that model when drawing Lewis Electron Dot structures for molecules. Your textbook outlines the procedure for drawing correct electron dot structures for common compounds. A few examples of correctly drawn Lewis structures are shown in figure 1: Remember that all non-bonding electron pairs must be shown in a correct Lewis Structure!

Molecular Geometries

Valence Shell Electron-Pair Repulsion theory or VSEPR is used to predict molecular geometries. VESPR theory proposes that the structure of a molecule is determined by the repulsive interaction of electron pairs in the valence shell if its central atom. In other words, the bonding pairs and the non-bonding (lone) pairs around a given atom are as far apart as possible. For example, methane CH₄, has four bonding electron pairs around the carbon atom. In order to maximize the distance between these bonding electrons (and minimize the repulsive forces) the hydrogen atoms will orient themselves forming a tetrahedron with 109.5^o bond angles as shown in figure 2.

If pairs of non-bonding, lone pairs also surround an atom they will also require space. An example of this is shown in both ammonia and water where the angles between the four sets of electron pairs are still approximately 109 as seen in figure 3.

A summary of the orbital geometries, molecular geometries, and approximate bond angles is illustrated in the table on the next page.

Valence shell electron pairs	Bonding electron pairs	Lone pairs		Orbital geometry	Approximate bond angles	Molecular geometry
2	2	0		Linear	180	Linear
3	3	0		Trigonal Planar	120	Trigonal Planar
	2	1				Bent
4	4	0		Tetrahedral	109	Tetrahedral
	3	1				Trigonal Pyramidal
	2	2				Bent

Ions

If you need to draw the Lewis Electron Dot structures for ions, add or subtract valence electrons based upon the charge on the molecule. For example OH- would have 6 valence electrons from the oxygen, 1 valence electron from the hydrogen, and 1 extra valence electron due to the charge, giving it a total of 8 valence electrons.

Procedure

Build and draw the molecules and ions listed in the tables on the following pages. Be sure to answer the questions regarding these molecules.

Molecular Models

Prelab Questions

1. How many valence electrons does an atom of nitrogen contain?

2. How many valence electrons are in a molecule of C₄H₆Br₂?

3. How many valence electrons are in a CH₃CO₂^-1 ion?

4. How many valence electrons are in a NH₃C₂H₅⁺¹ ion?

5. What is the abbreviation for the theory used to determine the geometry of molecules?

6. What is the orbital or electronic geometry of a molecule with 2 nonbonding electron pairs and 2 bonding electron pairs?

7. What is the orbital or electronic geometry of a molecule with 0 nonbonding electron pairs and 2 bonding electron pairs?

8. What is the molecular geometry of a molecule with 1 nonbonding electron pair and 2 bonding electron pairs?

9. What is the molecular geometry of a molecule with 1 nonbonding electron pair and 3 bonding electron pairs?

10. What is the approximate bond angle of a molecule with 1 nonbonding electron pair and 2 bonding electron pairs?

11. What is the approximate bond angle of a molecule with 0 nonbonding electron pair and 4 bonding electron pairs?

Molecular Models

Data Sheet

Lab Grade

Prelab	/11
Format/Signature (Models returned organized)	/17
Molecular model descriptions	/60
Post lab questions	/12
Total	/100

Fill in the sheet below for the molecules you build.

Molecule	CBr4	PH3	SiO2
Total # Valence Electrons
Lewis Structure
Number of Bonding pairs of electrons around central
Number of Nonbonding Electron Pairs on central
Electronic or Orbital Geometry
Molecular Geometry (Name)
3-Dimensional Sketch with Bond Angles Note
Bond Angles

Molecule	C2H6	C2H4	C2H2
Total # Valence Electrons
Lewis Structure
Number of Bonding Electrons Pairs on central
Number of Nonbonding Electron Pairs on central
Electronic or Orbital Geometry
Molecular Geometry (Name)
3-Dimensional Sketch
Bond Angle

Molecule	HF	C2H5OH	SO3
Total # Valence Electrons
Lewis Structure
Number of Bonding Electrons Pairs on central
Number of nonbonding Electron Pairs on central
Electronic or Orbital Geometry
Molecular Geometry (Name)
3-Dimensional Sketch
Bond Angle

Molecule	H3O^+1	CO3^-2	NH2^-1
Total # Valence Electrons
Lewis Structure
Number of Bonding Electrons Pairs on central
Number of Nonbonding Electron Pairs on central
Electronic or Orbital Geometry
Molecular Geometry (Name)
3-Dimensional Sketch
Bond Angle

Post Lab Questions

1. Why are the valence electrons of an atom the only electrons likely to be involved in bonding to other atoms?

2. Why do representative elements tend to form bonds giving them a total of 8 valence electrons?

3. How is the structure around a given atom related to repulsion between valence electron pairs on the atom involved?

4. Why are all diatomic molecules linear, regardless of the number of valence electron pairs on the atoms involved?