Predicting Molecular Shapes using models and  VSEPR

Background	Electron groups	Molecule Shapes	Paper Models
Pladoh Models	Triangular Planar	Trigonal Pyramid	Tetrahedral
Molecules with single bonds	Molecules with multiple bonds	Molecules with two central atoms

Report Sheet

Dr. Walt Volland All rights revised© May 9, 2002

 

This exercise is aimed at using Lewis dot structures and building molecular models to predict the shapes for small molecules and fragments that are building blocks for large molecules like DNA, RNA, proteins, fats, and carbohydrates. The basic model used in this exercise is the Valence Shell Electron Pair Repulsion Theory. It is essential to "build" the models so you have a 3D experience with these shapes. Two-dimensional Lewis dot formulas help us understand the bonding within a molecule or polyatomic ion, but they do not give us a sense of the 3-dimensional shape of the particle. Valence Shell Electron Repulsion Theory (VSEPR) is often used to predict particle shape from a Lewis dot formula. The principle is that the electron pairs repel one another to achieve a minimum in repulsion energy between the electrons. The VSEPR theory focuses on the idea that electrons repel one another and that these repulsions are smallest when the electron pairs or groups of electron pairs are as far apart as possible. This will then be the most stable form or shape of a particle. Electron groups in the VSEPR model are lone pairs, single bonds, double bonds, triple bonds and single electrons as in NO. The No molecule is an exception to the octet rule. There are only 11 valence electrons in the molecule and the nitrogen cannot "get" an octet. The oxygen has a higher electronegativity and does form an octet. We know from a study of Lewis formulas that molecules and polyatomic ions may contain single bonds, double bonds, triple bonds, and "lone pairs" of electrons that are not used for bonding. We also know that a particle contains one or more "central atoms" around which the rest of the atoms are arranged; we know that the rest of the atoms are bonded either directly or through other atoms to this center atom. Remember the central atoms in molecules usually have attained an octet of valence electrons in the molecule. This is the reason the central atom is bonded. It now has an octet around it, not just the original set of valence electrons. In the VSEPR theory approach to particle shapes, you focus on two things. the central atom the number of different electron groups around the central atom return to top

The arrangement in space (geometry ) of the electron groups around a center atom controls the overall shape of a particle because all bonds radiate out from the central atom of the particle. An electron group may be 1 pair of electrons (single bond or lone pair), 2 pairs (double bond) or 3 pairs (triple bond). The carbonate ion, for example, has one double bond and two single bonds attached to the center carbon atom. Thus, there are 3 groups of electrons around the C even though there are 4 pairs (an octet) of electrons on carbon. Two pairs of electrons point in the same direction, the double bond to O. The other two pairs go in two other directions, one pair to each remaining O. One double bond and two single bonds on the center atom are considered to be 3 electron groups. Remember the negative two charge is distributed over the whole ion. return to top

The VSEPR theory table below refers to electron groups around the center atom in a particle. There is a descriptive name for each electron group geometry or arrangement of the electron pairs around the center atom. The sketch indicates the electron groups around the center atom only.return to top

number of electron groups	2	3	4click for pladoh model
name of geometry of electron groups	Linear	Triangular planar	Tetrahedral
sketch of geometry -- electron groups represented by arrows
ideal angles between electron groups	180o	120o	109.5o

Molecule Shapes

The name for the overall shape of a particle may or may not be the same as the name for the geometry of its electron groups. This is an important fact because the shape is dictated by the positions of both unshared electron pairs and electrons in bonds. If all electron pairs on the center atom are used for single bonds, then the overall shape of the molecule or ion has the same name as the electron group geometry around the center atom.  If there are lone pairs, a double bond, or a triple bond on the center atom, then the name for the overall particle shape is different from the name of the electron group geometry. return to top

Geometry of electron groups	Molecule shapes* possible		Ideal bond angles
	Appearance	name of molecule shape
Linear		linear	180o

*All geometries have a possible molecule shape that is linear. All diatomic or 2-atom molecules are linear regardless of the number of electron groups. return to top

Geometry of electron groups	Molecule shapes possible		Ideal bond angles
	Appearance	name of molecule shape
Trigonal planar		triangular planar	120o
		angular	120o
		linear	120o
Tetrahedral		tetrahedral	109.5o
		pyramidal	109.5o
		angular	109.5o
		linear	109.5o

Paper Models

 

Materials list

Printed copies of templates in this exercise tape scissors return to top

Use a ruler and a ball-point pen to scribe the lines that mark where folds need to be made. You do the scribing by lining up the ruler along the fold line and running the ball point pen tip along the printed lines. This "etches" the paper. Scribing the edges makes it easier to have the right positions for the folds. Cut out each paper model. Remember, do not cut off the black lines. return to top

Triangular Planar Shape

 

Cut out the planar triangle. No folding is needed since this shape is flat. Boron trifluoride, BF3, is an example of the planar triangular shape. Boron, unlike most nonmetals, often has only 6 electrons in its valence shell, giving it only 3 pairs instead of 4. return to top

Tetrahedral Shape

Be careful to keep the A, B, and D tabs on the template when you cut out the tetrahedron. They will be folded against a corresponding face and taped down to maintain the shape of the model. Be sure to leave the black edges on the faces. Write your name on the line provided. Hold the cutout so you can read your name. Fold faces A, B, and D away from you. Fold tab B over face B and secure tab with transparent tape. Fold tab A over face A and secure with tape. Likewise, fold tab D over face D and secure with tape. You now have a paper model of a tetrahedron. Carbon tetrachloride, CCl4, is a molecule shaped like a tetrahedron. It has a chlorine atom at each of the four points of the tetrahedron. A carbon atom is in the center of the tetrahedron. In your model of the tetrahedron, the C atom would be hidden inside the paper model. The bonds from C to each Cl are also hidden inside. return to top

 

Write your name on the line. Hold the cutout so you can read your name. Fold faces A and B away from you. Hold face C up so you can read it. Fold the tab on face B over face A. Secure the tab and edges with transparent tape. You now have your trigonal pyramid molecule shape. return to top   The molecule, NCl3, has a trigonal pyramid shape. The nitrogen is at the top of the pyramid. The central nitrogen atom has an octet with 3 pairs of electrons used for the three N-Cl bonds and the other two electrons in a lone pair.

Pladoh Models of Electron-pair Geometries and of Molecules

Three dimensional models can be made using toothpicks and spheres made from Playdoh. The spheres represent the atoms in the particle. The toothpicks represent the electron pairs around the central atom. Open a can of Playdoh and remove a piece that is about one inch in diameter. Roll the Playdoh around between the palms of you hands, making a circular motion with your palms. The lump will gradually roll into a sphere. Repeat this process to make a total of 16 Play-doh spheres of this color . Open the other can of Play-doh, take out a lump that is about 1/2 inch in diameter, and roll this into a sphere. Make 41 or 42 of these spheres.

Make a model of the linear geometry of electron groups around a central atom. return to top Take a large sphere of Playdoh and stick a toothpick into it on the left side. Stick a second toothpick into the sphere on its right side; try to place the toothpicks in a straight line. The toothpicks represent the 2 electron groups around the central atom (Play-doh sphere). Repeat to make a total of 5 of these. You will use 1 now and 4 later.

 

Make a molecule with a linear shape. Now stick a small sphere onto the open end of each toothpick. You should be able to imagine a straight line from one small sphere, through the large sphere, and on to the other small sphere. This molecule shape is called "linear". Note there is a"large" sphere in the middle. click to see photo of model

Make a model of the triangular planar geometry of electron groups around a central atom. Take a large sphere of Play-doh and stick a toothpick into it on the left side. Stick a second toothpick into the sphere on its right side; place the toothpicks so they make an angle of 120o with each other. Stick a third toothpick into the sphere at its top. Place this toothpick so that it is 120o away from each of the others. The 3 toothpicks should be level with each other, i.e. in the same flat plane. The toothpicks represent the 3 electron groups around the central atom (Play-doh sphere).   Repeat until you have made 6 models of this electron group geometry. You will use 3 now and the other 3 later.

 

Make a molecule with a triangular planar shape. Now stick a small sphere onto the open end of each toothpick. You should be able to imagine connecting the small spheres to form a flat triangle. This molecule shape is called "triangular planar".

 

Make a molecule with an angular shape.  Use your second model of the electron pair geometry. Stick a small sphere onto the open end of 2 of the toothpicks. If you drew a line from one small sphere to the central atom and then on to the other small sphere, you would get an angle or a bent line with an angle of 120o. This molecule shape is called "angular".

 

Make a molecule with a linear shape.  Use your remaining model of triangular planar electron pair geometry. Stick a small sphere onto the open end of 1 toothpick. You could only draw a straight line from this small sphere to the large sphere. This molecule shape is called "linear".

 

Make a model of the tetrahedral geometry of electron groups around a central atom.  Take a large sphere of Play-doh and stick a toothpick into it on the top. Stick 3 more toothpicks into the bottom part of the sphere so they form "legs" below the sphere. The toothpicks should be about 109š apart. You should have something that looks like a camera tripod when you are done. It should stand on a table or countertop so that the sphere is above the table. You will have to utilize all 3 dimensions to make this model. The toothpicks represent the 4 electron groups around the central atom (Play-doh sphere). Repeat until you have made 8 of these models; you will use 4 now and the other 4 later.

Make a molecule with a tetrahedral shape. Using your first model of the tetrahedral electron pair geometry, stick a small sphere onto the open end of each toothpick. If you connected the small spheres, you would get a tetrahedron. The central atom and the bonds would be inside this tetrahedron. If you drew a line from one small sphere to the central atom and then on to another small sphere, you would have drawn an angle of 109.5o. This molecule shape is called "tetrahedral".

 

Make a molecule with a triangular pyramid shape. Stick a small sphere onto the open end of 3 toothpicks. If you connected the small spheres, you would get a triangle with 3 equal sides. If you then connected each small sphere to the big sphere, you would get a pyramid with this triangle for its base. The big sphere would be at the top of the pyramid. Notice that the pyramid molecule is not the same as the tetrahedral molecule. This molecule shape is a "triangular pyramid". There is one lone pair pointing to one of the corners of the tetrahedron.

Make a molecule with an angular shape.  Now stick a small sphere onto the open end of 2 of the toothpicks. If you drew a line from one small sphere to the central atom and then on to the other small sphere, you would get an angle or a bent line; the size of this angle is 109.5o. This molecule shape is called "angular". There are two lone pairs pointing to two of the corners of the tetrahedron.

Make a molecule with a linear shape. Stick a small sphere onto the open end of 1 toothpick. You could only draw a straight line from this small sphere to the large sphere. This molecule shape is called "linear". There are three lone pairs pointing to three of the corners of the tetrahedron.

 Applying VSEPR to Real Molecules

Your observations from the models of these molecules are needed to answer the questions in the report sheet. For each of the molecules listed below: Write the Lewis formula. Sketch the geometry of the electron pairs around the central atom, using lines to represent pairs of electrons. Name this geometry. Use one of the "extra" geometry models and your supply of "extra" small spheres to build a model of the molecule. Sketch this model of the molecule. Name this molecule shape and answer the questions on the report sheet.return to top

Hydrogen Compounds of C, N, O, and F

Methane, CH4

Ammonia, NH3

Water, H2O

Hydrogen Fluoride, HF

Compounds With Multiple Bonds

Carbon Dioxide, CO2	Ethylene, C2H4
Formaldehyde, H2CO	Acetylene, C2H2
Hydrogen Cyanide, HCN	nitric oxide, NO

When you are finished with the activities listed your report sheet, stuff the Play-doh™ back into its original containers and reclose it tightly so the Play-doh™ does not dry out. Follow the storage directions on the container. Save the Play-doh™ for other experiments. return to top

 

Complete the following table and turn send in your responses by email. Use the models from the earlier activities to help match shapes with the formulas that follow. Report is due as described in syllabus

VSEPR Theory Report Sheet return to top

-----------------------------------------------------------------------------------------------	Name______________

 Molecules with only single bonds

	Methane, CH4	Ammonia, NH3	Water, H2O	Hydrogen Fluoride, HF
Number of valence electrons in the molecule	___________	___________	___________	___________
Number of valence electrons around central atom in the molecule	___________	___________	___________	___________
# of single bonds on central atom	___________	___________	___________	___________
# of lone pairs on central atom	___________	___________	___________	___________
# of double bonds on central atom	___________	___________	___________	___________
# of triple bonds on central atom	___________	___________	___________	___________
# of electron groups on central atom	___________	___________	___________	___________
Name of geometry of electron pairs	___________	___________	___________	___________
Name of molecule shape	___________	___________	___________	___________

What molecule shape do you expect for each compound listed below? return to top

compound	molecule shape (bent, linear, tetrahedral, trigonal pyramidal)
Hydrogen Sulfide, H2S	___________
Hydrogen Chloride, HCl	___________
Phosphine, PH3	___________
Silane, SiH4	___________

Molecules with multiple bonds

	Nitrogen oxide, NO	Carbon Dioxide, CO2	Formaldehyde, H2CO	Hydrogen Cyanide, HCN
Number of valence electrons in the molecule	___________	___________	___________	___________
Number of valence electrons in the molecule	___________	___________	___________	___________
Number of valence electrons around central atom in the molecule	___________	___________	___________	___________
# of single bonds on central atom	___________	___________	___________	___________
# of double bonds on central atom	___________	___________	___________	___________
# of triple bonds on central atom	___________	___________	___________	___________
# of electron groups on central atom	___________	___________	___________	___________
Name of geometry of electron pairs	___________	___________	___________	___________
Name of molecule shape	___________	___________	___________	___________

Ethylene and acetylene molecules with two central atoms

Name the geometry of the electron pairs(groups) around each of the individual C atoms in Ethylene, C2H4.	____________________
Describe the overall shape of the molecule Ethylene, C2H4. Linear, tetrahedral, planar, bent, etc. Justify your answer.	____________________
Describe the overall shape of the molecule Acetylene, C2H2. Linear, tetrahedral, planar, bent, etc. Justify your answer.	____________________

All rights reserved revised May 9, 2002 return to top

Dr. Walt Volland