VSEPR Theory and Molecular Shapes

Dr. Walt Volland, All rights reserved 1998-2000

 

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-dimentional shape of the particle. Valence Shell Electron Repulsion Theory (VSEPR) is often used to predict particle shape from a Lewis dot formula. The VSEPR theory focuses on the idea that electron pairs and 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 molecule or ion. 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. 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 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 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.

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.

number of electron groups	2	3	4
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. 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.

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

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.

 

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.

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.

 

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.   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.

Play-doh Models of Electron-pair Geometries and of Molecules

Make a model of the linear 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; 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".

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 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".

Keep your models of molecules until you are finished with the activities on your report sheet. You will need them to check shapes

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.

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

Molecules with single bonds

	Methane, CH4	Ammonia, NH3	Water, H2O	Hydrogen Fluoride, HF
Number of valence electrons around central atom	___________	___________	___________	___________
# 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	___________	___________	___________	___________

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

	molecule shape
Hydrogen Sulfide, H2S	___________
Hydrogen Chloride, HCl	___________
Phosphine, PH3	___________
Silane, SiH4	___________

Molecules with both single and multiple bonds

	Carbon Dioxide, CO2	Formaldehyde, H2CO	Hydrogen Cyanide, HCN
Number of valence electrons around central atom	___________	___________	___________
# 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, a molecule with two central atoms

Name the geometry of the electron pairs around either C atom in Ethylene, C2H4.	____________________
Name the shape of the molecule Ethylene, C2H4.	____________________
Name the shape of the molecule Acetylene, C2H2.	____________________

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