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Predicting Molecular Shapes using models and  VSEPR

Background

Electron groups

Molecule Shapes

Pladoh or Marshmallow Models

Paper Models

 

Triangular Planar Trigonal Pyramid Tetrahedral
Molecules with single bonds Molecules with multiple bonds Molecules with 2 central atoms

AXE

AX4E0AX3E1, AX2E2, AX1E3,

 

ABE,  where  X =  B

AB4E0AB3E1, AB2E2, AB1E3

Report Sheet preparation for equiz

Dr. Walt Volland All rights revised© July 9, 2013 

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. Click to view a toothpick & pladoh model of a tetrahedral shape.

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(groups) repel one another to achieve a minimum in repulsion energy between the electrons. Click here for in depth VSEPR content.

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" . The lone pairs of electrons not used for bonding. We also know that a structure 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. This is the reason the central atom is bonded. The central atom in the structure now has an octet around it, not just the original set of valence electrons.

In the VSEPR approach molecule shapes, you focus on two things a

central atom and electron groups around it .

  1. the central atom, A
  2. the number of different electron groups(bonded atoms, X,
  3. and lone pairs, E) around the central atom return to top

Representations of the molecule use the symbol , AXnEm, or ABnEm

where A = central atom, X = atoms joined to A by bonds, E = lone pairs on A. The subscript n equals the count of bonded atoms and the subscript m equals the count of unshared pairs. The sum of  m and n  tells the number of electron clouds or groups around the central atom. This helps us predict the shape around the central atom.

 

The arrangement in space (geometry ) of the electron groups around a center atom controls the overall shape because all bonds and unshared pairs radiate out from the central atom.

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 central atom in a structure 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 central atom only.return to top

 

number of electron groups ----------------->

 

2

 

3

 

4

click for pladoh model

 

name of geometry of electron groups

 

Linear

 

Triangular planar

 

Tetrahedral

 

sketch of geometry -- electron groups represented by arrows

 

 

 

Format in AXnEm

 

AX2E0

 

AX3E0

 

AX4E0

 

 

ideal angles between electron groups

 

180o

 

120o

 

109.5o

Molecule Shapes

The name for the overall shape of a particle may not be the same as the name for the geometry of its electron groups. This is an important because the shape is dictated by the positions of both unshared electron pairs and electrons attaching atoms to the central atom .

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

AX2 or AX2E0

linear

180o

 

*All diatomic or 2-atom molecules are linear regardless of the number of electron groups around the "central" atom. return to top

 

Geometry of electron groups

Molecule shapes possible

Ideal bond angles

 

 

Appearance

 

name of molecule shape

 

 

Trigonal planar

AX3 or AX3E0

 

 

triangular planar

 

120o

 AX2E1

 

 

angular

 

120o

AX1E2

 

 

linear

 

120o

 

Tetrahedral

AX4E0

 

 

tetrahedral

 

109.5o

 AX3E1

 

 

pyramidal

 

109.5o

 AX2E2

 

 

angular, bent

 

109.5o

 AX1E3

 

 

linear

 

109.5o

 

Paper Models

 

Please read all these directions before doing any cutting.

 
Materials list

Printed copies of templates in this exercise

tape

scissorsreturn 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. AX3 or AX3E0

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 AX4E0click for pladoh model

 

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

 

 

Trigonal Pyramid Shape AX3E1 click for playdoh model

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.

 

 

 

 

 

 

 

Models of Electron-pair Geometries & Molecules

 

  
Materials list

Pladoh™, 2 cans Use 2 different colors of Pladoh

If you do not want to use Pladoh you can substitute marshmallows, gum drops, styrofoam balls or similar materials.

toothpicks return to top

 

Three dimensional models can be made using toothpicks, marshmallows or 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 10 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 16 or 20 of these spheres. The large number of balls (spheres) are needed if all the models are kept during the procedure.

 

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 model 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.

Thus the two electron clouds contained in a simple triatomic molecule AX2 will extend out in opposite directions; an angular separation of 180° places the two bonding orbitals as far away from each other they can get. 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 model 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 model with an angular or bent 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 model 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. AX4

  •  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 109 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). click for pladoh tetrahedron.
  • Repeat until you have made 8 of these models; you will use 4 now and the other 4 later.

 

Make a molecule model with a tetrahedral shape, AX4.click for AX4 pladoh mode

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, X, you would get a tetrahedron. The central atom, A, 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" . This is also represented by identifying the central atom represented by A and the attached atoms by X the representation AX4.

 

Make a molecule model with a triangular pyramid shape AX3E1

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 model with an angular shape or bent shape. AX2E2

 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 model with a linear shape. AX1E3

  • 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.

 

 

 

Keep your molecule models until you are finished with the activities on your report sheet. You will need these models for reference .

 Applying VSEPR to Real Molecules

 annimation-ammonia-AXE

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:

  1. Write the Lewis formula.
  2. Sketch the geometry of the electron pairs around the central atom, using lines to represent pairs of electrons.
  3. Name this geometry.Use one of the "extra" geometry models and your supply of "extra" small spheres to build a model of the molecule.
  4. Sketch this model of the molecule.
  5. 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 Both Multiple Bonds and Single 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 work sheet using your models and sketches. The report is submitted using the equiz in Angel.  The work sheet is supposed to help complete the equiz.

 

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 based on the models and examples above? return to top

compound

shape (bent, linear, tetrahedral, trigonal pyramidal)

AXnEm type (AX4E0, AX3E1, AX2E2, AX1E3)

Hydrogen Sulfide, H2S

___________

___________

Hydrogen Chloride, HCl

___________

___________

Phosphine, PH3

___________

___________

Silane, SiH4

___________

___________

 

Molecules with both single and multiple bonds

Nitrogen oxide, NO

Carbon Dioxide, CO2

Formaldehyde, H2CO

Hydrogen Cyanide, HCN

Total # valence electrons in the molecule

 

___________

 

___________

 

___________

 

___________

AXE classification of central atom

 

___________

 

___________

 

___________

 

___________

Total 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 linear, tetrahedral, planar, bent, etc

 

 ___________

 

 ___________

 

 ___________

 

 ___________

 

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.

ethylene

Justify your answer.

 

____________________

Describe the overall shape of the molecule Ethylene, C2H4. Linear, tetrahedral, planar, bent, etc. Justify your answer.

____________________

Name the geometry of the electron pairs(groups) around each of the individual C atoms in Acetylene, C2H2.

acetylene

Justify your answer.

____________________

Describe the overall shape of the molecule Acetylene, C2H2. Linear, tetrahedral, planar, bent, etc. Justify your answer.

____________________

All rights reserved revised July 9, 2013 return to top

Dr. Walt Volland

 

 

 

vsepr tetrahedral-pladoh-models