Shapes of Covalent Molecules

Description

To practice drawing Lewis structures for molecules, predicting shapes from these drawings, and building appropriate models.

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• All of the molecules studied in this experiment obey the octet rule, and are ones for which suitable Lewis structures are easily drawn.
• In octet-rule molecules, eight electrons on each atom are arranged in four pairs. Each pair of electrons is present either in a covalent bond or as a non-bonding pair.
• The electron pair repulsion rule will guide the decisions regarding structure that will be made for the molecules studied here. According to this rule, the regions of electron density around an atom move as far apart from one another as possible. The region of electron density means a single bond, a double or triple bond, or an unshared electron pair.
• If all of the four electron pairs around an atom are in single bonds, they are oriented toward the corners of a tetrahedron centered on the atom. In this arrangement the electron pairs are as far away from each other as possible, and so satisfy the electron pair repulsion rule.
• The description of the geometry of a molecule is made on the basis of the positions of its atoms, not its electrons. Suppose a molecule contains four single bonds each consisting of one shared electron pair. The arrangement in which the bonding electron pairs are as far apart as possible puts the bonded atoms on the corners of a tetrahedron. For this reason, the shape is said to be tetrahedral.
• If the central atom is bonded to three atoms, as in NH₃, three of the four pairs around the central atom are used in bonds, and the fourth pair is non-bonding. The NH₃ molecule is not said to be tetrahedral but, since no atom completes the tetrahedron; the molecule is said to be pyramidal. In a similar way, we conclude that the H₂O molecule is bent, and the HF molecule is linear.

  Regions	Example	Angle	Name of Shape
  4	CCl4	109.5	tetrahedral
  4	PCl3	~109.5	pyramidal
  4	SCl2	~109.5	bent
  3	BCl3	120	planar
  3	SnCl2	~120	bent
  2	BeCl2	180	linear

• A double or triple bond is counted as one region of electron density.
• From the geometry of a model one can predict whether a molecule is polar or nonpolar. This is done by examining the molecule for symmetry. Formal rules can be written down for this inspection, but the easiest way for beginners is to see if every polar bond is, somehow, counterbalanced by another identical bond in the molecule.
• A bond between unlike atoms will always be polar; it will have a positive end and a negative end.
• A diatomic molecule containing two different atoms will be polar. The HF, CO, and ICl molecules are all polar.
• N₂ and O₂ are nonpolar because both ends of these molecules are equivalent.
• A polyatomic molecule may be nonpolar even though it contains polar bonds. In such a molecule, the charge distributions in the bonds interact in such a way that the molecule has no + and - ends even though the bonds do. The CH₄ molecule is nonpolar because of this effect.

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Do NOT throw, roll, or damage the spheres. Follow routine laboratory precautions.

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1. Write the formula, and draw the Lewis dot structure for each of the following molecules: CH₄, H₃O⁺, N₂, C₂H₂, CH₂Cl₂, HF, Cl₂, SO₂, CH₄O, NH₃, C₂H₄, SO₄^2-, H₂O, H₂O₂, CH₂O, CO₂. Use the handout if available.
2. Build a template for the tetrahedral shape. From plane geometry one calculates that the distance between any two tetrahedral points on the surface of a sphere is given by the formula 0.82D where D is the diameter of the sphere.
3. Mark a point on a sphere. Set a compass to 0.82D for that sphere. The compass must be tight and able to hold this distance. Place the compass point on that point, and sketch a circle on the sphere. Mark a second point on the circle. Place the compass point there, and, using the same compass setting, mark two additional points on the circle. There are now four points on the sphere.
4. Holding the sphere so that a point is on top, and inserting a toothpick (bond) vertically for the point will ensure that all of the toothpicks (bonds) are at tetrahedral angles.
5. The teacher should build templates for a planar shape. Fold a piece of paper in half. Fold the paper again so that three equally sized sectors result. Open the paper, and make alternate fold lines. These are at 120° apart.
6. Build a model of the first molecule on the list given. When that model is finished, bring it to the instructor with your data table. It will be graded.
7. After the model is checked, fill in each column on the data table for the model. Then, take it apart and work on a model for the next molecule. Each time, take it to the instructor for grading. A movie is available for self-testing.
8. Makeup students should ask the instructor for models. The movie may be used to check your models. Or, if models are not available, try to identify each model of a molecule in the movie from the list below and then go to the next slide to check.

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Name _____________________________ Class _______

Teacher______________________________

DoChem 038 Shapes of Covalent Molecules

Makeup students should ask the instructor for models. The movie may be used to check your models.

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Purpose

• To predict shapes, bond angles, and polarities of some molecules.
• To build models of some simple molecules.

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(per ten students)

• 10 student set of ball and stick models, or styrofoam balls of various sizes and colors (about 40 3-inch balls and 75 2-inch balls. Use the dense type of styrofoam spheres; this materials holds up better in the student laboratory.)
• 200 pipe cleaner or toothpick
• 200 map tack, various colors
• 10 compass
• 10 ruler
• 10 model building set (optional)
• 10 sheet of paper

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• Assign the students the task of completing Lewis structures for homework the night before the lab.
• Fill a box or tray at each lab desk with about 8 3-inch balls and about 15 2-inch balls.
• Have the students bring the models up to you one at a time. Grade each one individually. If the first molecular model is not correct, give the student a second chance to complete the structure successfully. Have a model set with you in order to show students the correct shapes.

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Teacher preparation: 5 minutes

Class Time: 40-50 minutes

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There are no unusual hazards in this experiment.

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Remind students not to throw, roll, or damage the spheres. Follow routine laboratory precautions.

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Store materials for use in future years.

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Closure Questions:

1. Describe the polarity of the H₂O₂ molecule as rotation occurs around the oxygen-oxygen bond.
2. CO₂ is nonpolar, but SO₂ is polar. Explain.
3. Describe the electronic similarities between NH₃ and H₃O⁺.

Answers to Closure Questions:

1. When all four atoms are coplanar and the hydrogens are as far apart as possible, the H₂O₂ molecule is nonpolar. All other arrangements of the molecule are polar. (Different forms like this are called conformations.)
2. In CO₂ there are only 2 regions of electron density, both double bonds. The symmetrical linear model which results from these double bonds is nonpolar. In SO₂ there are 3 regions of electron density: a single bond, a double bond, and an unshared electron pair. The 3 regions force a bent shape on the molecule. The bent shaped moledule is polar.
3. Electronically, NH₃ and H₃O⁺ are identical. These structures are said to be isoelectronic. The only difference is in the number of protons inside the nucleus of the central atom. This difference is apparent in the volume occupied by these molecules. H₃O⁺ has a larger charge due to a larger number of protons and, therefore, would have a smaller volume than NH₃.

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• bond
• Lewis structure
• Lewis formula
• octet
• single bond
• double bond
• triple bond
• bond angle
• tetrahedral
• planar
• pyramidal
• bent
• linear
• polar
• nonpolar
• isoelectronic

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