Molecularity: Covalent Molecules

1 lab period; work in pairs. Complete the Preparation page before laboratory.

Goals

• To enable you to predict and visualize molecular shapes

The three-dimensional arrangement of atoms in a molecule--its shape--determines virtually all of the properties of not only the molecule, but of the substance that it represents. Thus the shape adopted by the molecules of a substance determines the melting and boiling points, solubility, strength, and reactivity of the substance.

Molecular shape is a consequence of the distribution of electrons in a molecule. It would seem to be a difficult matter to predict the distribution of electrons, and indeed to do so precisely requires sophisticated experimental and theoretical methods. However, it turns out to be quite simple to predict qualitatively how the electrons in a molecule will be arranged using a simple idea called the Valence Shell Electron Pair Repulsion Theory (VSEPR). The essential idea of the theory is that electron groups in the valence shell of an atom distribute to minimize repulsions between them. An electron group is any of the following things:

• a nonbonding electron pair
• a single bond
• a double bond
• a triple bond
• a single electron (rare)

Thus a double bond (triple bond) counts as one electron group even though it consists of two (three) pairs of electrons. To apply VSEPR requires only that we be able to generate a valid Lewis structure for the molecule, which is simple to do. Once the Lewis structure is available, the electron groups can be counted, and the distribution of groups about any chosen atom can be predicted using Table 1.

The purpose of this workshop is to familiarize you with the relatively limited number of molecular shapes by giving you an opportunity to build models.

Focus Questions are included in the Experimental Procedure, below.

A good commercial molecular model kit capable of simulating molecules in which the central atom has up to six electron groups.

Experimental

Some guidelines for building structures:

• First, short fat sticks are for single bonds, long thin sticks are for double bonds. To make a double bond between 2 atoms, use 2 long thin sticks and 2 adjacent holes on each atom; to make a triple bond, use 3 long thin sticks and 3 adjacent holes on each atom.
• Second, the ball representing the central atom must have a number of holes consistent with the number and nature of electron groups around the central atom in the Lewis structure. For example
  • If the Lewis structure shows 4 electron groups consisting of 3 single bonds and 1 lone pair, the central atom ball must have 4 holes;
  • If the Lewis structure shows 3 electron groups consisting of 2 single bonds and 1 double bond, the central atom ball must have 4 holes, because 2 holes will be used to make the double bond.
  • If the Lewis structure shows 3 electron groups consisting of 3 single bonds, the central atom ball must have 3 holes arranged in an equilateral triangle. A brown ball (5 holes) must be used here; the top and bottom holes remain unused.
• Third, you should explicitly represent lone pairs in your models using short, fat grey sticks, usually used for single bonds.

Activities

1. Develop Lewis structures for CH₄, NH₃, and H₂O; then build models for them. (Remember that you must explicitly put the lone electron pairs on oxygen in the water molecule, so even though the red balls are supposed to be oxygen, you can't use a red ball here because it has only 2 holes.) Make a stereochemical drawing of each molecule based on your model.
   • Describe the electron group distribution in each molecule.
   • Describe the shape of each molecule.
   • Predict the H-X-H bond angle for each molecule.
   
   
2. Draw a Lewis structure for ozone, O₃. Build a model, and make a stereochemical drawing.
   • Describe the electron group distribution in ozone.
   • Describe the shape of ozone.
   • What is the bond angle in ozone?
   • Based on your model, what would you predict about oxygen-oxygen bond lengths in ozone?
   • Experimentally, it is found that the oxygen-oxygen bond lengths in ozone are identical. How can this be explained?
   
   
3. Benzene consists of a ring of 6 carbon atoms, with one hydrogen atom bonded to each carbon. Draw the Lewis structure for benzene. Build a model.
   • What is the stereochemistry at each carbon atom?
   • Is benzene a flat (planar) molecule? Explain.
   • Based on your model, what would you predict about carbon-carbon bond lengths in benzene? About C-C-C and H-C-C bond angles? (C-C bonds have length of about 0.154 nm; C=C bonds have length about 0.133 nm.)
   • Experimentally, benzene is found to have all carbon-carbon bond lengths the same at 0.139 nm. Explain.
   
   
4. Develop Lewis structures for
   • BF₃
   • CH₂O
   • C₂H₄
   • C₂H₂Cl₂ (put one H and one Cl on each C)
   • N₂O₄
   
   

   For each molecule, build a molecular model based on the count of electron groups on the central atom(s). For each molecule, determine the following:

   • Central atom electron group distribution
   • Molecular shape
   • Values of the X-A-X and X-A-A (last 4 molecules only) bond angles (X = terminal atom, A = central atom(s))
   • Whether or not resonance forms are appropriate. If they are, draw them
   • The A-X and A-A (where relevant) bond orders (bond order is the number of bonds between a pair of atoms)
   • Whether the molecule is planar
   • Whether there is more than one way to build C₂H₂Cl₂.
   • Whether the molecule is polar. In what direction does the molecular dipole point? (Answer this question at the discretion of the instructor.)
   
   
5. (Optional--consult the instructor) Develop Lewis structures for
   • PF₅
   • IF₅
   • SF₄
   • XeF₄
   • ClF₃
   • XeF₂

   For each molecule, build a molecular model based on the count of electron groups on the central atom(s). For each molecule, determine the following:

   • Central atom electron group distribution
   • Molecular shape
   • Values of the X-A-X bond angles (X = terminal atom, A = central atom(s))
   • Are all F atoms in PF₅ the same? If not, label the different categories on your drawing.
   • Are all F atoms in SF₄ the same? How about in IF₅?
   • Whether the molecule is planar.
   • Whether the molecule is polar. In what direction does the molecular dipole point? (Answer this at the discretion of the insructor.)
   
   
6. Join another group and pool your kits. Build a model of the dipeptide of alanine and leucine, amino acids with these structures. To make the dipeptide, carry out the following steps:
   • Remove the double-bonded oxygen atom from the carboxyl (COOH) end of one of the amino acids. Don't remove the 2 thin bond sticks.
   • Remove the hydrogen atom from the other oxygen atom of this amino acid.
   • From the other amino acid, remove one H atom + bond, and remove the fat stick representing the nonbonding pair on the nitrogen atom.
   • Make a water molecule from the oxygen atom, H atoms, and fat sticks that you have removed. This is one of the products of dipeptide formation.
   • Attach the N atom of the second amino acid to the carboxyl C atom of the first amino acid using the 2 thin sticks from the C atom. This gives a C=N double bond.

   Discuss in writing the stereochemistry at each atom in the main dipeptide chain.

Molecularity: Covalent Molecules
Preparation

Preparation Questions