Materials Included In Kit

Additional Materials Required

Safety Precautions

Although this activity is considered nonhazardous, observe all normal laboratory safety guidelines.

Lab Hints

• This activity is written for 10 groups of three students. Each student in the group will construct two of the six structures. This will allow the students to complete the procedure and answer the questions in one class session. We suggest pairing up forms 1 and 6, 2 and 5, and 3 and 4. Students may get frustrated without having done a simpler pattern first.
• Students may have difficulty with folding the paper back and forth between solid and dashed lines. Using the edge of a ruler to burnish the creases makes this task easier and produces more accurate folds.
• The completed molecular models are pictured in the Supplementary Information part of the Teacher PDF. Additional plastic models can be built using the “Shapes of Molecules Model Set” (Catalog No. AP5456) available from Flinn Scientific. The “Shapes of Molecules” set may be used to prepare one model for each of the principal types of molecular geometries (linear, bent, trigonal planar, pyramidal, tetrahedral, trigonal bipyramidal, octahedral, square pyramidal and square planar).

Teacher Tips

• A periodic table master sheet has been included to provide a copy of the table for each student.
• According to the most recent official IUPAC recommendations, the columns in the periodic table are numbered continuously from 1 to 18, with no breaks for the transition metals. The A/B Roman numeral numbering system is often still shown in textbooks, probably because it helps students predict valence electrons and ionic charges.
• Many teachers have developed their own protocols for drawing Lewis structures. There are no hard-and-fast rules. One approach that works well is to study the typical number of bonds that an atom will form when it has zero formal charge (e.g., carbon forms four bonds, nitrogen three bonds, oxygen two bonds, fluorine one bond). Drawing Lewis structures always requires trial-and-error, however, no matter how detailed the “rules” that the students are given.
• Explain to students that lone pair electrons, as a rule, require more space than bonded pairs. When looking at molecules that can have more than one spatial arrangement of atoms (i.e., XeF₄), select the geometry that allows the lone pairs the greatest separation.
• Students may refer to their textbooks for additional examples, drawings, and models of molecular geometry. We recommend, however, that students start with the actual physical models to learn about VSEPR theory and molecular geometry. Words and pictures on the printed page are not an adequate substitute for students holding the models in their hands as they try to visualize the shapes of molecules.

Answers to Prelab Questions

1. Write the Lewis electron-dot symbol for each of the following atoms: hydrogen, boron, nitrogen, silicon, sulfur and bromine.
   {12598_PreLabAnswers_Figure_8}
   Note: There are no rules for where the dots are drawn.
2. What information about a molecule does its Lewis structure provide? What information is neither shown nor implied in the Lewis structure?

   The Lewis structure shows all of the atoms in a molecule and how they are connected via single, double or triple bonds. It also shows any unshared pairs of valence electrons on each atom in the structure. The Lewis structure does not provide any information concerning the three-dimensional structure of the molecule or bond angles between atoms. The structures are drawn in two dimensions and are not meant to be perspective drawings.

3. There are several exceptions to the octet rule.
   1. Based on its electron configuration, explain why hydrogen can only have two valence electrons around it when it bonds to other atoms. What is the maximum number of bonds hydrogen will form?

      Hydrogen has one valence electron in a 1s orbital. The 1s orbital can accommodate only two electrons and there are no p orbitals in the n = 1 principal energy level. Therefore, hydrogen can have only two electrons around it when it bonds to other atoms. Hydrogen can form a maximum of one (single) covalent bond.

   2. Neutral compounds of boron may be described as “electron-deficient.” Based on its electron configuration, predict how many covalent bonds boron will form. Is this the maximum number of bonds boron will form? Hint: Boron forms polyatomic ions.

      The valence electron configuration of boron is 2s²2p¹. Boron has three valence electrons available for bonding and thus should form only three covalent bonds by sharing these electrons with other atoms. In neutral compounds, if one “counts” bonding electrons as belonging to both atoms in the bond, then boron would have six electrons around it, not eight. Because the outermost n = 2 principal energy level for boron will accommodate a total of eight electrons, it is possible for boron to form four covalent bonds if it “accepts” additional electrons. The borohydride (BH₄^–) and fluoroborate (BF₄^–) anions are examples of polyatomic ions in which boron forms four bonds.

   3. Many elements in the third row and beyond in the periodic table may form more than four bonds and thus appear to have “expanded octets.” Phosphorus and sulfur, for example, may form five and six covalent bonds, respectively. Count up the total number of valence electrons in PCl₅ and draw its Lewis structure. How many valence electrons are “counted” toward the central P atom?
      {12598_PreLabAnswers_Figure_9}

      PCl₅ has a total of 40 valence electrons distributed as shown in the Lewis structure at the right. The phosphorus atom forms five bonds to chlorine atoms and by the electron counting scheme appears to have 10 electrons in its valence shell. This is permitted because the P atom has empty 3d orbitals.

Answers to Questions

1. Draw the Lewis structures for each of the following.
   1. BF₃
      {12598_Answers_Figure_10}
   2. NH₃
      {12598_Answers_Figure_11}
   3. SO₃
      {12598_Answers_Figure_12}

   Which molecule(s) has the pyramidal geometry?
   NH₃—There are three bonding electron pairs and one non-bonding electron pair.

2. Draw the Lewis structures for each of the following.
   1. XeF₄
      {12598_Answers_Figure_13}
   2. SF₄
      {12598_Answers_Figure_14}
   3. CH₄
      {12598_Answers_Figure_15}

   Which molecule(s) has the tetrahedral geometry?
   CH₄—There are four bonding electron pairs and no non-bonding pairs around carbon.

3. Draw the Lewis structures for each of the following.
   1. NH₄⁺
      {12598_Answers_Figure_16}
   2. SF₄
      {12598_Answers_Figure_17}
   3. CCl₄
      {12598_Answers_Figure_18}

   Which molecule(s) has the seesaw geometry?
   SF₄—There are four bonding electron pairs and one non-bonding electron pair around sulfur.

4. Draw the Lewis structures for each of the following.
   1. PF₅
      {12598_Answers_Figure_19}
   2. ClF₅
      {12598_Answers_Figure_20}

   What is the molecular geometry for each of these molecules?
   PF₅ has five bonding electron pairs around phosphorus and no non-bonding pairs. The molecular geometry is trigonal bipyramidal.
   ClF₅ has five bonding electron pairs and one non-bonding electron pair around chlorine. The molecular geometry is square pyramidal.

Teacher Handouts

12598_Teacher1.pdf

References

Hanson, Robert M. Molecular Origami; United Science Books: Sausalito, CA, 1995.

Introduction

Molecules have shape! The structure and shape of a molecule influences its physical properties and affects its chemical behavior as well. In this activity, you will examine the structures of molecules by creating their geometric shapes from paper using origami techniques. VSEPR theory offers a useful model for visualizing the structures of covalent compounds.

Concepts

• Valence electrons
• Covalent bonding
• VSEPR theory

Background

According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the valence electron pairs that surround an atom repel each other due to their like negative charges. In order to minimize this repulsion, the electron pairs are positioned around the atom so that they are as far apart as possible. The resulting symmetrical arrangement of electron pairs around atoms can be used to predict molecular geometry—the three-dimensional structure and shape of a molecule. Two pairs of electrons around an atom should adopt a linear arrangement, three pairs a trigonal planar arrangement and so on.

The three-dimensional structure of a molecule is affected by the spatial arrangement of all the electron pairs—both bonding and nonbonding—around the central atom. However, only the physical arrangement of the atoms is used to describe the resulting molecular geometry. This is best illustrated using an example. The Lewis structure of the water molecule is shown as the first example in Figure 1—there are four pairs of valence electrons around the central oxygen atom. Two pairs of electrons are involved in bonding to hydrogen atoms, while the other two electron pairs are unshared pairs. Four pairs of electrons around an atom will adopt a tetrahedral arrangement in space, to be as far apart in space as possible, as depicted in the second example in Figure 1. For this representation, the symbol

shows one lone pair of electrons extending behind the plane of the paper. The symbol shows one lone pair of electrons extending in front of the plane of the paper, while the symbols —— represent the hydrogen–oxygen bonds positioned in the plane of the paper. As a result, the two hydrogen atoms and the oxygen atom occupy a “bent” (inverted-V) arrangement, as seen in the third example in Figure 1. When two atoms are linked via a double or triple bond (with two or three bonding pairs of electrons, respectively), the multiple electron pairs between the atoms must be considered together when determining the shape of the molecule. Carbon dioxide provides a good example (see Figure 2). The central carbon atom is linked to two oxygen atoms by two double bonds. The resulting arrangement of atoms is linear—both electron pairs in each double bond are considered to be one electron group that must be in approximately the same region, near the oxygen atom.

Experiment Overview

The purpose of this activity is to construct six models of VSEPR molecular geometrics from paper using origami folding techniques.

Materials

Prelab Questions

1. Write the Lewis electron-dot symbol for each of the following atoms: hydrogen, boron, nitrogen, silicon, sulfur and bromine.
2. What information about a molecule does its Lewis structure provide? What information is neither shown nor implied in the Lewis structure?
3. There are several exceptions to the octet rule.
   1. Based on its electron configuration, explain why hydrogen can only have two valence electrons around it when it bonds to other atoms. What is the maximum number of bonds hydrogen will form?
   2. Neutral compounds of boron may be described as “electron-deficient.” Based on its electron configuration, predict how many covalent bonds boron will form. Is this the maximum number of bonds boron will form? Hint: Boron forms polyatomic ions.
   3. Many elements in the third row and beyond in the periodic table may form more than four bonds and thus appear to have “expanded octets.” Phosphorus and sulfur, for example, may form five and six covalent bonds, respectively. Count up the total number of valence electrons in PCl₅ and draw its Lewis structure. How many valence electrons are “counted” toward the central P atom?

Safety Precautions

Although this activity is considered nonhazardous, observe all normal laboratory safety guidelines.

Procedure

Part A. MX₃ Geometry—Trigonal Pyramidal

1. Cut out form.
2. Make all folds. For dotted lines, - - -, fold faces together; for solid lines, ——, fold faces apart.
3. Fold dark, excess flaps between faces labeled 1 and labeled 3 together and tape the flap to the back of face 3. Make sure sides 1, 2 and 3 face each other.
4. Fold dark, excess flaps between faces labeled 1 and 2 together and tape the flap to the back of face 2. Make sure sides 1, 2 and 3 face away from each other.
5. Fold sides labeled 1 together. Tape the edges of sides labeled 2 and the sides labeled 3.

Part B. MX₄ Geometry—Tetrahedral

1. Cut out form.
2. Make all folds. For dotted lines, - - -, fold faces together; for solid lines, ——, fold faces apart.
3. Fold the dark excess flaps between faces 2 and 5 together and tape the flap to the back of side 5. Make sure sides 2, 5 and 6 face each other.
4. Repeat step 3 for each of the three remaining dark flaps.
5. Fold faces 1 together, then fold faces 2 together. Tape edges of sides 5 together.
6. Fold faces 3 together. Tape edges of sides 6 together, then tape edges of sides 4 together.

Part C. MX₄ Geometry—Seesaw

1. Cut out form.
2. Make all folds. For dotted lines, - - -, fold faces together; for solid lines, ——, fold faces apart.
3. Spread the form out flat.
4. Pick up the form and fold faces labeled 1 together.
5. Fold the dark areas between faces 3 and 4 together.
6. Place the excess fold flush with the back of face 3 and tape the flap in place. Sides 1, 3 and 4 should face each other (see Figure 3).
   {12598_Procedure_Figure_3}
7. Rotate the form around face 1 and repeat steps 5 and 6 for faces 5 and 2, placing the excess fold flush with the back of face 5. Sides 1, 2 and 5 should face each other.
8. Fold faces 2 together, then fold faces 3 together, making sure the dark excess area is folded in (see Figure 4).
   {12598_Procedure_Figure_4}
9. Repeat step 8 for faces 4 and 5.
10. Tape faces 4 and 3 together, then faces 5 and 2.

Part D. MX₅ Geometry—Square Pyramidal

1. Cut out form.
2. Make all folds. For dotted lines, - - -, fold faces together; for solid lines, ——, fold faces apart.
3. Fold the dark, excess flaps between faces 6 and 1 together, then bend the flap flush against the back of face 6 and tape. Sides 7, 6 and 1 should face each other (see Figure 5).
   {12598_Procedure_Figure_5}
4. Repeat step 3 for the other 3 outside squares.
5. Place faces 2 together, then faces 3 together, then faces 4 and faces 1. Tape faces 5 edges together, then face 7 edges, then 6 edges, then 8 edges.

Part E. MX₅ Geometry—Trigonal Bipyramidal

1. Cut out form.
2. Make all folds. For dotted lines, - - -, fold faces together; for solid lines, ——, fold faces apart.
3. Fold the excess flaps between faces 6 and 8 together, then bend the flaps flush against the back of face 8 and tape. Sides 8, 6 and 2 should face each other (see Figure 6).
   {12598_Procedure_Figure_6}
4. Repeat step 3 for the other three corners.
5. Fold the excess flaps between faces 5 and 2 together, then bend the flaps flush against the back of face 5 and tape. Sides 3, 5 and 2 should face each other.
6. Repeat step 5 for faces, 1, 3 and 4.
7. Fold faces 8 together, then tape the edges together.
8. Fold faces 9 together, then tape the edges together.
9. Tape edges of 7 together, then edges 6.

Part F. MX₄ Geometry—Octahedral

1. Cut out form.
2. Make all folds. For dotted lines, - - -, fold faces together; for solid lines, ——, fold faces apart.
3. Fold faces 1 together.
4. Close the dark excess flaps between faces 6 and 4 together and the excess flaps between 3 and 5. Fold the flaps over and secure with tape to the back of faces 4 and 5, respectively.
5. Repeat steps 3 and 4 for faces labeled 2.
6. On the end with two faces labeled 9, fold the dark area along the dotted line so that sides 7, 12 and 9 face each other and sides 8, 9 and 10 also face each other (see Figure 7).
   {12598_Procedure_Figure_7}
7. Repeat step 6 for the other end with two faces labeled 11.
8. Fold faces 7 and faces 8 together, then tape faces 9 together along the outside.
9. Fold faces 6 and faces 5 together, then tape faces 11 together along the outside edge.
10. Tape faces 10 together along edge.
11. Tape faces 12 together along edge.

Student Worksheet PDF

12598_Student1.pdf