PerfecTIC General description Title Building Lewis diagrams and visualizing molecules or polyatomic atoms in 3D Teaching goals Develop the ability to formulate an efficient algorithm for the building of Lewis structures and to deduce, according to these diagrams, the geometrical characteristics, the different covalent bonds, and the polarity of the molecules or polyatomic ions studied. Learning goals For any empirical or partly developed formula for a molecule or a polyatomic ion, to be able to write the electron configuration of valence electrons in ground-state atoms in the case of a polyatomic ion, distribute electric charges among atoms, according to their electronegativity excite atoms or ions as necessary build a Lewis diagram in accordance with the molecule or polyatomic ion under study for each atom or ion, calculate the number of free electron pairs, bonded electron pairs, and packets of electrons identify the geometry of packets of electrons surrounding an atom or ion identify the type of covalent bond evaluate the bond angle between three pre-selected atoms identify the geometry of the molecule or polyatomic ion when there is only one central atom predict whether the molecule will be polar or nonpolar draw the three-dimensional structure or make a molecular model of the molecule or polyatomic ion Summary The teacher gives to each student a different sheet containing six chemical formulas: three model I molecules, one ion, and two model II molecules. As homework, students must build the Lewis structures for the formulas, analyse them, and do the 3-D drawings. At a pre-determined time, the teacher and students go to a college computer laboratory. There, students in pairs use the Lewis Structures II application to check their answers and view their molecules or ions in 3-D. Since students will likely only have enough time to check their model I molecules and ions, they should return to the lab on their own or purchase the software in order to check their answers for
the model II molecules and to practise with the greatest possible number of chemical formulas. Certain exam questions are similar to the ones in this activity. Students demonstrate that they have met the objectives of this activity when they can build a molecular model from any chemical formula and answer questions related to the objectives. Duration About 8 hours (including 5 hours of theory) Technological skill required By the teacher and students: Basic working knowledge of a PC or Macintosh computer Material needed By the students: Access to a college computer lab equipped with one workstation per pair of students Lewis Structures II software installed on the network or on each workstation (Optional) personal Macintosh or PC computer with 8 MB of RAM, equipped with a CD-ROM drive, and a color monitor (Optional) Printer Advantages of using ICT This could simply be a written activity. However, in this case, since all students receive different chemical formulas, the teacher would have to spend too much time checking student answers, even if assisted by a resource person. With the Lewis Structures II application, students can correct their homework themselves by entering their answers and having them checked. This gives them immediate feedback. The software also allows them to view the three-dimensional structure of the analysed substances and to print a report in order to save the picture. Lastly, under the supervision of the software, students can perfect their knowledge autonomously by building and analysing other chemical formulas. PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 2
Step-by-step sequence Preliminary pedagogical preparation By the teacher, before the activity: Prepare a course of about 5 hours on the concepts related to covalent bonds. Prepare a different sheet for each student containing six chemical formulas: three model I molecules, one ion, and two model II molecules (see examples in Appendix A). Prepare answer sheets for the homework (see example in Appendix B). By the students: None. Preliminary technical preparation By the teacher, before the activity: Book a college computer lab for 1 hour on the required date. Ask a college technician to install Lewis Structures II on the college intranet or on the hard drive of each workstation in the computer lab. Print the sheets containing the chemical formulas for the homework for each student (see examples in Appendix A). Print the answer sheets for the homework (see example in Appendix B). By the students: (Optional) Install Lewis Structures II on the hard drive of their PC or make a reservation for 1 or 2 hours of computer time in a college lab outside of class hours. Step 1 (5 hours) Teach the concepts relating to covalent bonds. Give each student a different sheet containing six chemical formulas: three model I molecules, one ion, and two model II molecules (see examples in Appendix A). Give each student six answer sheets (see example in Appendix B). Step2 (1 hour) At home, students complete the answer sheets for each chemical formula received (see Appendices A and B). Step 3 (1 hour) The teacher and students go to a college computer laboratory. There, students in pairs use the Lewis Structures II application to check their answers and view their molecules or ions in 3-D. (Students will likely only have enough time to check their model I molecules and ions.) PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 3
Step 4 (1-2 hours) Students should return to the lab on their own or purchase the software in order to check their answers for the model II molecules and to practise with the greatest possible number of chemical formulas. Step 5 (about 3 hours) In a first exam, students must build a molecular model from the chemical formulas assigned them and answer related questions. A second exam comprises theoretical questions related to this activity. For more information Workshop author Serge Bazinet Institution Collège de Maisonneuve Web site: http://www.cmaisonneuve.qc.ca Tel.: (514) 254-7131 E-mail address sbazine@cmaisonneuve.qc.ca Main reference Lewis Structures II (Software). Montreal: Centre collégial de développement de matériel didactique, 1996. Created by S. Bazinet and the Micro-Intel Group. 1 CD-ROM. Configuration: Macintosh computer or PC, 8 MB, CD-ROM drive, 256-color monitor, and printer. Suggested reference Nil PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 4
Appendix A Examples of sheets containing the chemical formulas for the homework N. B. Each rectangle corresponds to a sheet given to a student. Model I Molecules comprising only one central atom (of course, there is no central atom in the few cases of molecules comprising only two atoms): SnCl 4 CH 2 FBr SF 6 Ion (with model I if time permits, with model II if not): S 2 2- Model II H 2 SO 4 C 4 N 2 H 6 (with two coordinate bonds) (cycle containing the 4 C's and the 2 N's) Model I Molecules comprising only one central atom (of course, there is no central atom in the few cases of molecules comprising only two atoms): GeH 2 CFBrCl 2 PCl 5 Ion (with model I if time permits, with model II if not): NH 2 - Model II NO 2 - N 2 O 5 (with two double bonds N = ) and two coordinate bonds N - 0) Model I Molecules comprising only one central atom (of course, there is no central atom in the few cases of molecules comprising only two atoms): NH 3 BeCl 2 XeF 4 Ion (with model I if time permits, with model II if not): TeCl 3 - Model II POCl 3 NCCHO (with one coordinate bond) PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 5
Appendix B Examples of answer sheets for the homework Molecular formula: MODEL I MOLECULES 1. What is the valence electron configuration for the central atom in its ground state? Specify if it is 1s, 2s, 3d, 4p, etc. Symbol of the central atom: s p d 2. Does the formation of the molecule require the advancement of an electron into the central atom? Yes No If yes, what is the configuration of the valence electrons of the central atom after this advancement? s p d 3. Draw the Lewis structure of the molecule: PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 6
4. How many electron packets are there around the central atom? 5. According to which geometry are the electron packets placed around the central atom? Give the name of this geometry and draw a diagram to represent it. 6. How many free electron pairs are there around the central atom? 7. What is the geometry of the molecule? Give the name of this geometry and draw a diagram to represent it. PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 7
8. For each different atom (except hydrogen) surrounding the central atom, state the number of packets around the atom and identify the geometry of these packets. Symbol of the atom Number of packets Geometry of the packets Linear Triangular Tetrahedral Trigonal-bipyramidal Square-bipyramidal Pentagonal-bipyramidal PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 8
MODEL II MOLECULES Formula of the molecule or ion: 1. Write the electron configuration of the different atoms (only one configuration per element) making up the structure of this molecule or ion. Write only the part of the configuration that represents the valence electrons and illustrate this using quantum boxes. (N.B. In the case of an ion, write the configuration of the atom carrying the charge, taking into account this charge.) 2. To build this molecule (or this ion), did you have to excite certain atoms? Yes No If yes, give the configuration of these atoms in their excited state. (Only the valence electron part) 3. Illustrate the Lewis structure of this molecule (or of this ion) using different symbols for the electrons of each atom. For example: x X ox Y x Z : x PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 9
4. First enter the name of each atom, then give the information requested in each column. (N.B. If the molecule contains several atoms of the same element, indicate these atoms using the same index as in the Lewis diagram and group the indexes of the identical atoms as follows: C 1, 2, 3_.) Name of the atom Number of Number of Total number Respects octet free pairs bonded pairs of packets rule? (Yes or No) 5. How many of each of the following types of bonds does this molecule (or ion) comprise? Simple Double Triple Sigma (σ) Pi (π) Coordinate covalence 6. Build this molecule (or ion) to scale using appropriate plastic stems and metallic hearts. 7. Draw this molecule (or ion) on the back of this sheet. Indicate on the drawing each atom core, each bond (s π) and each free pair. PerfecTIC Self-evaluation tools for students to measure their understanding of subject matter on different cognitive levels. 10