Geometry of Covalent Compounds

Transcription

1 Geometry of Covalent Compounds Introduction This laboratory exercise will give you experience working with molecular model sets so you will better understand the geometries of small covalent molecules. Since building accurate molecular representations requires Lewis Dot Structures, you will also get extensive experience building Lewis Structures. Using molecular model kits: The most common type of molecular models are those using balls and sticks. Each ball represents an atom, and each stick represents a bond between two atoms. The convention is to use different colored balls to represent different elements. Below are the most commonly used colors for a few elements and ways to help you remember the identity of each color: carbon = black (color of coal) nitrogen = blue (color of the sky -- nitrogen makes up 80% of air) oxygen = red (fire is red requires oxygen) hydrogen = white (color of clouds hydrogen is the lightest element) halogen = green (color of chlorine gas) Note: If a halogen is a central atom, green will not work sulfur = yellow (color of elemental sulfur) In general, single bonds are the longest; double bonds are shorter; and triple bonds are the shortest. Although single bonds are actually longer than double bonds and triple bonds, use the short grey sticks for the single bonds and the longer flexible sticks to make the multiple bonds (use two sticks for double bonds, three for triple bonds). For more information: Chemistry: Atom s First by OpenStax sections 4.4 Lewis Symbols and Structures, Formal Charges and Resonance, 4.6 Molecular Structure and Polarity and 5.2 Hybrid Atomic Orbitals. Materials: molecular model kit colored pencils Procedure This week work in groups of 2-3 students per group. Each person should build a decent portion of the models. 1. Calculate the total number of valence electrons for all atoms in the compound: Example CH3Cl valence e for C + 3 (valence e for H) + valence e for Cl = 4 + 3(1) + 7 = 14 e 2. Divide the total number of valence electrons by 2 for the number of electron pairs: - 14 e 2 = 7 electron pairs 3. The central atom is usually the least electronegative. Although H is less electronegative than C, H must always be an outer atom. Thus, C is the central atom, and the Cl and H atoms are all outer atoms. GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 1 of 9

2 4. Next, connect all the atoms with single bonds. 7 electron pairs 4 bonding pairs = 3 electron pairs left 5. The C atom already has an octet, and each H atom has a pair of electrons, so they need no additional electrons. The last three pairs are put around the Cl, so it also has an octet. Now C and Cl each have an octet, and each H has the pair of electrons needed. Thus, the Lewis electron-dot structure for CH3Cl is shown below. 6. Confirm that electrons were neither created nor destroyed that there are in fact 14 electrons, no more or less, in the final Lewis structure. 7. To finish off a polyatomic ion, place square brackets around the Lewis structure, and show the charge in the upper right-hand corner outside the brackets. 8. Show all possible resonance structures. When resonance structures are needed to correctly represent a molecule or polyatomic ion, all of the resonance structures are shown with double arrows between them, as shown below. Use formal charges to help determine the best resonance structure. 9. Use the molecular model kit to make the molecule. When making the model, the best structure to use is the one that obeys the octet rule first. If the model has a central atom that needs to expand the octet rule, use the brown and silver atoms for the expanded central atom only. Do not expand the octet rule on the outer atoms. Once the molecule is made, answer questions and have your instructor sign off on your model. 10. Apply Valence Shell Electron Pair Repulsion Theory (VSEPR) to determine the electron domain geometry, molecular geometry (shape) and bond angle(s) for the Lewis Structure. 11. Indicate if the overall molecule is polar or nonpolar. Use the trend for electronegativity values to determine the more electronegative atom for each bond on the central atom. Draw a dipole arrow from the less electronegative atom to the more electronegative atom. If the dipole arrows completely cancel one another, the molecule is nonpolar. If the dipole arrows add up to give an overall or net dipole, the molecule is polar. 12. Determine the hybridization on the central atom. Clean-Up: Put all equipment back exactly where you found it. GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 2 of 9

3 Name: Data and Results Tables: Partners: Geometry of Covalent Compounds Lab Report Turn in Pages 3-8 as Your Lab Report Water Sulfur dioxide GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 3 of 9

4 Carbonate Chlorate GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 4 of 9

5 Sulfur hexafluoride Phosphate GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 5 of 9

6 Triiodide I 3 - Pentachloroantimonate SbCl 5 2- GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 6 of 9

7 Difluoromethane CH2F2 Xenon tetrafluoride GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 7 of 9

8 Phosphorus pentachloride Sulfur tetrafluoride *There is NO conclusion for this lab report. GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 8 of 9

9 Post-Lab Questions These questions will not be graded as part of your lab report grade. You will be responsible for the information in these questions and able to answer these or similar questions on the post-lab quiz at the start of next week s lab period. Questions will also be similar to your lab report data, observations, calculations, and results. 1. Be prepared to draw the Lewis Structure (electron dot structure) for any molecule or ion and answer the following questions: a. Number of valence electrons b. Electron domain geometry c. Molecular geometry (shape) d. Bond angle(s) e. Polarity of individual bonds between atoms f. Overall polarity of molecule g. Hybridization of the central atom 2. Draw the electron dot structure for nitrogen dioxide, NO2. Explain why this molecule is reactive in terms of the features of its electron dot structure. 3. When are the electron domain geometry and the shape of a molecule the same? Explain. 4. Fulminates are explosive. Draw the electron dot structure of fulminate (CNO - ) and explain the explosive reactivity of fulminate in terms of formal charges for this ion. GCC CHM 151LL: Geometry of Covalent Compounds GCC, 2019 page 9 of 9