Bond Order and Molecular Geometry, chemistry homework help

1 Recitation: Molecular Geometry | Arizona State University @ Polytechnic Name: ___________________________________ Date: ________________________ Partners Name______________________________ Partners Name_____________________________ Partners Name_____________________________ Bond Order and Molecular Geometry Part 1: Bond Order _______________________________________________________ Bond energy is defined as the energy required to break the bond that holds two adjacent atoms together in a molecule. This energy is usually expressed on a per mole basis. Bond order is directly related to bond energy. 1. There are three types of bonds illustrate d in Table 1. Give an example of each kind of bond. 2. Using Table 1, what is the relationship between bond order and bond type? 3. What is the relationship between bond order and the number electrons shared in that bond? 4. Rank the bond orders in order of increasing strength. (Note: use <, > symbols when making the ranking) 2 Recitation: Molecular Geometry | Arizona State University @ Polytechnic 5. Using Model 2, consider the hydrogen - halogen compounds (H — X). a. What is the bond order for the H — X compounds? b. What is the trend of bon d energy in relation to the halogen group? c. What is the trend of atomic radii for the halogen group? d. What trend can you theorized between atomic radii and bond energy? 6. How does bond length relate to bond energy? 7. Using Table 3, why is the bond order for ethene listed as “2”? 8. How do the calculated bond orders for ethane, ethene, and ethyne compare with the bond orders predicted by Lewis structures? 3 Recitation: Molecular Geometry | Arizona State University @ Polytechnic 9. Why is th e C -C bond order (Lewis) for ethene given as a “2” in Table 3? 10. How do the calculated bond orders for ethane, ethene, and ethyne compare to bond orders predicted by the Lewis structures? 11. How does the calculated bond order for benzene compare to the bond orders predicted by the Lewis structures? Table 3: Lewis Bond Orders and Calculated Bond Orders 4 Recitation: Molecular Geometry | Arizona State University @ Polytechnic Lewis Formula (electron dot formula) A representation that uses lines and dot pairs to represent the bonding and nonbonding valence electrons in a molecule Octet Rule The tendency for an atom to achieve an electron configuration having 8 valence electrons ( hydrogen is an exception since it only needs 2 electrons to be like helium . Boron is another exception with only 6 electrons. ) Steps for Drawing Lewis Formulas: 1. Determine the valance electrons available from each atom in the chemical formula 2. Draw a skeletal structure by placing the first atom in the formula in the center unless it is H, then use the second atom 3. Draw a dash (bond) between the outer atoms to the central atoms 4. Add electrons to the outer atoms to complete the octet 5. Count how many electron used so far in the illustration 6. Compare with the amount of valance electrons available as determined in the beginning 7. If there are any extra electrons, place them on the central atom as non -bonding electrons pairs 8. Determine if all atoms in the structure have an octet. If the central atom does not have an octet, move non -bonding electrons from an outer atom into a bonding position creating a multiple bond . Repeat, if necessary, until the central atom has an octet. Part 2: Draw ing Lewis Structures__________________________________________ Molecular Formula Calculate Number of Valence Electrons Draw Lewis Formula (Lewis Structure) Number of Valence Electrons in Structure Octets on each Atom except H? CH 4 (Example) C (4) 4H 4(1) 8 Valence e - 8 (4 bonding pairs & 0 nonbonding pairs) (Needs to match column 2 ) Yes NH 3 H2O C H H H H 5 Recitation: Molecular Geometry | Arizona State University @ Polytechnic Groups of valence electrons around atoms are called electron domains . The following count as single electron domains: a single bond, double bond, triple bond, or a lone pair of electrons. Since like charges repel, these areas will try to get as far away from each other as possible. This is the basis for Valence Shell Electron Pair Repulsion (VSEPR) The ory. Using the Lewis structure and VSEPR, you can establish the electron -domain geometry and the molecular geometry. 12. In the above grid, how many electron domains does each of the compounds have? a. CH 4 b. NH 3 c. H 2O 13. Using the Tables 9.2 and 9.3 on the back pages, what is the electron domain geometry for the above 3 compounds? a. CH 4 b. NH 3 c. H 2O 14. Continue using the Tables 9.2 and 9.3 to determine the molecular geometry for the 3 compounds. a. CH 4 b. NH 3 c. H 2O 15. Using Table 9.4 on the back page, determine the hybrid orbital set for each the 3 compounds. a. CH 4 b. NH 3 c. H 2O 6 Recitation: Molecular Geometry | Arizona State University @ Polytechnic In some compounds, there are more electron s than needed to make an octet on each atom but all of the valence electrons must be used therefore these molecules expand the octet. To have an expended octet, the d subshell is used. Since atoms in periods 1 and 2 do not have a d subshell, they will not form expanded octets. Using the same Lewis structure rules, draw the Lewis structure for the following compounds 16. In the above grid, how many electron domains does each of the compounds have? a. SF 6 b. Xe F4 c. PCl 5 17. Using the Tables 9.2 and 9.3 on the back pages, what is the electron domain geometry for the above 3 compounds? a. SF 6 b. Xe F4 c. PCl 5 Molecular Formula Calculate Number of Valence Electrons Draw Lewis Formula (Lewis Structure) Number of Valence Electrons in Structure Octets on each Atom except H? SF 6 (Example) S (6) 6F 6(7) 48 Valence e - 48 (6 bonding pairs & 18 nonbonding pairs) (Needs to match column 2 ) Yes on F No on S Xe F4 PCl 5 F F F F F S F 7 Recitation: Molecular Geometry | Arizona State University @ Polytechnic 18. Continue using the Tables 9.2 and 9.3 to determine the molecular geometry for the 3 compounds. a. SF 6 b. Xe F4 c. PCl 5 19. Using Table 9.4 on the back page, determine the hybrid orbital set for each the 3 compounds. a. SF 6 b. Xe F4 c. PCl 5 8 Recitation: Molecular Geometry | Arizona State University @ Polytechnic 9 Recitation: Molecular Geometry | Arizona State University @ Polytechnic 10 Recitation: Molecular Geometry | Arizona State University @ Polytechnic