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Lab- Molecular Geometry PSI Chemistry Name____________________________ Pre-Lab 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 the Lewis structure provide? What information is neither shown nor implied in the Lewis structure? 3. There are several exceptions to the octet rule. a. Based on the electron configuration, explain why hydrogen can have two valance electrons around it when it bonds to other atoms. What is the maximum number of bonds hydrogen will form? b. 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 will form polyatomic ions. c. Many elements in the third row and beyond in the periodic table may form more than 4 bonds and thus appear to have “expanded octets.” Phosphorous and sulfur, for example, may form 5 and 6 covalent bonds, respectively. Count up the total valance electrons in PCl5 and draw the Lewis structure. How many valance electrons are “counted” towards the central P atom? Procedure 1. On Data Table 1, draw the Lewis structure for the molecule or polyatomic ion listed. 2. On Data Table 2, note the total number electron domains around the central atom, the total number of electron domains that are bonding pairs and the total number that are lone pairs. 3. On Data Table 2, note the VSEPR number. 4. Use the table below to predict the molecular geometry of the molecule or polyatomic ion. 5. Build physical models of the molecules or polyatomic ions using your model kits. VSEPR Number Shape (Molecular Geometry) 220 Linear 330 Trigonal Planar 321 Bent 440 Tetrahedral 431 Trigonal pyramidal 422 Bent 550 Trigonal bipyramidal 541 See-saw 532 T-shaped 523 Linear 660 Octahedral 651 Square pyramidal 642 Square planar Data Table 1 Molecule or Polyatomic ion CH4 H2O NH4+ XeF4 SO32- BH3 SF4 CO2 Lewis Structure Data Table 2 # of Electron Domains Molecule or around the Polatomic ion Central Atom # of Bonding Pairs # of Lone Pairs VSEPR # VSEPR Predicted Geometry CH4 H2O NH4+ XeF4 SO32BH3 SF4 CO2 Post lab Questions 1) Complete a Lewis structure for each of the following molecules or polyatomic ions: CH3Br, H2S, SF6, HCl, IF5 and NO2 -. Complete a table like Data table 2 above and predict the molecular shape of each. 2) Do all of the compounds in the question above and those you worked on in the lab adhere to the octel rule? If not, why not? 3) List the polar molecules from your lab and from the list above. What makes a substance polar? How can a substance contain polar bonds but yet be non-polar? 4) Did any of the substances have more than one valid Lewis structure? What is it called when a substance has more than one valid Lewis structure?
