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CMY 117 Tutorial 4 Molecular Geometry 1

Section A: Bent structures: Water (AX2E2) and ozone O3, (AX2E1) Trigonal Pyramidal: Ammonia (AX3E1) Trigonal Bipyramidal: SOF4 (AX5) See-saw: SF 4 (AX4E1) T-structure: CF3 (AX3E2) Electron pair geometry = Parent structure i) Models: Each student must build the following electron pair geometries, as well as molecular geometries belonging to the specific electron pair geometry (13 models). ii) Complete only the answer sheet and submit. The Linear structure. Build a linear molecule, using as central atom the black atomic centre with two arms (this represents a divalent eg Be atom), attach a green straw (use the short ones) to each arm and to the other end of the straw, a white atom with one arm (mono- or univalent, eg H atom). Look at the structure and draw it. What is the bond angle if you move from white to black to white centre? (These will represent atomic nuclei.) Build and draw the electron pair geometries (or parent structures) and offspring structures for the following structures. Use the models and include the bond angles. Trigonal Planar structure, (a black three armed central atom). How can you use the trigonal planar structure to represent the bent structure? Tetrahedral Structure, (a black four armed central atom). How can you use the tetrahedral structure to represent a trigonal pyramidal and bend structures? Trigonal Bipyramidal Structure, (a black five armed central atom). How can you use this structure to represent See-saw, T- and linear structures? Octahedral structure, (a black six armed central atom). How can you use this structure to represent square pyramidal and square planar structures? Draw the Lewis line and Lewis dot structures of (i) SOF4 (AX5 ) (ii) ClO2 F (AX3 E1) 1

Section B: Model exercises, 3-D Perspectives in Molecular Geometry: Judge the molecules in each instance: i) Decide which atom is the central atom (convention the atom written first, eg for SOF 4 it is S, not O.) ii) Recognize in which period you find the central atom, for period 2 use octet rule, for period 3 or higher either electron octet or the expanded octet. iii) Recognize the terminal atoms, H and F atoms form only single bonds, they are never central atoms. Halogens e.g. Cl and Br only when terminal, single bonds, (when central, either octet or expanded octet). iv) When O is terminal and bonded to a central atom with d-orbitals, the shorter double bond is measured. (O in 2nd period, itself only octet.) v) Identify the group in which the atoms belong to determine valence electrons. vi) Form bonds with valence electrons. The following groups of compounds must be compared with each other: Group A: Group B: Group C: Group D: Group E: Group F: Group G: Group H: Group I: Group J: Group K: Group L:

BF3 , NF3, ClF 3 − − O3, N3 , I3 , I3 + SnF4, XeF4 , SF 4 PCl 3, PCl5, PCl 6 − H2O, H 2S, OF 2 CH4 , CH3CH 3 (ethane), CH 3CH2CH2 CH3 (butane) CH2 CH2 (ethene, eteen), CH 3CHCH2 (propene, propeen), CHCH (ethyne) CH3 CH2OH (ethyl alcohol), CH3CHO (acetaldehyde) NH3 , N2 H4 , NO2, N 2 O4 HCOOH (formic acid), CH3 COOH (acetic acid) C6H 12 (cyclohexane), C 6H6 (benzene). H2 SO3, HClO3 , HClO2

Draw the Lewis structure of each compound and answer the questions. 1. 2. 3.

4. 5. 6.

7. 8.

Apply the VSEPR model to predict the molecular geometry and electron pair geometry of each compound. Build each compound with your model kit and keep your model for comparison with others in the same group Draw a 3D (also sometimes called a Lewis line or Couper structure) sketch of each compound. USE THE MODELS. Compare the structures of groups A-E. Do compounds with related molecular formulas necessarily have similar structures? For the organic molecules, groups F-K: identify the molecular geometry around each C-, O- or N-atom. Bond angle prediction: in the group E, which compound will have the largest and which will have the smallest bond angle? Explain your reasoning. Predict a possible bond angle for OCO in HCOOH. For the organic acids, one H atom is bonded to an oxygen atom, this will make the H-atom acidic, compare with the inorganic oxo-acids eg H2 SO4 or HClO4 For group L, the oxo-acids, predict the HO-central atom (the acidic H atom) as well as O- central atomO bond angles. Polarity: In the groups A to E, predict which compounds will be polar. Indicate the direction of a dipole moment next to the 3D structure. Show clearly in each compound the polarity of each bond.

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