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Unit 6

Unit 6. Lewis Structures and VSEPR. Quick Review. What is electron configuration? What are valence electrons?. Lewis Structures. Show the valence electrons and bonds for a compound. 1. Draw the electron dot for each element. 2. Determine if the bond will be ionic or covalent.

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Unit 6

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  1. Unit 6 Lewis Structures and VSEPR

  2. Quick Review • What is electron configuration? • What are valence electrons?

  3. Lewis Structures • Show the valence electrons and bonds for a compound. • 1. Draw the electron dot for each element. • 2. Determine if the bond will be ionic or covalent. • 3. If Ionic: Draw arrows showing the electrons transferring. • 4. If Covalent: Draw a line showing the bond (shared electrons).

  4. Lets make some Lewis Structures! Draw a Lewis Dot for Na & Cl.

  5. Lets make some Lewis Structures! Draw a Lewis Dot for H2S.

  6. Lewis Structures 1. Write the symbols for the elements in the correct order. - The central atom is the 1st element in the formula. - It will never be hydrogen. Carbon is always the central atom. 2. Calculate the total number of valence electrons. 3. Place one pair of electrons between each pair of bonded atoms. 4. Beginning with the outside atoms, place the remaining electrons around each atom until there are 8 around each or all electrons have been used. - If there are extra, place them on the central atom.

  7. More Lewis Structures 5. If not enough electrons are available to give all atoms an octet, move unshared pairs to form double or triple bonds. 6. Examine your Lewis structure to determine if there is resonance. 7. Check your answer: - Correct number of atoms? - Correct number of electrons? - Octet rule for all atoms?

  8. - - - - - - Li H He VSEPR Theory • new theory: • Valence Shell Electron Pair Repulsion • “push” electron pairs as far away from each other as possible • electrons repel each other • like Thomson model of atom

  9. look at groups of electrons • any collection of valence electrons localized around central atom • each electron group repelled by every other e- group • can be comprised of • unpaired electrons • bonding electrons • bonding • 1 group = single bond • 1 group = double bond • 1 group = triple bond • unpaired • 1 group = a single electron • e.g. free radical • 1 group = 1 lone pair

  10. VSEPR “rules” for predicting geometry: • 1. start with valid Lewis dot structure • in case of resonance structures, any valid structure will give same geometry • 2. count number of electron groups around central atom • 3. determine number of bonding vs. unbonded electron groups • 4. describe molecular geometry • learn chart • see Table 9.2

  11. 180o 2 2 0 linear CO2 line 120o 3 3 0 trigonal planar BF3 triangle •• 120o trigonal planar bent 3 2 1 SO2 boomerang • possible geometries: total # e- groups # bond # lone pairs molecular geometry structure e.g.

  12. 109.5o CH4 pyramid 4 4 0 tetrahedral total # e- groups # bond # lone pairs molecular geometry structure e.g.

  13. total # e- groups # bond # lone pairs molecular geometry structure e.g. 109.5o CH4 pyramid 4 4 0 tetrahedral •• <109.5o 4 3 1 tetrahedral trigonal pyramidal NH3 tripod •• <109.5o tetrahedral bent 4 2 2 H2O “v” ••

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