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Lewis Structures

Lewis Structures. 1. Start by determining the formula and the total number of valence electrons. 2. The central atom is the 1 st element in the formula. - It will never be hydrogen. Carbon is always the central atom. 3. Connect the central atom to the outer atoms with a single bond.

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Lewis Structures

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  1. Lewis Structures 1. Start by determining the formula and the total number of valence electrons. 2. The central atom is the 1st element in the formula. - It will never be hydrogen. Carbon is always the central atom. 3. Connect the central atom to the outer atoms with a single bond.

  2. Lewis Structures 4. Determine the number of remaining electrons. • Number from #1 – (# Bonds x 2) 5. Place the remaining electrons around the outer atoms so each obeys the octet rule. • A bond counts as 2 electrons.

  3. Lewis Structures 6. Any remaining electrons go on the central atom. 7. Check to make sure each atom obeys the octet rule. - If the central atom does not, take a pair from an outer atom and make a double bond with the central atom.

  4. - - - - - - 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

  5. 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

  6. 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

  7. 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.

  8. 109.5o CH4 pyramid 4 4 0 tetrahedral total # e- groups # bond # lone pairs molecular geometry structure e.g.

  9. 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” ••

  10. TrigonalBipyramidal • 5 Total electron groups • 5 bonding groups • 0 lone pairs

  11. Octahedral • 6 Total electron groups • 6 Bonded • 0 Lone Pairs

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