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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
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.


Lewis structures1
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.


Lewis structures2
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.


- -

-

- -

-

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


  • 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


  • 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


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.


109.5o

CH4

pyramid

4

4

0

tetrahedral

total #

e- groups

#

bond

#

lone pairs

molecular

geometry

structure

e.g.


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”

••


Trigonal bipyramidal
TrigonalBipyramidal

  • 5 Total electron groups

  • 5 bonding groups

  • 0 lone pairs


Octahedral
Octahedral

  • 6 Total electron groups

  • 6 Bonded

  • 0 Lone Pairs


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