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Lewis Structures - PowerPoint PPT Presentation

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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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.

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.

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.

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- -

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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.

CH4

pyramid

4

4

0

tetrahedral

total #

e- groups

#

bond

#

lone pairs

molecular

geometry

structure

e.g.

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”

••

TrigonalBipyramidal

• 5 Total electron groups

• 5 bonding groups

• 0 lone pairs

• 6 Total electron groups

• 6 Bonded

• 0 Lone Pairs