Lewis Structures and VSEPR

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Lewis Structures and VSEPR

• You need to write everything in white!

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Reminder Lewis Dot Structures

• electron-dot structure consists of elements
  symbol surrounded by dots that represent the
  atoms valence e

• Examples
• Chlorine
• Aluminum
• Sodium
• Oxygen

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• Covalent Bond formed by shared pair of valence
  e between 2 nonmetal atoms
• by sharing, both atoms get to count the electrons
  toward a noble gas configuration
• Valence electrons e in the highest energy
  level of an atom
• Remember valence electrons can be determined
  based on the group an element is in.
• bonding pair e pair shared by 2 atoms
• nonbonding, lone, or unshared pair e pair not
  shared between atoms

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• single bond sharing one e pair between 2 atoms
  
• Example iodine
• I I
• double bond sharing of two e pairs between 2
  atoms
• Example sulfur
• SS or S S

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• triple bond sharing of three e pairs between 2
  atoms
• Example phosphorus
• PP
• or
• P P

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Drawing Lewis structures for molecules and ions

1. Determine the total number of valence e for the
   molecule.
2. Connect each atom to the central atom with a dash
   (representing a shared pair of electrons.)
3. The central atom is the least electronegative
   atom - often the first in the chemical formula
   (except H)
4. Draw e as dots around all atoms so that atoms
   have 8 valence e (except H)
5. If there are not enough e to give each atom 8,
   then shift e to make double or triple bonds

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Example

• NH3, ammonia
• N has 5 valence e
• H has 1 (x3) valence e
• total e is 53 8 e

N
H
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Example

• Place N in the middle
• Place three H on sides with bonding e pair
  between atoms
• Count to be certain all atoms have full octet

H
N
H
H
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• can use a line to indicate a bond
• each line 2 valence e

H
H
O
H
H
O

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VSEPR Theory

• Valence Shell Electron Pair Repulsion Theory
• because like negative charges of e- repel each
  other, molecular shape adjusts so the valence e-
  are as far apart as possible
• repulsions cause bonds to be angled and give
  molecules a 3-D shape (geometry)
• molecular shape is based on number of bonding and
  lone e- pairs on the central atom

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Shapes to Know

• Tetrahedral
• Bent
• Linear
• Trigonal planar

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