Molecular Geometry

1
Molecular Geometry

• Lewis Structures and VSEPR Theory

2
Lewis Structures

• Also known as electron dot structures
• Shows the positions and bonds of each atom in a
  molecule
• Example below will be used to illustrate the 6
  steps for creating Lewis Structures.
• Draw the Lewis structure of iodomethane, CH3I.

3
Lewis Structures Step 1

• Determine the type and number of atoms in the
  molecule.

• CH3I
• 1 carbon atom
• 3 hydrogen atoms
• 1 iodine atom

4
Lewis Structures Step 2

• Write the electron-dot structure for each type of
  atom in the molecule.

I
C
H
5
Lewis Structures Step 3

• Determine the total number of valence electrons
  in the atoms to be combined.

6
Lewis Structures Step 4

• Arrange the atoms to form a skeleton structure
  for the molecule. If carbon is present, it is
  the central atom. Otherwise, the
  least-electronegative atom is central (except for
  hydrogen, which is never central). Then connect
  the atoms by drawing straight-line bonds. Each
  line represents two valence electrons.

H
C
H
H
I
7
Lewis Structures Step 5

• Make sure the central atom has four bonds going
  to it if it doesnt add double or triple bonds
  to get there. Add unshared electrons so that
  each hydrogen atom shares a pair of electrons and
  each other nonmetal is surrounded by eight
  electrons.

H
C
H
H
I
8
Lewis Structures Step 6

• Count the electrons in the structure to be sure
  that the number of valence electrons used equals
  the number available.

H
C
H
H
I
9
Resonance Structures

• Sometimes we can draw two or more equally valid
  Lewis structures for a given molecule.

O O O
O O O
10
Resonance Structures

• In the past, chemists thought the actual molecule
  switched between these two structures the
  molecule resonated between the two options
• Modern tests show that the molecule has two bonds
  of equal length (a 1.5 bond)
• We cant draw a 1.5 bond, so we draw the two
  resonance structures instead
• Put a double-head arrow between the two choices

11
VSEPR Theory

• Valence Shared Electron Pair Repulsion theory
• Electrons have negative charges
• Negatives repel other negatives
• This repulsion means the electron pairs around an
  atom spread out as much as possible
• We can use this to predict the shape of any
  molecule

12
Number of atoms bonded to the central atom Number of lone pairs on the central atom Shape
4 0 tetrahedral




13
Tetrahedral
14
Number of atoms bonded to the central atom Number of lone pairs on the central atom Shape
4 0 Tetrahedral
3 0 Trigonal planar



15
Trigonal Planar
16
Number of atoms bonded to the central atom Number of lone pairs on the central atom Shape
4 0 Tetrahedral
3 0 Trigonal planar
3 1 Trigonal pyramidal


17
Trigonal Pyramidal
18
Number of atoms bonded to the central atom Number of lone pairs on the central atom Shape
4 0 Tetrahedral
3 0 Trigonal planar
3 1 Trigonal pyramidal
2 0 Linear

19
Linear
2 atoms only
20
Number of atoms bonded to the central atom Number of lone pairs on the central atom Shape
4 0 Tetrahedral
3 0 Trigonal planar
3 1 Trigonal pyramidal
2 0 Linear
2 2 Bent
21
Bent
22
Predict these shapes
NH3
H2O
CO2




N
H
H




O
H
H


C
O
O




H
trigonal pyramidal
bent
linear