Molecular Shape

1
Molecular Shape

• The Geometry of molecules

2
Molecular Geometry

• The shape of a molecule is determined by where
  the nuclei are located.
• But the nuclei go to certain locations
  because of the electron pairs.
• Goal minimize electron-pair repulsions.

3
Molecular Shape

• Electron pairs repel each other. They want to be
  as far apart from each other as they can.
• Nonbonding pairs take up a little more room than
  bonding pairs They will repel with a greater
  force.

4
To determine molecular geometry start with the
Lewis e- dot Structure

• Lewis dot structures are 2-D, but they can
  help you figure out the 3-D shape.
• To help us predict the shape of molecules we will
  use
• the VSEPR Theory.

5
VSEPR Theory Valence Shell Electron Pair
Repulsion

• 1) Draw the Lewis structure.
• 2) Identify the regions of high electron density
  Bonding and nonbonding Sites on the central
  atom.
• a) Each single, double, /or triple bond counts
  as 1 region of bonded electron density.
• b) Each nonbonding electron pair counts as 1
  region of non-bonded electron density.
• c) An unpaired electron counts as 1 region of
  non-bonded electron density.
• 3) Assign a VSEPR formula according to the of
  regions.
• Resonance structures will fluctuate their
  regions.

6
VSEPR Theory

• The shape is always referenced around the central
  atom. Determine the formula using
• A as the central atom
• X as the bonded atoms or shared pairs around
  the central atom(A).
• E as the unshared electron pairs.
• Example H2O would be AX2E2
• ? A O X2 H2 E2 2 unshared
  pairs of e-
• .

7
Most Common VSEPR Shapes
Memorize
VSPER Formula bonding sites Most stable arrangement
AX2 2 Linear
AX3 3 Trigonal Planar
AX4, AX3E, AX2E2 4 Tetrahedral, Trigonal Pyramidal or Bent
AX5 5 Trigonal bipyramidal
AX6 6 Octahedral
8
TWO Share Pairs. LINEAR

• 2 bonding regions.
• 0 unshared pairs.
• Linear.
• Bond angle 180?.
• Central atom regions of electron density
  arranged in a straight line.

AX2


Cl?Be?Cl


9
Three Shared pairs Trigonal Planor

• Bond angle 120?.

AX3

F
?
B
?
?
F
F

• 3 bonding regions. 0 unshared pairs.

10
3 regions of high electron density.

• 1 unshared pair.
• 2 Shared pairs( one single or double bonds).

• NO2-
• Lewis Structure

AX2E
1-



O?N?O


3 unequal regions of electron density Since only
2 are shared, the molecule will look bent.
What is the bond angle?
11
Bent slightly lt 120
AX2E
12
Four shared pairs
Tetrahedral Bond angles 109.5?.

• 4 shared pairs
• 0 unshared pairs
• Lewis structure

AX4
13
Trigonal Pyramidal
AX3E
107

• 3 shared pair
• 1 unshared pair.

?
4 regions of electron density would be 109.5
apart. But only 3 end in atoms, not all 4. The
molecule will look like a squashed pyramid.
Trigonal pyramidal. Bond angles?
14
4 regions of electron density.

• 2 Shared pairs 2 unshared pairs
• NH2- with 8 valence electrons

4 regions of electron density but only 2 end in
atoms. Molecule will look bent.
AX2E2
Bent 104.5
15
5 regions of electron density
of bonding regions of lone pairs Shape
5 0 Trigonal Bipyramid
4 1 See-Saw
3 2 T-Structure
2 3 Linear
16
Trigonal Bipyramidal
17
See-Saw
18
T-shaped
19
Linear
20
6 regions of electron density
of bonding regions of lone pairs shape
6 0 Octahedral
5 1 Square Pyramid
4 2 Square Planar
21
Octahedral
22
Square Pyramid
23
Square Planar