MOLECULAR GEOMETRY
Determining the Structure of
Molecules
Molecular Structures
Molecular Formula
H2O
Electron Dot Structure
H:Be:H
Structural Formula
Ball and Stick
Molecular Model
H – Be – H
MOLECULAR GEOMETRY

Structural formulas, such as NH3, provide
information about bonding only. It does not
provide direct information about the shape of
the bond or the shape of the molecule.
 The repulsion between charge clouds in the
outer levels of atoms determines the
arrangement of the orbitals. The orbital
arrangement determines the shape of the
molecules.
VSEPR





Valence Shell Electron Pair Repulsion
theory
is based on the number of regions of high
electron density around a central atom.
can be used to predict structures of molecules
or ions by minimizing the electrostatic
repulsion between the regions of high electron
density.
can also be used to predict structures of
molecules or ions that contain multiple bonds
or unpaired electrons.
does fail in some cases.
VSEPR
In small molecules, electron pairs will
spread as far apart as possible to
minimize repulsive forces.
 Two electron pairs = 180 apart
 Three electron pairs = 120 apart
 Four electron pairs = 109.5 apart
SHAPES WE WILL LEARN
Linear
 Trigonal Planar
 Tetrahedral
 Pyramidal
 Bent

LINEAR
Atoms are connected in a straight line.
 180 bond angles
 One or two bonded pairs of electrons
 Examples:
HCl
CO2

TRIGONAL PLANAR
Atoms are connected in a flat equilateral
triangle
 Three bonded pairs of electrons
 120 bond angle
 Example:
BCl3

TETRAHEDRAL
Atoms are connected in a shape with
four surfaces.
 Four bonded pairs of electrons
 109.5 bond angles.
 Example: CH4

PYRAMIDAL

Atoms are in the shape of a pyramid.
 Similar to tetrahedral but only has three
bonded pairs of electrons, not four; has one
unshared/lone pair of electrons.
 Less than 109.5 bond angle due to unshared
pair of electrons.
 Example:
NH3
BENT
Atoms are close to the shape of a
tetrahedral, but the two unshared pairs of
atoms exert a greater repulsive force
than the two sets in the bonds.
 Two shared pairs of electrons and two
unshared pairs of electrons
 105 bond angle
 Example: H2O

Other Shapes
VSEPR TABLE
Look on the back of your Molecular Geometry worksheet
HOW TO DETERMINE THE
GEOMETRY OF A MOLECULE
USE ELECTRON DOT DIAGRAMS
1. Look at the chemical formula. Figure out the
location of the atoms


Hydrogen is always on the outside
The least electronegative atom is the central atom
(most “electropositive”).
2. Draw the Electron Dot Diagram for each atom.
3. Count up the total amount of valence
electrons for all the atoms involved.
HOW TO DETERMINE THE
GEOMETRY OF A MOLECULE
4. Determine the number of bonding pairs of
electrons by dividing the total # of electrons by
two.
5. Arrange a skeletal diagram of the molecule
by placing the other atoms around the central
atom. Place a bonding pair of electrons (2)
between the central atom and each of the
terminal atoms.
HOW TO DETERMINE THE
GEOMETRY OF A MOLECULE
6. Subtract the number of bonded pairs of
electrons for the central atom from the total
number of electrons. If there are any
electrons left over, these electrons are
lone pairs to be placed around the central
atom.
HOW TO DETERMINE THE
GEOMETRY OF A MOLECULE
7. If the terminal (outside) atoms do not have a
full octet, place lone pairs of electrons around
them. The rest go around the central atom.
8. If the central atom does not have four pairs of
electrons around it (and it had a least four
valence electrons to begin with), try converting
some of the lone pairs to double or triple bonds.
(Carbon, nitrogen, oxygen, and sulfur like to form
multiple bonds)
EXCEPTIONS
There are exceptions to the OCTET rule:
Atoms with less than an Octet:
 Hydrogen – only 2 valence electrons
 Group 2A – only 4 valence electrons
 Group 3A – only 6 valence electrons
Atoms with more than an Octet:
 Sulfur and phosphorus – 10+ valence electrons
 Krypton, xenon, iodine, and others with “d” orbitals will
accept more than 8.
PRACTICE
Fill in the following chart and predict the molecular shape for the following substances:
Molecule
E- Dot diagram
# of shared/
# of lone/
Electron Dot
Shape of
Bond Type
Formula
all elements
bonded e-
unshared e-
Structure
Molecule
(e-negativity)
PRACTICE
H2O
 SiCl4
 NH3
 Cl2
 N2
 GaF3

MOLECULAR POLARITY
This is a result of bond dipoles (areas of
unequal polarity) that do not cancel each
other out.
 This is the polarity of the MOLECULE
not the BOND.

BOND POLARITY
You can determine the polarity of
BONDS by determining the
electronegativity differences of the two
atoms involved.
 C – C nonpolar cov. e-neg diff = 0
 Na – F ionic
e-neg diff – 3.05
C-H
nonpolar cov. e-neg diff = 0.35

MOLECULAR POLARITY
But, take those same molecules and the
polarity of the molecule will depend on
the whole molecule, not just the bond.
 C – C nonpolar equal sharing
 Na – F polar
unequal sharing
C-H
polar
unequal sharing
 For a molecule, you must consider the
shape and the terminal atoms.

MOLECULAR POLARITY
LINEAR
 If the terminal atoms are the same, there
are equal forces, so it is NONPOLAR. If
they are not the same, it is POLAR.
 BeF2 – nonpolar
 HCl - polar
MOLECULAR POLARITY
TETRAHEDRAL
 If the terminal atoms are the same, there
are equal forces, so it is NONPOLAR. If
they are not the same, it is POLAR.
 CCl4 – nonpolar
 CHCl3 - polar
MOLECULAR POLARITY
TRIGONAL PLANAR
 If the terminal atoms are the same, there
are equal forces, so it is NONPOLAR. If
they are not the same, it is POLAR.
 BCl3 – nonpolar
 BHCl2 - polar
MOLECULAR POLARITY
PYRAMIDAL
 Because of the unshared pair, there are
unequal forces, so the molecule is
POLAR.
 NH3
MOLECULAR POLARITY
BENT
 Because of the unshared pairs, there are
unequal forces, so the molecule is
POLAR.
 H 2O
MOLECULAR POLARITY
What is the Molecular Polarity
for these molecules?
Molecular
Polarity
What about these?
REMEMBER!
To determine BOND POLARITY,
calculate the electronegativity
differences.
 To determine MOLECULAR POLARITY,
look at the shape of the molecule and
the terminal atoms.

ORBITAL HYBRIDIZATION
This is the mixing of atomic orbitals in an
atom to generate a new set of atomic
orbitals.
S and p orbitals merge and there no longer
are distinct orbitals.
They merge to form sp orbitals.
ORBITAL HYBRIDIZATION
ORBITAL HYBRIDIZATION
Determine the Hybridization:
sp3
sp
sp2
Do this now – scrap paper
Molecule
Formula
AlH3
PH3
CS2
E- Dot
Diagram
for all
atoms
# shared/
bonded
electrons
# lone/
unshared
electrons
E- Dot
Structure
Shape of
molecule
Bond Type
(electronegativity)