Topic: Molecular Shape
Do Now: Draw Lewis Dot Diagram
for the following two compounds
1. CH2F2
2. CaO
[ Ca
+2
]
Use the Lewis Structure
• Lewis structure is 2-D, but can
help figure out 3-D shape
H O H
REMINDER:Bonding Capacity
Atom
H
F, Cl, Br, I
C, Si
N, P
O, S
Ne, Ar, Kr
Lewis
Structure
.
H
..
·F:
..
.
·C·
.
..
·N·
...
·O:
···
:Ne:
··
# Unpaired
Electrons
Bonding
Capacity
1
1
1
1
4
4
3
3
2
2
0
0
Molecular Shape
• Determined by overlap of orbitals
• Shape determined by two factors:
1. # e- groups (pairs) around the central atom –
pay close attention to the “free electron pairs”
2. total # atoms bonded to central atom
Free electron pairs
Central Atom
VSEPR
(A model used to predict the shape of individual
molecules based on the extent of electron-pair
electrostatic repulsion)
• VSEPR= Valence shell electron
pair repulsion:
–Basically free electrons will repel one
another causing the molecule to
bend
2 atoms (no central atom)
• Always linear
– Examples
•
•
•
•
H2
O2
N2
HCl
3 atoms
• Can be linear or bent
– Must look at central atom
• If free electrons, then will be
bent
Central atom
Free electrons
so bent
Central atom
No free electrons so linear
4 atoms that form triangles
• Look at central atom
– No free electrons =
trigonal planar
- Free electrons =
trigonal pyramidal
4-Atom Molecules:
Planar = no
real central
atom, no free
electrons
either
5 atom molecules
• Always tetrahedral
– Examples
• CCl4
• CH4
• CH2I2
Summary of Molecular Shapes
• Start with Lewis Structure!
• Look at total number of atoms
• Look at # of atoms bonded to
central atom
• Look at free pair(s) of electrons
around central atom


Nonpolar
Symmetrical
Molecules
 Nonpolar
Asymmetric Molecules
 Polar
 Bent & trigonal pyramidal are
always polar
Let’s try a few…
Name the SHAPE and
POLARITY
BENT
POLAR
Trigonal Planar
POLAR
Tetrahedral
POLAR
Trigonal Pyramidal
POLAR
linear
POLAR
linear
POLAR
bent
POLAR
tetrahedral
NONPOLAR