Chem 121 Determination of Molecular Geometry & Hybridization Based on the VSEPR Theory
It is assumed that you already know how to write Lewis structures, which do not necessarily show the correct molecular geometry. The following table shows how to tell, from a given Lewis structure, what the molecular geometry and hybridization are of the central atom(s). We begin with defining the number of charge clouds on a central atom. The number of charge clouds is the number of lone pairs ( lp ) and number of atoms directly attached to the central atom. We do not count the number of bonds. For example, in the structure below, N has 3 charge clouds (2 atoms + 1 lone pair), C has 3 charge clouds (3 atoms), and O has 4 charge clouds (2 atoms + 2 lone pairs). ..
H
..
..
# of charge
# atoms + # lp on central atom
Hybrid- zation
Sketch
Molecular Geometry
& approx.. bond angle
Example atoms sp linear bond angle = 180 °
H–C ≡ N:
2 1 atom + 1 lp sp : linear bond angle = N/A
:N ≡ N:
3
3 atoms sp 2
2 atoms + 1 lp
1 atom + 2 lp sp 2 sp 2
..
:
: trigonal planar bond angle = 120 ° bent (or angular) bond angle = 120 ° linear bond angle = N/A
:
:
..
..
..
..
: :
..
..
:
2
_
..
..
..
..
..
:
_
4 atoms sp 3
3 atoms + 1 lp sp 3
..
..
tetrahedral bond angle = 109 ° trigonal pyramidal bond angle = 109 °
H
H C H
H
..
H N H
H
4 2 atoms + 2 lp sp 3
: bent or angular bond angle = 109 °
..
..
..
4 1 atom + 3 lp sp 3
..
: linear bond angle = N/A
..
..
:
_

Chem 121
# of charge clouds
5
5
Determination of Molecular Geometry & Hybridization Based on the VSEPR Theory
# atoms + # lp on central atom
Hybrid- zation sp 3 d
Sketch
Molecular Geometry
& approx.. bond angle trigonal bipyramidal bond angle = 90°,
120°
Example
SbCl
5
4 atoms
+ 1 lp sp 3 d
:
See-saw shaped bond angle = 90°,
120°
SF
4
..
3 atoms
+ 2 lp sp 3 d
:
T-shaped bond angle = 90°
ClF
3
5
2 atoms
+ 3 lp sp 3 d ..
..
:
Linear bond angle = 180°
I
3
−
3 d 2 octahedral bond angle = 90°,
180°
SF
6
6
5 atoms
+ 1 lp sp 3 d 2
Square-pyramidal bond angle = 90°
SbCl
5
2 −
..
..
6
4 atoms
+ 2 lp sp 3 d 2
Square bond angle = 90°
XeF
4
..
You are expected to know all the names of the geometries (tetrahedral, bent, trigonal planar, etc.), but by now you should know in General Chemistry you should not be blindly memorizing them. Although there is certainly some memory work involved it is much easier if you learn the logic behind it:
Electron pairs want to be as far apart as possible because of repulsion of like charges (negative to negative).
First you must know how to count the number of charge clouds as defined at the beginning of this tutorial. Next, you should note that the number of charge clouds tells you a great deal of information.
There is a direct correlation of the number of charge clouds (1 st column) and the hybridization (3 rd column). For example, whenever there are 4 charge clouds, the hybridization is always sp 3 , regardless of how many lone pairs or covalent bonds are involved.
The hybridization tells you how to draw the sketch. Whenever you have sp 3 , you should draw a "threelegged rocket" and put in the lone pairs and atoms. Looking at your sketch, you can then name the

Chem 121 Determination of Molecular Geometry & Hybridization Based on the VSEPR Theory geometry accordingly.
Only after you have determined the # of charge clouds and hybridization should you determine the geometry, which does depend on how many lone pairs and how many atoms are attached to the central atom.
What is the geometry of NH
3
?
Given the molecular formula, first you must draw the Lewis structure. [See separate tutorial on how to draw Lewis structures.]
For NH
3
, the Lewis structure is…
..
H N H
H
The central atom, N, has 4 charge clouds (1 lp + 3 atoms).
This tells us the hybridization is sp 3 , and we know the orientation of the sp 3 orbitals is like a 3-legged rocket:
Next, you put in the lone pairs and atoms:
..
Lone pairs are "invisible" when you describe the geometry. So describe only the central atom and the
3 atoms attached to it. This is called "trigonal pyramidal."
What is the geometry of CHO?
First work out the Lewis structure:
..
..
H
The central atom, C, has 3 charge clouds (3 atoms, no lone pairs).
This tells us its hybridization is sp 2 , and we know the orientation of the sp 2 orbitals are planar with the three charge clouds spread out as far apart as possible (divide 360° by 3 and you have 120°).
Next, put in the 3 atoms to give you
The geometry is trigonal planar.

Chem 121 Determination of Molecular Geometry & Hybridization Based on the VSEPR Theory
What is the geometry of SF
4
?
First work out the Lewis structure of SF
4
.
:
..
..
..
S
..
F :
..
..
:
The central atom, S, has 5 charge clouds (1 lp + 4 atoms).
This tells us the hybridization is sp 3 d, and we know the orientation of the sp 3 d orbitals is as follows:
Next you put in the lone pairs and atoms, but you must remember the lone pairs go into the triangle first !
F
F
:
F
F
The geometry is called "see-saw shaped."
There are no practice problems in this tutorial. Please go to your textbook and find problems in the back of the chapter for practice.