CHEM 121
Introduction to Fundamental Chemistry
Summer Quarter 2008 SCCC
Lecture 6
http://seattlecentral.edu/faculty/lcwest/CHE121
The simplest polyatomic ion or molecule is made of two atoms:
What is the shape of this type of molecule or ion?
The only one way to join two atoms is with a line. All diatomic
molecules and ions have a linear geometry.
Valence shell electron pair repulsion theory (VSEPR theory)
allows us to predict the 3 dimensional shape of molecules and
polyatomic ions with >2 atoms.
VSEPR theory states that electrons in lone pairs and bonds
move as far away from one another as possible to minimize
repulsive interactions.
To determine the correct shape of a molecule we must first begin
with the correct Lewis structure.
We then need to determine how many regions of electron
density are around the central atom. This is the number of bonds
and electron pairs.
O S O
H O H
H N H
H
3 regions
4 regions
4 regions
The arrangement of outer atoms is determined by how many lone
pairs there are around the central atom.
If there are 2 or less electron regions then the arrangement will
always be linear.
The situation becomes more complex if there are more than 2
electron regions around the central atom.
For a central atom with three electron regions there are two
possibilities.
For a central atom with four electron regions there are three
possibilities.
Use VSEPR theory to determine as much as possible about the
structure of N2, H2O, SO3, CH4, NH4+ and NH3