Chapter 8 Molecular Shape
The shape of a molecule can be
important in determining its
chemical reactions
Molecular shape is often very
important in the chemistry of living
things.
Molecular geometry also helps
determine the polarity of molecules
High polarity – higher boiling and melting
points
CO2 gas is a nonpolar molecule (low
boiling point)
H2O is a polar molecule (high melting point)
Valence-shell electron pair
repulsion theory (VSEPR theory)
Atoms and electron pair groups around
an atom repel each other
A molecule will have a geometry that
will keep these groups as far apart as
possible.
To determine geometry draw an electron
dot structure and count the groups around
the central atom. (note – a nonbonding
electron pair counts as a group.)
Two Groups
Example: CO2 (see electron-dot)
two groups around the central C atom
180o allows these groups to be as far
apart as possible.
Will be linear (180o bond angle).
Three Groups
Example: CH2O see electron-dot structure
Three groups form a triangle around the
central C atom.
Called “trigonal planer” geometry (120o
bond angle)
4 groups around central atom
Example CH4 (see electron-dot structure)
Form a tetrahedron around the central C
atom (109.5o bond angle)
If all 4 groups are atom groups, the
geometry is called tetrahedral.
If 3 atom groups and one electron pair, the
geometry is called pyramidal. (NH3)
If 2 atom groups and 2 electron pair
groups, the geometry is called bent (H2O)
Bond Hybridization
The valence electrons are the s and p
electrons.
There is one s orbital and three p orbitals
on the outer energy level.
When bonding occurs the orbitals
rearrange (hybridize) to create the most
stable geometry.
2 groups around atom – sp hybridization
1 s orbital combines with 1 p orbital to make
2 sp hybrid orbitals.
3 groups around atom – sp2 hybridization
1 s orbital combines with 2 p orbitals to
make 3 sp2 hybrid orbitals.
4 groups around atom – sp3 hybridization
1 s orbital combines with 3 p orbitals to
make 4 sp3 hybrid orbitals.
http://www.mhhe.com/physsci/chemistry/a
nimations/chang_7e_esp/bom5s2_6.swf
Molecular Polarity
There are two requirements for a
molecule to be polar.
1. The molecule must have polar bonds.
(check the electronegativity)
2. The geometry must allow for a net
dipole.
Example – CO2
Example – H2O