Discussion: Lewis Structures
and VSEPR Theory
OBJECTIVE: Students Will
Understand Why Elements Form
Bonds With Other Elements
What are valence electrons?
Electrons in the highest energy level
 Elements in the same group on the
periodic table have the same number
 A valence of 8 electrons is called a octet
 Atoms try to get a valence of 8 electrons

Not all elements try to get a
valence of eight electrons
Hydrogen needs only two.
 Beryllium needs on four
 Boron needs only six.
 However, most elements like to have eight
electrons in their valence shell.

How the Valence Electrons Fit
Around Group #1 Atoms
Valence Electrons for Group #2
Valence for the rest
How Atoms Gain an Octet by
Losing Electrons

Calcium atom loses two electrons to
achieve an octet
How Atoms Gain an Octet by
Gaining Electrons

Chlorine has seven electrons. It gains one
electron to make eight. The addition of an
electron make the ion negative.
Nobel Elements already
have an octet.
Neon for example has 10 electrons
 Notice it already has an octet. That is
why it doesn't react with other elements

Ions almost always have an octet
Na+ ion for example, has 10 electrons just
like Neon

2 8
Na+ |
|
lost 1

2 8
 O2|
| gained 2

2 8
 Ne
|
| Atoms try to get this
electron configuration

AlCl3

Draw on board
PCl3

Count all the valance electrons
Determine the central atom.
 Connect the ligands to the central atom.

VSEPR Theory
Valence
 Shell
 Electron
 Pair
 Repulsion

The VSEPR Theory

Explains the geometric shape of a
molecule based solely on the repulsion of
electron pairs around the central atom.
Geometric Shapes

Linear

Planer Triangular

Tetrahedral
Polar molecules

Molecules that are unbalanced are polar.
Planer Triangular






Draw SO2
Total Electrons = 18
Central Atom = S
Connect the two oxygen
atoms to the central atom.
Subtract 2 for each bond.
Put six around the O. Put the
extra around the central
atom.
Make the legends double
share until all have octets.
The Shape of SO2

Notice there are 3 pairs
of electrons around the
central atom.
The Shape of H2O

Draw the Lewis
structure for
Water
NH3

Draw the Lewis
structure for ammonia
CH4

Draw the Lewis
structure for methane