4f
5f
Ionization Energy Trends
Electronegativity
Decreasing Electronegativity
Increasing Electronegativity
Lewis Structure Assumptions
• Only valance electrons are involved in bonding.
• Atoms react to form molecules, so to achieve
stable noble gas electron configurations.
• Atoms in molecules want eight valance electrons
(octet rule) except for hydrogen which wants two
electrons (duet rule).
• In covalent compounds atoms share electrons to
form bonds in order to achieve stable noble gas
electron configurations. In ionic compounds
electrons are transferred from one atom to
another to achieve stable noble gas electron
configurations.
Rules for Drawing Lewis Structures
Step 1: Count the total number of valance electrons.
Step 2: Identify the central atom (the first atom written unless that atom
is hydrogen). Place all terminal atoms around that atom.
Hydrogen atoms NEVER have more than one bond.
Step 3: Complete the octet for all atoms in the Lewis structure with
lone pairs of electrons (except hydrogen).
Step 4: Check your structure by counting the number of valance
electrons used (they will match step 1 if the structure is
correct). If your valance electrons don’t match you will need
to tweak your structure as follows.
Tweaking Lewis Structures
Too Many Electrons Initially: Redraw the
Lewis structure from step 2 adding a double
bond. If you still have too many electrons
add another multiple bond and repeat.
Always add double bonds before triple
bonds. Every double bonds effectively
remove two electrons from the structure while
triple bonds effectively remove 4 electrons.
“Exceptions” to the Octect
Two Few Electrons: If you only have four or
six valance electrons initially you can’t
possibly fill the octet rule (usually BeH2 or
BH3). Just place hydrogens around central
atoms and call it done.
“Exceptions” to the Octect
Odd number of electrons: One atom will
have to have less than eight electrons.
Draw the Lewis structure as if it had one
more valance electrons than it actually does.
Then subtract one electron from the least
electronegative element (often the central
atom).
“Exceptions” to the Octect
Exceeding the Octet Rule: If the central atom is in
the third row or below it may be possible to exceed
eight electrons around the central atom. But only if
you absolutely have no choice. If you can give
everything eight and only eight do that first. When
you must break the octet rule draw the structure as
you would in steps 1-4 and the place the extra
electrons around the central atom. Even when
breaking the octet rule no atom will ever have
more than 12 electrons.
Example
Total # of
Things
Lone
Pairs
Geo.
Shape
3-D Lewis Bond
Structure Angles
Hybrid
Orbitals
CO2
2
0
Linear
Linear
180
sp
CH2S
3
0
Trig.Planar
Trig.
Planar
120
sp2
SeO2
3
1
Trig.Planar
Bent
120
sp2
SiCl4
4
0
Tetra.
Tetra.
109.5 sp3
PCl3
4
1
Tetra.
Trig.
Pyramid.
109.5 sp3
H2S
4
2
Tetra.
Bent
109.5 sp3
3
Tetra.
Linear
180
HI
4
sp3
Example
Total # of
Things
Lone
Pairs
Geo.
Shape
3-D Lewis Bond
Structure Angles
Hybrid
Orbitals
PF5
5
0
Trig.
bipyramid
Trig.
bipyramid
120, 90,
180
dsp3
SeBr4
5
1
Trig.
bipyramid
See-saw
120, 90,
180
dsp3
BrCl3
5
2
Trig.
bipyramid
T-shaped
90, 180
dsp3
XeCl2
5
3
Trig.
bipyramid
linear
180
dsp3
TeF6
6
0
Octahed.
Octahed.
90, 180
d2sp3
IF5
6
1
Octahed.
Square
pyramid
90, 180
d2sp3
RnBr4
6
2
Octahed.
Square
planar
90, 180
d2sp3
Organic Lewis Structure Rules
• Based on formal charge arguments
Atom
Bonds Formed in
Lewis Structure
Lone Pairs on Atom
in Lewis Structure
C
4
0
N
3
1
O
2
2
F, Cl, Br, I
1
3
p-orbitals are 90 degrees from
each other
1 s + 3 p = 4 sp3 orbitals
1 s + 2 p = 3 sp2 orbitals
1 s + 1 p = 2 sp orbitals
Unhybridized p-orbitals delocalize
the -electrons
ALL atoms are sp2 hybridized in NO31-.
There are NO sp3 hybridized orbitals.