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Lewis structures and VSEPR
Lewis
structures
CO2
CH4
CH4
H2O
NH3
Please draw the valence
shell picture for NH3
and CO2.
H
N
H
H
NH3
N needs three covalent bonds to fill its valence shell.
C needs four covalent bonds
to fill its valence shell.
CO2
O requires two covalent bonds
to fill its valence shell
O
C
O
maximum number of pairs of electrons two main group atoms can share is 3 pairs.
Lewis structures show the
bonding.
Lewis structures do not
show the molecular shape.
Valence Shell Electron Pair
Repulsion or VSEPR
VSEPR model gives molecular shape.
L. S. Bartell
J. Chem. Educ.,
1968, 45 (12),
p 754
Bonds formation, not electron pair repulsion,
is responsible for molecular geometries.
VSEPR:
(1) Determine steric number (SN)
(2) Use SN to predict molecular shape
SN
2
3
SN
5
equitorial
site
more electronegatve atom
always chooses axial site.
6
4
axial site
two methods to determine SN for VSEPR
1) central atom method
2) Lewis structure method
Central atom method of determining SN
(easiest method when there is one atom
in the center of the molecule/ion bonded
to all the other atoms)
Central atom method is based on the periodic table.
H contributes 1 e- Br contributes 1 e-.
to central atom
electron count.
O contributes 0 e-.
Z = Zentralatom
N takes 1 e- away
from central atom
count.
Use central atom method to determine SN
and shape of:
Determine SN and shape of:
General rule: (99+% true in stable molecules/
ions) The least electronegative main group
atom (excluding H) is at the center.
Determine SN and shape of
:
XeF5+ XeO3
IOF4-
SOCl2
c(F) > c(O) > c(N) = c(Cl)
The remarkable molecule OFCl
two methods to determine SN for VSEPR
1) central atom method
2) Lewis structure method
As the molecules get less central-atom-like
it is easiest to use the Lewis-structure method
for determining the steric number, SN
Use the Lewis-structure
method to determine
a plausible molecular
shape for pyro-sulfuric
acid.*
*method requires one to determine
Lewis structure before determining
molecular shape
sulfuric
acid
pyrosulfuric
acid
compound XnYm
Have you seen XnYm before?
no
atoms have 10-16
bonds each, often
in closest packing
Use metallic
bonding
yes
Are X and Y
both metals
no
Locate X and Y on
the periodic table.
Is Dc small or are X
Are X and Y from
Are X and Y
no
yes
or Y 1st and 2nd row yes
opposite sides of
both p-block
elements?
the periodic table?
elements?
yes
yes
no
no
Use covalent
bonding model
or
Use ionic bonding model
central atom
method
Ionic metal atoms often 6 coordinate
determine
Lewis
structure
Determine a plausible molecular/extended solid
shape. If a portion of the compound might be
covalent use covalent model for that portion.
Is XnYm a molecule or extended solid?
determine
Lewis structure
apply
VSEPR
Is XnYm a molecule or extended solid?
the shape of XnYm
Determine shapes of:
I3
-
NH3
CO2
TeF6
formal charge
Two standard methods for electron counting
a) formal charge method: Divide equally the
shared electrons in bonds between the bonded
atoms. Lone pairs ( ) are assigned to host
atom.
oxygen starts
b) oxidation state method
O
with
six outer shell
electrons
C
carbon starts
with
four outer shell
electrons
Two standard methods for electron counting
a) formal charge method: Divide equally the
shared electrons in bonds between the bonded
atoms. Lone pairs ( ) are assigned to host
atom.
oxygen starts
O
C
and ends with
six outer shell
electrons. O formal
charge is zero.
carbon starts
and ends with
four outer shell
electrons. C formal
charge is zero.
Please draw a Lewis structure for each of the the above three
species. Where ever an atom has non-zero formal charge
please note the non-zero charge next to the atom.
Please draw a Lewis structure for each of the the above three
species. Where ever an atom has non-zero formal charge
please note the non-zero charge next to the atom.
Correct Lewis structures require placement of all non-zero formal charges on the Lewis diagram.
octet rule
If formal charge is zero :
If formal charge is +1 then atom behaves as
if it is an atom one column to the left.
If formal charge is -1 then atom behaves as
if it is an atom one column to the right.
and so forth.
assuming atom has eight valence shell electrons.
assuming atom has eight outer shell electrons.
modern and traditional Lewis structures
deducing modern Lewis structures
1. if at all possible, all main group atoms (besides H) have an outer
(valence) shell with eight electrons. Hydrogen just needs two. This is the
octet rule. Outer shell electrons are called valence electrons.
2. having done step 1, make formal charges as small as possible.
3. no more than 3 bonds (multiple bonds) allowed between two
main group atoms. One of the two atoms in this multiple bond is
typically a second row element.
it's a puzzle and needs to be solved like a puzzle
draw Lewis structures for all the ions below:
carbonate has 3 resonance
structures
how many resonance
structures does bicarbonate
have?
draw Lewis structures
for all the ions to the
left:
deducing traditional Lewis structures
1. make formal charges as small as possible.
2. second row main group elements almost always obey the octet
rule.
3. other rows obey octet rule if it doesn't interfere with rules 1 and 2.
4. multiple bonds: allowed to make up to 3 bonds between a pair of
main group atoms. One of the two atoms in multiple bond is typically
a second row element.
it's a puzzle and needs to be solved like a puzzle
please deduce both modern and traditional
Lewis structures for the molecules and ions below
Please also deduce SN and geometry:
SO42-, the sulfate ion
XeO4
PF3Cl2
the modern Lewis structure of PF3Cl2
is a resonant ''detached'' diagram where
each resonant structure looks like the
molecule is separated in two. But the two
resonant structures taken together show a
single cohesive whole molecule.
Detached diagrams like this always require the
three atoms to be in a straight row, a syzygy.
The bonding is called the 3-center 4-electron
bond.
comparative studies of modern and
traditional Lewis structures show the
modern method is more correct.
comparative studies of modern and
traditional Lewis structures show the
modern method is more correct.