1. Lewis structures and VSEPR

2. Lewis structures CO2 NH3 CH4 H2O CH4
Please draw the valence shell picture for NH3 and CO2.
CH4
CH4

3. NH3 N H
N needs three covalent bonds to fill its valence shell.

4. C O C needs four covalent bonds to fill its valence shell.
O requires two covalent bonds
to fill its valence shell
CO2 C O
maximum number of pairs of electrons two main group atoms can share is 3 pairs.

5. Lewis structures show the
bonding.
Lewis structures do not
show the molecular shape.

6. Valence Shell Electron Pair Repulsion or VSEPR
VSEPR model gives molecular shape.

7. Bonds formation, not electron pair repulsion,
is responsible for molecular geometries.
L. S. Bartell
J. Chem. Educ., 1968, 45 (12),
p 754

8. VSEPR:
Determine steric number (SN)
Use SN to predict molecular shape

9. SN SN 2 5 3 6 4 axial site equitorial site more electronegatve atom
always chooses axial site.

10. two methods to determine SN for VSEPR
1) central atom method
2) Lewis structure method

11. 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)

12. Z = Zentralatom Central atom method is based on the periodic table.
H contributes 1 e-
to central atom
electron count.
Br contributes 1 e-.
O contributes 0 e-.
N takes 1 e- away
from central atom
count.
Z = Zentralatom

13. Use central atom method to determine SN
and shape of:

14. Determine SN and shape of:

15. Determine SN and shape of: XeF5+ XeO3 IOF4- SOCl2
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)

16. The remarkable molecule OFCl

17. two methods to determine SN for VSEPR
1) central atom method
2) Lewis structure method

18. As the molecules get less central-atom-like
it is easiest to use the Lewis-structure method
for determining the steric number, SN

19. Use the Lewis-structure method to determine a plausible molecular
shape for pyro-sulfuric
acid.*
sulfuric
acid
*method requires one to determine
Lewis structure before determining
molecular shape
pyro-
sulfuric
acid

20. compound XnYm Have you seen XnYm before? Use metallic bonding
yes
Have you seen XnYm before? no
atoms have 10-16
bonds each, often
in closest packing
Use metallic
bonding
yes
Locate X and Y on
the periodic table.
Are X and Y
both metals no
Are X and Y from
opposite sides of
the periodic table?
Are X and Y
both p-block
elements?
Is Dc small or are X
or Y 1st and 2nd row
elements?
Use covalent
bonding model no
yes
yes or
yes no no
Use ionic bonding model
central atom
method
determine
Lewis structure
Ionic metal atoms often 6 coordinate
determine
Lewis
structure
apply
VSEPR
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?
Is XnYm a molecule or extended solid?
the shape of XnYm

21. Determine shapes of:
I CO2
NH TeF6

22. formal charge

23. 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.
b) oxidation state method
oxygen starts
with
six outer shell
electrons O
carbon starts
with
four outer shell
electrons C

24. 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
and ends with
six outer shell
electrons. O formal
charge is zero. O
carbon starts
and ends with
four outer shell
electrons. C formal
charge is zero. C

25. 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.

26. 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.

27. Correct Lewis structures require placement of all non-zero formal charges on the Lewis diagram.

28. octet rule

29. If formal charge is zero :

30. 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.

31. assuming atom has eight valence shell electrons.

32. assuming atom has eight outer shell electrons.

33. modern and traditional Lewis structures

34. deducing modern Lewis structures
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

35. draw Lewis structures for all the ions below:

36. carbonate has 3 resonance
structures
how many resonance
structures does bicarbonate
have?

37. draw Lewis structures
for all the ions to the
left:

38. deducing traditional Lewis structures
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

39. 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.

40. comparative studies of modern and
traditional Lewis structures show the
modern method is more correct.

41. comparative studies of modern and
traditional Lewis structures show the
modern method is more correct.