Lewis Dot Structures

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Lewis Dot Structures: Electron dot structures of compounds
Q: What is the formula for water?
H2 O
Q: How many valence electrons does each hydrogen have? 1
Q: How many valence electrons does the oxygen have?
6
Q: How many valence electrons are there in each
water molecule?
#VE = 2H + 1 oxygen = 2(1) + 6 = 8
More examples
1. How many VE in H2O2?
2. How many VE in H2SO4?
H
O
#VE = 2(1) + 2(6) = 14
#VE = 2(1) + 6 + 4(6) = 32
H S
O
The ‘Keep ‘em Happy’ approach to Lewis structures:
1. Add up the number of valence electrons present
2. Draw the stick-skeleton of the molecule
3. Satisfy the octet rule for all atoms in the molecule
Exceptions: H only needs 2e- and B only needs 6e4. Count up the number of electrons present in the Lewis
structure
- If there aren’t enough e- (the molecule is ‘unhappy’),
add the missing electrons to the central atom
- If there are too many e- (the molecule is ‘too happy’),
take the excess away from the central atom and then
form double bonds with the terminal atoms to satisfy the
octet.
Examples:
SF4
#VE = 34
Note: 1 stick = 2 electrons in a bond
F
S
F
F
F
32
34
SO2
#VE = 18
O
20
S
O
18
This molecule isn’t ‘happy’
because it doesn’t have
enough electrons.
This is the only possible
structure because Fluorine
NEVER forms double bonds
This molecule is too happy,
take away an electron pair
from the central atom…
…but now sulfur is unhappy
with only 6 eMake sulfur happy by using
one of the pairs on oxygen
to form a double bond.
Recall that the shape of a
molecule can play a very
important role in determining its
properties.
Example: The odor of the compounds outlined below depend
upon their 3D shape
Molecules will adopt whatever shape allows them to minimize
the repulsion between electrons in adjacent bonds.
H
H C H
H
Ex: CH4
109.5°
90°
You might think this
is the farthest that
the hydrogens can
get away from each
other
But if you think in 3-dimensions,
this shape actually causes less
repulsion between the bonding
pairs of electrons.
A useful model for predicting the shape of molecules is the…
•Molecules will adopt a shape that is lowest in energy
•A low energy shape is one that minimizes the valence
shell electron pair repulsion (VSEPR) between adjacent
atoms (electrons in bonds and in lone pairs repel each
other).
The 5 Main Shapes
Linear
180°
Trigonal planar Tetrahedral
120°
109.5°
Octahedral
90°, 180°
Trigonal bipyramidal
120°, 180°
Molecules adopt a geometry that minimizes electron-electron
repulsions  this occurs when e- pairs are as far apart as
possible.
Steps to determining molecular geometry:
1. Draw a Lewis structure
2. Determine the “AXE” notation
•A = central atom
•X = # atoms bonded to the central atom
•E = # of lone pairs on central atom
3. Determine the geometry using the AXE chart
Examples:
PH3
X
H
E
P
A
X
H
AX3E
Trigonal
pyramidal
H
AX2E2
bent
HX
H2 S
H
S
Let’s look at a few examples…
AX3
AX2E
Trigonal planar
bent
H O
H
AX3E
AX2E2
Trigonal pyramidal
bent
Going from AXE notation to hybridization of central atom:
1. Add up X and E subscripts on AXE notation
Ex: AX5E1
5+1=6
 you have to have 6 orbitals
to hold 6 “things”
2. Combine orbitals until the superscripts add up to the same
amount (start with s, then p, then d; Maximum of s1, p3, d5)
Ex: s1 p3 d2
1+3+2=6
 You have 6 hybrid orbitals
A few more examples:
AX3E 
s1p3
AX2E3  s1p3d1
AX3 
s1p2
AXE 
s1p1
The Name Game: Covalent Molecules
1. The first element in the formula is also first in the
name and retains the name of the element.
2. The ending of the second element is changed to –ide.
3. Prefixes are used to indicate the number of each
element in the molecule.
Mono-
1
Hexa-
6
Di-
2
Hepta-
7
Tri-
3
Octa-
8
Tetra-
4
Nona-
9
Penta-
5
Deca-
10
Working out the kinks with a few covalent compounds:
1. CO2
Carbon dioxide
2. N2O4
Dinitrogen tetroxide
3. SF6
Sulfur hexafluoride
4. PCl5
Phosphorus pentachloride
5. SO3
Sulfur trioxide
6. H2S
Dihydrogen monosulfide
7. BCl3
Boron trichloride
8. NO
Nitrogen monoxide
9. CF4
Carbon tetrafluoride
10. XeF4
Xenon tetrafluoride
Now…change it up a bit and write the formula from the name:
1. Sulfur dioxide
SO2
2. Diphosphorus tetroxide
P 2 O4
3. Nitrogen trichloride
NCl3
4. Krypton tetrafluoride
KrF4
5. Dinitrogen trioxide
N 2 O3
6. Dihydrogen monoxide
H2O
7. Boron tribromide
BBr3
8. Carbon monoxide
CO
9. Silicon tetraiodide
SiI4
10. Disulfur tetroxide
S2O4

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