Octet Rule, Ion Formation, &Writing Lewis Dot Structures
Courtesy Christy Johannesson
The Octet Rule
lMany elements have a tendency to gainor lose enough electrons to attain thesame number of electrons as the noblegas closest to them in the periodic table.
lAtom’s want to have a stablearrangement of e-’s
lThey will lose, gain or share e-’s to have8 valence e-
Writing Lewis Dot Structures
ELEMENTS
lDetermine how many valence electronsare in the atom. (This is what oxidation# is based on.)
lPlace the electrons around the elementsymbol as shown….
Mg
Write the electron dot diagram for
lNa
lMg
lC
lO
lF
lNe
lHe
Na
C
O
F
He
Ne
Bond Formation
An ion will form bonds based on howmany valence e-’s (dots) it has:
lLess than 4 dots, each dot is anavailable bonding site.
l4 or more dots, each missing dot is anavailable bonding site.
K
1bond
3 bonds
P
DOWRITING LEWIS DOT STRUCTURES#1 & #2
IONS
lAdd one dot for every negative, subtractfor each positive.
lDesignate the charge of the ion andplace the entire structure in brackets.
Draw the Lewis Structure for I-1.
I
-1
Extrae-
DOWRITING LEWIS DOT STRUCTURES#3
Ionic Bonding
Na
Cl
t
r
a
n
s
f
e
r
o
f
e
l
e
c
t
r
o
n
+
-
NaCl
Ca
+2
P
-3
Ca
+2
P
lA
l
l
t
h
e
e
l
e
c
t
r
o
n
s
m
u
s
t
b
e
a
c
c
o
u
n
t
e
d
f
o
r
!
+2
Ionic Bonding
Ca
-3
Ionic Bonding
C
a
3
P
2
Formula Unit
Ca2+
Ca2+
Ca2+
P3-
P 3-
Ca2+
P3-
Ca2+
P3-
Ca2+
DOWRITING LEWIS DOT STRUCTURES#5
How does H2 form?
+
+
The nuclei repel
But they are attracted to electrons
They share the electrons
Hydrogen Bond Formation
0.74 A
- 436
0
H – H distance
Energy (KJ/mol)
Brown, LeMay, Bursten, Chemistry The Central Science, 2000, page 318
no interaction
increased
attraction
balanced attraction
& repulsion
increased
repulsion
Potential Energy Diagram - Attraction vs. Repulsion
(internuclear distance)
Covalent bonds
lNonmetals hold onto their valenceelectrons.
lThey can’t give away electrons to bond.
lStill want noble gas configuration.
lGet it by sharing valence electrons witheach other.
lBy sharing both atoms get to count theelectrons toward noble gasconfiguration.
Covalent bonding
F
F
lF
l
u
o
r
i
n
e
h
a
s
s
e
v
e
n
v
a
l
e
n
c
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c
t
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o
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s
lA
s
e
c
o
n
d
F
a
t
o
m
a
l
s
o
h
a
s
s
e
v
e
n
lB
y
s
h
a
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c
t
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lB
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b
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a
l
s
(
s
t
a
b
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)
8 Valenceelectrons
8 Valenceelectrons
Single Covalent Bond
lA sharing of two valence electrons.
lOnly nonmetals and Hydrogen.
lDifferent from an ionic bond becausethey actually form molecules.
lTwo specific atoms are joined.
lIn an ionic solid you can’t tell whichatom the electrons moved from or to.
How to show how they formed
lIt’s like a jigsaw puzzle.
lI have to tell you what the final formulais.
lYou put the pieces together to end upwith the right formula.
lFor example - show how water isformed with covalent bonds.
Water
H
O
E
a
c
h
h
y
d
r
o
g
e
n
h
a
s
1
v
a
l
e
n
c
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t
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o
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E
a
c
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h
y
d
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g
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w
a
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t
s
1
m
o
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T
h
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x
y
g
e
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h
a
s
6
v
a
l
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c
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c
t
r
o
n
s
T
h
e
o
x
y
g
e
n
w
a
n
t
s
2
m
o
r
e
T
h
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y
s
h
a
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m
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k
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p
p
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Water
lPut the pieces together
lThe first hydrogen is happy
lThe oxygen still wants one more
H
O
Water
lThe second hydrogen attaches
lEvery atom has full energy levels
lA pair of electrons is a single bond
H
O
H
H
H
O
Lewis Structures
1) Count up total number of valence electrons
2) Connect all atoms with single bonds
-
“
m
u
l
t
i
p
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e
”
a
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o
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s
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a
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s
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a
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l
y
i
n
c
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r
;
C always in center,
H always on outside.
3) Complete octets on exterior atoms
(
n
o
t
H
,
t
h
o
u
g
h
)
4
)
C
h
e
c
k
- valence electrons math with Step 1
- all atoms (except H) have an octet;
if not, try multiple bonds
- any extra electrons?
Put on central atom
DOWRITING LEWIS DOT STRUCTURES#4 & 6
Multiple Bonds
lSometimes atoms share more than onepair of valence electrons.
lA double bond is when atoms share twopair (4) of electrons.
lA triple bond is when atoms share threepair (6) of electrons.
Carbon dioxide
lCO2 - Carbon is centralatom ( I have to tell you)
lCarbon has 4 valenceelectrons
lWants 4 more
lOxygen has 6 valenceelectrons
lWants 2 more
O
C
Carbon dioxide
lAttaching 1 oxygen leaves the oxygen 1short and the carbon 3 short
O
C
Carbon dioxide
lAttaching the second oxygen leavesboth oxygen 1 short and the carbon 2short
O
C
O
lThe only solution is to share more
lRequires two double bonds
lEach atom gets to count all the atoms inthe bond
8 valenceelectrons
8 valenceelectrons
8 valenceelectrons
Carbon dioxide
O
C
O
How to draw them
lAdd up all the valence electrons.
lCount up the total number of electronsto make all atoms happy.
lSubtract.
lDivide by 2
lTells you how many bonds - draw them.
lFill in the rest of the valence electronsto fill atoms up.
Examples
lNH3
lN - has 5 valence electronswants 8
lH - has 1 valence electronswants 2
lNH3 has 5+3(1) = 8
lNH3 wants 8+3(2) = 14
l(14-8)/2= 3 bonds
l4 atoms with 3 bonds
N
H
N
H
H
H
Examples
lDraw in the bonds
lAll 8 electrons are accounted for
lEverything is full
Examples
lHCN C is central atom
lN - has 5 valence electrons wants 8
lC - has 4 valence electrons wants 8
lH - has 1 valence electrons wants 2
lHCN has 5 + 4 + 1 = 10
lHCN wants 8 + 8 + 2 = 18
l(18 - 10) / 2= 4 bonds
l3 atoms with 4 bonds -will requiremultiple bonds - not to H
HCN
lPut in single bonds
lNeed 2 more bonds
lMust go between C and N
N
H
C
HCN
lPut in single bonds
lNeed 2 more bonds
lMust go between C and N
lUses 8 electrons - 2 more to add
N
H
C
HCN
lPut in single bonds
lNeed 2 more bonds
lMust go between C and N
lUses 8 electrons - 2 more to add
lMust go on N to fill octet
N
H
C
Another way of indicatingbonds
lOften use a line to indicate a bond
lCalled a structural formula
lEach line is 2 valence electrons
H
H
O
=
H
H
O
Structural Examples
H CN
C O
H
H
lC has 8 electronsbecause eachline is 2 electrons
lDitto for N
lDitto for C here
lDitto for O
Coordinate Covalent Bond
lWhen one atom donates both electronsin a covalent bond.
lCarbon monoxide
lCO
O
C
Coordinate Covalent Bond
lWhen one atom donates both electronsin a covalent bond.
lCarbon monoxide
lCO
O
C
Coordinate Covalent Bond
lWhen one atom donates both electronsin a covalent bond.
lCarbon monoxide
lCO
O
C
How do we know if…?
lHave to draw the diagram and see whathappens.
lOften happens with polyatomic ions andacids.
DOLEWIS DOT STRUCTURES ANDMOLECULAR SHAPES#1