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May 9, 2016
CHEM 121
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Ex: Na
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Complete transfer of e - from Na to F (more EN)
• Covalent bond: results from the sharing of electrons between two nonmetals
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Focuses on groups II-VIII
F-F
no difference in EN , NONPOLAR covalent bond, no one wins the tug of war
F-Cl nonzero difference in EN, POLAR covalent bond, one wins the tug of war
F is δCl is δ+
• Lewis structures can have both covalent and ionic bonds, but mostly covalent
Relative strengths of bonds

nonpolar covalent> polar covalent> ionic
Examples of each:
• Non polar:diamond (Carbon and coal), strongest, takes the most energy to break the bond
• Polar covalent: H2O
• Ionic: NaCl, weakest, takes the least amount of energy to break bond

Lewis Theory
• Based on postulate that valence electrons are the most important electrons in and largely
responsible for bonding
 
Lewis Dot Symbols show valence electrons in atoms (Groups I-VIII)
••
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:8:
:7:
:6•
•5•
•4•
3• 2
1•
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••
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•
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RULES:
MAX TWO ON EACH SIDE
KEEP UNPAIRED IF POSSIBLE
DOESN’T MATTER WHERE
Now try drawing Lewis Dot Symbols for Ca, H, O, and F

Use lewis dot symbols of elements to draw molecules

Let’s DRAW the PERIODIC TABLE & LABEL…

• Use lewis dot symbols of elements to draw molecules
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O: 6 valence
F: 7 valence
+1 Electron
= 14 electrons in total

BASIC RULES:
1
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Add/subtract for charges
3
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Give octets to “everything” (not H or B)
5
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Which one is better?

F-O-F
0 0 0

F-F-O
0 +1 -1

 
• Use formal charges to decide which structure is more valid
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First
structure better due to lower formal charges
• FC= group # (electrons available)- # in lewis structure (only use those that specific element
contributed to the bond)
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• For NH4 - what are the formal charges on each atom?
Notice the FC sum = charge on molecule
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• Explanation C2H2: There were too many electrons in the sketch without the double bond
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The central atom is usually the first listed, unless
it is an acid with H in front because H can never be a central atom due to its ability to have only
one valence electron

HCN

CH
2

NH
2

2

2

COCl

2

 
N

2

• N tends to make triple bonds, or a double and three single
• O tends to make double bonds or single bonds with FC -1
• C tends to make all three- hence organic chemistry
Multiple Central atoms
• C2H6
• C2H6O (isomers)
Exceptions to Octet Rule
1
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Elements in period n=3 or higher n can have expanded octets due to d-orbitals
SF6

F F F
\| /
S
/| \
F F F

 
EX:

BrF3

XeF4

(F needs octets)

I3

PCl5

Resonance Structures
• ISOMERS- atoms in different relative positions
• RESONANCES-same atom positions, different electron
Ex: NO 2

Not really a double and single
bond
More like 1

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