Covalent Bonds
• Covalent bond is a bond formed when 2
electrons are shared between atoms
• H + H forms H2
• Lewis Structure of H2
• Lewis Structure of F2
• A lone pair is any electrons not shared in a
covalent bond (a.k.a nonbonding electrons)
Drawing Lewis Structures
• Follow the octet rule - an atom wants to
have 8 valence electrons.
CH4
H2O
Rules for Drawing Lewis Structures
Example CCl4
• 1. Count total number of valence electrons from
all atoms plus electrons from charge of molecule
–C =
4e
– 4Cl = 4x7= 28 e
– charge = 0
0e
–
32 total e
• 2. Set the least electronegative non-H atom at the
center
C
– C is 2.6
– Cl is 3.2
Rules for Drawing Lewis Structures
3. Place other atoms around it with a bond connecting
each atom to the central atom
(1 bond = 2 electrons)
4. Put any remaining electrons around atoms as lone
••
pairs or double bonds to make octet
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C
Cl
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24e /2 = 12 lone pairs
Cl
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Cl
••
32 total e
- 8 bonding e
24 nonbonding e
••
••
Cl
Rules for Drawing Lewis Structures
• 5. Check Structure with Formal Charge
– F.C. = # original electrons - #bonds - 2(# lone pairs)
– F.C = 0 is best, but close to zero (+1, -1) is acceptable
••
••
C
••
Cl
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Cl
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Cl
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F.C. Cl = 7 original e
- 1bonds
- 2(3) lone pairs
Chlorine 0 good
Cl
••
F.C. C = 4 original e
- 4 bonds
- 2(0) loan pairs.
Carbon 0 good
••
Rules for Drawing Lewis Structures
• CO2
• ClO4• NF3
Group Work
• Draw the Lewis structures of the following
–
–
–
–
–
H2O
PCl3
CS2
NO3IF3