CHEMICAL BONDING

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CHEMICAL BONDING

• Cocaine

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Ionic bonding Li O
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Ionic bonding Al Cl
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Chemical Bonding

• Problems and questions
• How is a molecule or polyatomic ion held
  together?
• Why are atoms distributed at strange angles?
• Why are molecules not flat?
• Can we predict the structure?
• How is structure related to chemical and physical
  properties?

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Review of Chemical Bonds

• There are 3 forms of bonding
• ioniccomplete transfer of 1 or more electrons
  from one atom to another (one loses, the other
  gains) forming oppositely charged ions that
  attract one another
• covalentvalence electrons shared between atoms

Most bonds are somewhere in between ionic and
covalent.
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Bond Formation

• A bond can result from an overlap of atomic
  orbitals on neighboring atoms.

Note that each atom has a single, unpaired
electron.
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Review of Valence Electrons

• Number of valence electrons of a main (A) group
  atom Group number

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Strategy for Writing Lewis Structures
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Steps for Building a Dot Structure

• Ammonia, NH3
• 1. Decide on the central atom never H. Why?
• If there is a choice, the central atom is atom
  of lowest affinity for electrons. (Most of the
  time, this is the least electronegative
  atom Therefore, N is central on this one
• 2. Add up the number of valence electrons that
  can be used. If structure charged, add one
  electron for each unit of negative charge or
  subtract one for each positive charge.
• H 1 and N 5
• Total (3 x 1) 5
• 8 electrons / 4 pairs

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Building a Dot Structure

• 3. Form a single bond between the central atom
  and each surrounding atom (each bond takes 2
  electrons!)

4. Remaining electrons form LONE PAIRS to
complete the octet as needed (or duet in the case
of H).
3 BOND PAIRS and 1 LONE PAIR.
Note that N has a share in 4 pairs (8 electrons),
while H shares 1 pair.
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Building a Dot Structure

1. Check to make sure there are 8 electrons around
   each atom except H. H should only have 2
   electrons. This includes SHARED pairs.

6. Also, check the number of electrons in your
drawing with the number of electrons from step 2.
If you have more electrons in the drawing than
in step 2, you must make double or triple bonds.
If you have less electrons in the drawing than in
step 2, you made a mistake!
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Carbon Dioxide, CO2

• 1. Central atom
• 2. Valence electrons
• 3. Form bonds.

C 4 e-O 6 e- X 2 Os 12 e-Total 16 valence
electrons
This leaves 12 electrons (6 pair).
4. Place lone pairs on outer atoms.
5. Check to see that all atoms have 8 electrons
around it except for H, which can have 2.
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Carbon Dioxide, CO2
C 4 e-O 6 e- X 2 Os 12 e-Total 16 valence
electrons How many are in the drawing?
6. C does not have an octet. We must form DOUBLE
BONDS between C and O. Instead of sharing only 1
pair, a double bond shares 2 pairs. So one pair
is taken away from each atom and replaced with
another bond.