Molecular Shape

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Chapter 8

• Molecular Shape

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Section 8-1 The Shape of Small Molecules

• If you look at the Lewis structure for a
  molecule, you dont usually get an idea of what
  the molecule really looks like.
• H-N-H
• H
• An ammonia molecule isnt really T-shaped.

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• Chemists use a structural formula to show us what
  molecules really look like in 3-D the
  ball-and-stick model.

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• molecules look as symmetrical as possible. This
  is because of their electrons.
• Covalent bonds are just shared pairs of
  electrons, and every electron has a negative
  charge. So shared pairs will repel other shared
  pairs of electrons.
• This is actually called the Valence-Shell
  Electron Pair Repulsion theory, or VSEPR theory.

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• The VSEPR theory states that in a small molecule,
  the pairs of valence electrons are arranged as
  far apart from each other as possible.

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• GET OUT YOUR MOLECULAR SHAPE DATA TABLE. THE
  INFORMATION THAT WILL HELP YOU FILL IT OUT IS
  FOUND ON THE NEXT FEW SLIDES.

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LINEAR

• In a linear molecule, the atoms can be connected
  in a straight line.
• bond angle 180
• center atom connected to 2 other atoms
• center atom has no unshared electrons

Even the electron pairs that make up double bonds
will repel each other.
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TRIGONAL PLANAR

• Molecules that are trigonal planar have a flat,
  triangular shape.
• bond angle 120
• center atom connected to 3 other atoms
• center atom has no unshared electrons

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TETRAHEDRAL

• bond angle 109.5
• center atom connected to 4 other atoms
• center atom has no unshared electrons

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PYRAMIDAL

• Pyramidal molecules are shaped like a pyramid,
  hence the name.
• bond angle 107
• center atom connected to 3 other atoms
• center atom has unshared electrons

Unshared electron pair
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BENT

• bond angle 105
• center atom is connected to 2 other atoms
• center atom has unshared electrons

2 unshared electron pairs
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• Notice that for the tetrahedral shape, the
  pyramidal shape, and the bent shape, the atom in
  the center is surrounded by 4 pairs of electrons.
  So why are the bond angles different???

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• ANSWER Unshared electron pairs exert a greater
  repulsive force than shared pairs.
• TRANSLATION Unshared pairs take up more room
  than shared pairs.
• So the bond angle in a pyramidal molecule is a
  little smaller than a tetrahedral molecule, and
  the bond angle in a bent molecule is a little
  smaller than a pyramidal molecule.

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Hybrid Orbitals

• We learned in Chapter 4 that electrons live in
  different orbitals in an atom, like the only
  orbital in the 2s sublevel, or the 3 orbitals in
  the 2p sublevel. This only applies to unbonded
  electrons.
• When atoms are getting ready to share electrons,
  they combine all of their valence electron
  orbitals and create new hybrid orbitals.

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• If an atom has 2 electrons it wants to donate to
  covalent bonding, it combines 1 s orbital and 1 p
  orbital to create 2 sp hybrid orbitals. If an
  atom has 3 electrons it wants to donate to
  covalent bonding, it combines 1 s orbital and 2 p
  orbitals to create three sp2 hybrid orbitals.

p
p
sp2
sp2
sp2

s

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• Linear molecules have sp hybrid orbitals.
  Trigonal planar molecules have sp2 hybrid
  orbitals. Tetrahedral molecules, pyramidal
  molecules, and bent molecules have sp3 hybrid
  orbitals.

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Bond Length

• bond length the length of the bond between two
  atoms that are sharing electrons
• There are two important trends in bond length
• As you go down a group in the periodic table, the
  bonds get longer.
• Multiple bonds are shorter than single bonds.

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Section 8-2 Polarity

• Remember in a polar bond, electrons are not
  being shared equally. That makes one end of the
  bond slightly negative and the other end slightly
  positive.
• A polar molecule (a.k.a. dipole) is a molecule
  that has two oppositely charged ends. Polar
  molecules act like little magnets.

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• If a molecule contains only nonpolar covalent
  bonds, it is automatically a nonpolar molecule.
• If a molecule contains some polar bonds or all
  polar bonds, it may or may not be polar. The
  polarity of a molecule is determined by the
  polarity of its bonds AND by the shape of the
  molecule.

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Example 1 Formaldehyde

• Formula CH2O

The difference in electronegativity between
Oxygen and Carbon is 1.0, so the bond is clearly
polar.
O
The three green arrows show the direction of the
shift in negative charge based on the polarity of
each bond. All three arrows point up toward the
top of the molecule, so overall, the molecule is
polar. The oxygen end is negative, and the
hydrogen end is positive.
C
H
H
The difference in electronegativity between
Carbon and Hydrogen is 0.4, so these two bonds
are just barely polar.
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Example 2 Carbon Dioxide

• Formula CO2

C
O
O
The difference in electronegativity between
Oxygen and Carbon is 1.0, so both bonds are polar.
The two green arrows show the direction of the
shift in negative charge based on the polarity of
each bond. The arrows point in opposite
directions, so there is no way the molecule can
be polar.
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Example 3 Water

• Formula H2O

O
The two green arrows show the direction of the
shift in negative charge based on the polarity of
each bond. Both arrows point toward the oxygen,
so the oxygen end of the molecule is negative,
and the hydrogen end is positive. This molecule
is polar.
H
H
The difference in electronegativity between
Oxygen and Hydrogen is 1.4, so both bonds are
polar.
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BIG SHORTCUT!!!

• Linear, trigonal planar, and tetrahedral
  molecules will be nonpolar AS LONG AS the atoms
  around the center are all the same element.
• Bent and pyramidal molecules are always polar.

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Large Molecules

• For small molecules, the shape helps determine
  the polarity. The opposite is true for large
  molecules the polarity helps determine the
  shape.

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• One example of a large molecule is a protein.
  Proteins are long chains of subunits. Some
  subunits have side-chains that are polar, and
  other subunits have side-chains that are
  nonpolar. The polar side-chains on different
  subunits act like magnets to attract each other
  and stick together. This makes the whole chain
  kink and bend, kind of like the cord on your
  telephone.

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subunit
polar side-chain
nonpolar side-chain
Because the polar side-chains are attracted to
each other, the whole protein strand will kink up.