9.4 Molecular Shape

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9.4 Molecular Shape

• VSEPR Model
• (Valence Shell Electron Pair Repulsion)
• Used to show the molecular shape
• Based on arrangement that minimizes repulsion of
  shared unshared pairs of e- around central
  atom.
• Repulsions result in atoms existing a t fixed
  angles to each other
• Bond angle formed by any two terminal atoms and
  the central atom
• Shared e- repulse each other
• Lone pairs will occupy a slightly larger orbital
  than shared e-
• Example PH3 has one lone pair and 3 bonds. Q
  What shape should it have
• A trigonal pyramid
• Table 9-3

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• Hybridization
• Proces in which atomic orbitals are mixed to form
  new, idential hybrid orbitals.
• Each hybrid orbital will contain one electron
  that it can share with another atom.
• Carbon is best example that undergoes
  hybridization
• Lone pairs can occupy hybrid orbitals. Compare
  BeCl2 and H20.
• Both contain 3 atoms, but they occupy different
  orbitals due to the one pairs on water.

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Polarity

• Polar Covalent Bonds
• A covalent bond formed between atoms of diff
  elements, no equal sharing of electrons due to
  electronegativtiy (tendency accept an electron)
• This results in a polar covalent bond and sharing
  is not equal
• Molecular Polarity
• Molecules are either nonpolar are not attracted
  by an electric field or polar align with an
  electric field
• Polar molecules have a partial negative charge on
  one side, while the other side has a positive
  charge.

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Examples

• Water
• Is bent in shape
• A polar molecule
• Because the H-O bonds are not symmetric, the
  molecule has a positive end and a negative end.
• Carbon tetrachloride
• Is tetrahedral in shape
• It is symmetric and all charges are balanced.
• It is a nonpolar molecule
• Ammonia
• This is polar covalent.
• The charge distribution is unequal because it is
  not symmetric

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Properties

• Covalent molecular solids
• Due to intermolecular forces (attraction between
  molecules)
• The force between polar molecules is stronger and
  is called a dipole-dipole force.
• Nonpolar substances, the attraction between
  molecules are weak and called dispersion forces
• The third force is in a hydrogen bond. It is
  formed with H on the end of a dipole , and also a
  F,O, or N atom on another dipole
• This is why melting points and boiling points are
  lower in covalent compounds and higher in ionic
  compounds