VSEPR

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VSEPR

• Valence shell electron pair repulsion
• Repulsion between the sets of valence-level
  electrons surrounding an atom causes these sets
  to be oriented as far apart as possible.

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Two atoms

• Linear
• Electron pairs spread out as far as possible to
  minimize repulsive forces
• Draw picture of example

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AB2

• Be is used, it is an exception to the octet rule.
• No unshared pairs of electrons.
• Linear shape

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AB3

• Equilateral triangle with all atoms on one plane
• Trigonal planar
• Angles 120 degrees apart
• Example -- GaF3
• No unshared pairs

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AB4

• Tetrahedron shape
• Tetrahedral shape
• No unshared pairs
• Example CH4

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Now, with unshared electron pairs

• An unshared pair of electrons is associated with
  the central atom.
• It is like an electron cloud shaped like a pear
  with one end attached to the nucleus.
• FYI (a shared pair moves between two nuclei and
  therefore forms a more slender, stretched pear
  shaped cloud.)

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continued

• the unshared pair does not literally occupy
  space.
• The unshared pair shows greater repulsion than
  shared electron pairs.
• For geometry purposes only, we think of the
  unshared electron pair as occupying space
  around the nucleus.

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AB3E

• E represents the unshared pair.
• Trigonal Pyramidal
• Base forms a triangle, the unshared electron pair
  forms the top of the pyramid.

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AB2E2

• Water --H2O
• Bent, angular shape
• 105 degree between the H atoms

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Table on page 186
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Hybridization

• Carbon
• Only has 2 free p electrons, but bonds with 4
  atoms.
• Forms tetrahedral shaped molecules

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sp3

• The s orbital merges with the p orbitals to make
  a sp3 orbital
• Hybrid orbitals
• All 4 are identical

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Other types of hybridization

• Hybridization explains the geometry of many group
  15 and 16 elements
• sp linear
• Sp2 trigonal planar
• Sp3 tetrahedral
• Table 6.6 page 189

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Polarity

• In a covalent bond the more electronegative atom
  will pull on the electron more.
• So, the electron will spend more time around the
  more electronegative atoms nuclei.
• With geometry this creates a polar molecule.

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Polar molecule

• If you can draw a line through it making a
  partial negative and a partial positive side.
• Polar molecule uneven distribution of charge.
• Polar molecules cause dipoles.

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Intermolecular forces

• Force of attraction between two molecules
• types
• dipole - dipole
• induced dipole
• hydrogen bonding
• London dispersion

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Intermolecular forces

• Forces of attraction between two molecules.
• Measured by boiling point- energy required for a
  molecule to break away from the other molecules.
• Higher boiling point, stronger the attraction

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Polar molecules

• Strongest intermolecular forces
• Form dipoles
• Dipole
• Arrow indicates the direction of the dipole
• Positive to negative pole
• Indicated on the bonds

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Dipole-dipole forces

• The forces of attraction between polar molecules
• Short range, acts on nearby molecules
• Larger the dipole dipole attraction, stronger the
  intermolecular forces

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Induced dipoles

• Short range
• Weaker than dipole- dipole between polar
  molecules
• Why some non-polar substances dissolve in polar
  water

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Induced continued

• A polar molecule comes into contact with a
  non-polar molecule.The partial charge either
  attracts or repels the electrons of the non-polar
  molecule. Thus creating a temporary dipole.

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Hydrogen bonding

• Some hydrogen containing compounds have unusually
  high boiling points.
• Explained by a strong dipole dipole force called
  hydrogen bonding.
• Represented by dashed lines----
• Intermolecular force

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Hydrogen Bonding definition

• The intermolecular forces in which a small
  hydrogen atom, that is bonded to a highly
  electronegative atom, is attracted to an unshared
  pair of electrons of an electronegative atom in a
  nearby molecule.

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London dispersion forces

• Electrons are in continuous motion.
• At any given instant, the physical distribution
  of electrons could be uneven.
• Momentary imbalance can cause a positive and
  negative pole.
• This can then induce a dipole in a neighboring
  molecule!

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London dispersion forces

• The intermolecular attraction resulting from the
  constant motion of electrons and the creation of
  instantaneous dipoles.

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• Weak
• Intermolecular
• More electrons more possibility stronger they
  are.
• More electrons generally means more mass.