Chemistry 545 Inorganic Chemistry Lecture 1'

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Chemistry 545Inorganic ChemistryLecture 1.

• VSEPR

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VSEPR.

• The octet rule represents Valence Bond (VB)
  theory at its simplest level, and applies largely
  to compounds containing light central atoms such
  as B, C, N, O, and F. Heavier atoms such as Si,
  Ge, P, As, S, Se, Cl, Br and I are able to expand
  their valence shells, which leads to the familiar
  VSEPR (Valence Shell Electron Pair Repulsion)
  approach to molecular structure, which was
  developed by Ronald Gillespie. The basic idea is
  that electron pairs, whether present as lone
  pairs, or present in bonds between atoms, occupy
  space around a central atom in such
• a way as to minimize electrostatic repulsion
• between them.

Ronald Gillespie.
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Electron domains and molecular geometry
each pair of electrons both the lone pair, and
the pairs of electrons in each bond constitute
an electron domain
observed geometry is that where the
electron domains are as far apart as possible
lone pair of electrons
N
H
H
H
Lewis dot diagram of ammonia
Ammonia (derived from Tetrahedral
geometry)
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Using VSEPR

• In order to use VSEPR to predict molecular
  structure
• Draw up Lewis dot diagram for the molecule or
  ion. The first atom (e.g. N in NH3) is usually
  the central atom (H2O, HF are exceptions). Place
  the other atoms around the central atom.
• If these are single bonds, contribute one
  electron per attached atom. Then add the valence
  electrons for the central atom 5 for N.
• 2) Work out number of electron domains valence
  electron pairs (n) plus attached atoms on
  central atom. For NH3 n 4.
• 3) Relate n to the type of structure predicted
  for that value of n. n 4 tetrahedral.
• 4) Place lone pairs in expected positions,
  maximizing separation of lone pairs. For NH3,
  there is one lone pair, so mol. structure
  trigonal pyramidal.

place 3 H atoms around central N
red 5 valence electrons for N
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The structure of H2O from VSEPR
Lewis dot diagram
molecular structure bent
parent structure
lone pairs
n 4 from two attached atoms plus two
electron pairs
n 4, parent structure tetrahedral, but one
site occupied by a lone pair
molecular or final structure disregard the two
lone pairs
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Parent shapes for EXn molecules (n 2-5)

• Formula n shape shapes of structures
• EX2 2 linear
• EX3 3 trigonal planar
• EX4 4 tetrahedral

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Parent shapes for EXn molecules (n 6-8)

• Formula n shape shapes of structures
• EX5 5 trigonal
• bipyramidal
• EX6 6 octahedral

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Final structures for VSEPR theory.
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More final structures for VSEPR.
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Examples
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Ozone a bent molecule

• The structure of the O3 (ozone) molecule can be
  predicted using VSEPR. First draw up the Lewis
  dot diagram

Note that two pairs of es still count as only
one electron domain one attached O-atom
For the valence shell of the central oxygen atom
n 3, so parent geometry trigonal. The final
structure is thus two-coordinate bent, as seen
for the ozone molecule below
Central atom (red valence electrons)
Structure of the ozone molecule (oxygens red
atoms)
ozone
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Some more bent molecules/anions
N
sulfur dioxide the nitrite
anion
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Some linear molecules
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Trigonal planar molecules