Chemistry 1011

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Chemistry 1011

• TOPIC
• Physical Properties of Matter
• TEXT REFERENCE
• Masterton and Hurley Chapter 9

2
Review

• YOU ARE EXPECTED TO BE ABLE TO
• Draw electron dot diagrams to represent bonding
  in molecules and ions define electronegativity
  determine whether a covalent bond will possess a
  dipole moment determine the geometric shape of
  molecules using VSEPR theory determine whether a
  given covalent molecule will possess a net dipole
  moment. (REVIEW FROM CHAPTER 7)

3
REVIEW

• In order to understand intermolecular forces and
  their effects, you need to be able to
• Draw electron dot diagrams to represent bonding
  in molecules and ions
• define electronegativity
• determine whether a covalent bond will possess a
  dipole moment
• determine the geometric shape of molecules using
  VSEPR theory
• determine whether a given covalent molecule will
  possess a net dipole moment

4
REVIEW Drawing Electron Dot Diagrams

• Electron dots represent valence electrons
• In simple molecules, elements share pairs of
  valence electrons to form covalent bonds.
• Atoms other than hydrogen tend to from bonds
  until they are surrounded by eight electrons
  the octet rule e.g. CH4, NH3, H2O, HF
• A non-metal element normally forms a number of
  covalent bonds equal to
• 8 the number of valence electrons
• Double bonds, triple bonds are formed when two or
  three covalent bonds exist between a pair of
  atoms

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REVIEW Steps for Writing Electron Dot Diagrams

• Identify the central atom
• Draw an electron dot diagram for the molecule
  showing only single bonds
• Determine the number of bonds in the molecule
• bonds ½ (atoms x 8 actual valence
  electrons)
• Add any required double or triple bonds
• Draw in any remaining unshared pairs of electrons

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Electron Dot Diagram for NF3

• N is Central atom
• Actual valence electrons (3 x 7) 5 26
• Maximum valence electrons 4 x 8 32
• Difference 32 26 6 \ 3 bonds
• Distribute remaining electrons

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Electron Dot Diagram for NO2-

• N is Central atom
• Actual valence electrons (2 x 6) 5 1 18
• Maximum valence electrons 3 x 8 24
• Difference 24 18 6 \ 3 bonds
• There must be a double bond
• Distribute remaining electrons

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Electron Dot Diagram for BF3

• Boron trifluoride is electron deficient
• The octet rule does not apply
• Boron is the central atom
• Actual valence electrons (3 x 7) 3 24
• Maximum valence electrons 4 x 8 32
• Difference 32 24 8 4 bonds should be
  present, but only 3 are possible

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Electron Deficient Molecules

• In a few molecules, there are less than 8
  electrons around the central atom
• F Be F F B F

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Expansion of the Valence Level - Electron Dot
Diagram for SF6

• Sulfur (3s2, 3p4)has 6 valence electrons and
  would normally form 2 bonds
• However, two valence electrons can be promoted to
  the 3d orbitals (3s1, 3p3 ,3d2)
• In this way, sulfur can form additional bonds

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REVIEW Electronegativity

• The electronegativity of an element is a measure
  of the ability of an atom of the element to
  attract a shared electron pair in competition
  with an atom of a different element
• Electronegativities vary in much the same way as
  ionization energies
• They DECREASE going DOWN a Group
• They INCREASE going ACROSS a period from left to
  right
• E.g. FgtOgtNgtC and FgtClgtBrgtI

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REVIEW Bond Dipole Moments

• The valence electrons that make up a covalent
  bond between two atoms of the same element will
  be shared equally
• When bonds form between different elements, with
  different electronegativities, the bonding
  valence electrons will be attracted more towards
  one atom than the other
• The result is the existence of a bond dipole
• E.g. HF

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REVIEW VSEPR Theory and Molecular Shapes

• The pairs of electrons that surround the central
  atom repel each other and arrange themselves in
  space in such a way that they are as far apart as
  possible

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Molecular Shapes

• Four bonding pairs CH4
• Three bonding pairs, one non-bonding pair NH3
• Two bonding pairs, two non-bonding pairs H2O
• Electron deficient - three bonding pairs only
  BF3
• Expanded valence level six bonding pairs - SF6
• Two bonding pairs CO2

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Geometries of AX2-AX6 molecules
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Geometries of AX2-AX6 molecules
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REVIEW Molecular Dipole Moments

• Unsymmetrical molecules with polar bonds will
  have a molecular dipole
• HF
• H2O
• Symmetrical molecules with polar bonds will not
  have a molecular dipole
• BF3
• SF6

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Steps in Determining Molecular Polarity

• Electron dot diagram
• Bond dipoles?
• Molecular shape
• Symmetrical or non-symmetrical molecule?
• Molecular dipole?