Chemistry 1011
TOPIC
Physical Properties of Matter
TEXT REFERENCE
Masterton and Hurley Chapter 9
Chemistry 1011 Slot 5
1
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)
Chemistry 1011 Slot 5
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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
Chemistry 1011 Slot 5
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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
Chemistry 1011 Slot 5
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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
Chemistry 1011 Slot 5
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Electron Dot Diagram for NF3
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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
Chemistry 1011 Slot 5
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Electron Dot Diagram for NO2•
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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
Chemistry 1011 Slot 5
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Electron Dot Diagram for BF3
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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
Chemistry 1011 Slot 5
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Electron Deficient Molecules
In a few molecules, there are less than 8 electrons
around the central atom:
••
••
••
F
– Be – •F•
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••
••
••
••
••
F
–
B
–
F
••
••
••
F ••
••
Chemistry 1011 Slot 5
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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
Chemistry 1011 Slot 5
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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.
F>O>N>C and F>Cl>Br>I
Chemistry 1011 Slot 5
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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
Chemistry 1011 Slot 5
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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
Chemistry 1011 Slot 5
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Molecular Shapes
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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
Chemistry 1011 Slot 5
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Geometries of AX2-AX6 molecules
Chemistry 1011 Slot 5
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Geometries of AX2-AX6 molecules
Chemistry 1011 Slot 5
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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
Chemistry 1011 Slot 5
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Steps in Determining Molecular
Polarity
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Electron dot diagram
Bond dipoles?
Molecular shape
Symmetrical or non-symmetrical molecule?
Molecular dipole?
Chemistry 1011 Slot 5
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