AP Chemistry
Ch 8 Bonding
Vocab (Ch 8)
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Isoelectronic series
Dipole
Dipole moment
Formal charge
Resonance structure
Bond enthalpy
Lewis Dot Structures
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Create structures that satisfy the Octet Rule.
This shows us how the electrons are
distributed within a molecule.
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Page 278
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Formal Charge
The Formal Charge of an atom is the charge
that an atom in a molecule would have if all
atoms had the same electronegativity.
There may be instances when you can draw
2 structures for the same molecule. Then you
would want to use the Formal Charge Method
to determine which structure is the most
feasible.
Calculating Formal Charge
1.
All of the unshared (nonbonding) electrons are assigned to
the atom on which they are found.
2.
Half of the bonding electrons are assigned to each atom in
the bond.
3.
Formal Charge =
# of valence e- in the isolated atom
minus
# of electrons assigned to the atom in the Lewis
Structure.
See pages 280-281 for examples.
Resonance Structures
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These are individual Lewis dot structures in
cases where 2 or more Lewis structures are
equally good descriptions of a single
molecule.
The resonance structures are equivalent
except for the placement of electrons.
Resonance Structures
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Use double-headed arrows to indicate that the
structures are resonance structures.
Molecules that have resonance structures can not
be described accurately using only one Lewis
structure. The true arrangement of the electrons
must be considered as a “blend” of 2 or more Lewis
structures.
The molecule does not “oscillate” between 2 or more
Lewis structures.
See page 283
Resonance Structures
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commons.wikimedia.org/wiki/File:Nitrate_ion_r...
Exceptions to the Octet Rule
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There are 3 main types of exceptions:
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Molecules with an odd number of electrons.
Molecules in which an atom has less than an
octet.
Molecules in which an atom has more than an
octet.
Page 285-287
Strengths of Covalent Bonds
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The stability of a molecule is related to the
strengths of the covalent bonds it contains.
The strength of a covalent bond between 2
atoms is determined by the energy required
to break that bond.
We will use bond enthalpy to determine the
strength of the bonds.
Enthalpy and Bond Strength
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The bond enthalpy is always a positive value.
Energy is always required to break chemical bonds.
Energy is released when a bond forms between 2
gaseous atoms or molecular fragments.
The greater the bond enthalpy, the stronger the
bond.
A molecule with strong chemical bonds generally
has less tendency to undergo chemical change than
a molecule with weak bonds.
Bond Enthalpy and
Bond Strength
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ΔH = Σ(bond enthalpies of bonds broken)
minus
Σ(bond enthalpies of bonds formed)
One method of doing this is to break all of the bonds on the
reactant side and add them up. Then break all of the bonds
formed on the product side and add them up. Subtract
products from reactants.
Use the chart on page 289 for Average Bond Enthalpy values.
Exam Topics
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Concept Ionic vs Covalent vs Metallic bonding
Lattice energy problems
Isoelectronic series (nuclear charge)
Electronegativity, bond polarity, dipole, dipole
moment
Bond length
Formal charge
Resonance structures
Exceptions to the Octet Rule
Bond Enthalpy Calculations
Draw Lewis Dot Structures
Problems to Try
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Ch 8
# 9-11, 19, 20, 23, 27, 33, 34, 40, 41, 43, 44, 46-48,
50, 51, 54, 60, 61, 63, 64, 66-68, 79, 81, 85, 87
AP Exam Problems to Try
1999 # 8
2000 # 7 (last section)
2002 # 6
2003 # 8
2004 # 7 & # 8