Transcript Chapter 11 Chemical Bonds: The Formation of Compounds from

Chapter 11
Chemical Bonds:
The Formation of Compounds from Atoms
Objectives:
Describe the trends in the periodic table
Know how to draw Lewis Structures of atoms
Understand and predict the formation of ionic bonds
Understand and predict covalent bonds
Describe electronegativity
Know how to draw complex lewis structures of compounds
Understand the formation of compounds containing polyatomic ions
Describe molecular shape, including the VSEPR model
Periodic Trends in Atomic
Properties
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Periodic table designed to show trends
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Use trends to predict properties and
reactions between elements
Trends include:
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Metals, nonmetals, metalloids
Atomic radius
Ionization energy
Electronegativity
Metals, Nonmetals and Metalloids
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Metals:
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Lustrous, malleable, good conductors of heat
and electricity
Left-hand side of table
Most elements are metals
Tend to lose electrons and form positive ions
Metals, Nonmetals and Metalloids
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Nonmetals:
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Nonlustrous, brittle, poor conductors
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(Hydrogen displays nonmetallic properties
under normal conditions but is UNIQUE
element)
Metals, Nonmetals and Metalloids
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Metalloids
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Found along border between metals and
nonmetals
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Metal + Nonmetal
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Usually electrons are transferred from metal
to nonmetal…
Atomic Radius
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Increases down each group
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Decreases from left to right across a
period
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Increase in positive charge = stronger pull on
electrons = gradual decrease in atomic radius
Atomic Radius
Ionization Energy
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The energy required to remove an
electron from the atom
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Ionization Energy
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Ionization energy in Group A elements
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Ionization energy
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Nonmetals – tend to gain electrons (rather
than give them up)
Ionization Energy
Lewis Structures
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Diagram that shows valence electrons
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Dots = number of s and p electrons
Paired dots =
Simple way of showing electrons
Lewis Structures
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When drawing:
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“3, 6, 9” =
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Just like orbital filling diagram…
Examples: draw Lewis Structures of B, N,
F, Ne
Lewis Structures
B
F
N
Ne
The Ionic Bond
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Ionic bond:
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Attraction between electrostatic charges is a
The Ionic Bond
The Ionic Bond
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NOT A MOLECULE
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Bond not just between
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The Ionic Bond
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Typically metal + nonmetal
Predicting Formulas of Ionic
Compounds
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In almost all stable chemical compounds
of representative elements, each atom
attains a noble gas electron configuration.
This concept forms the basis for our
understanding of chemical bonding.
Predicting Formulas of Ionic
Compounds
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How many electrons must be gained or
lost to achieve noble gas configuration?
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Predicting Formulas of Ionic
Compounds
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Elements in a family usually form
compounds with the same atomic ratios
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Predicting Formulas of Ionic
Compounds
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The formula for sodium oxide is
Predict the formula for
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Sodium sulfide
Sodium [Ne]3s1 must
 Sulfur [Ne]3s23p4 must
 So…formula must
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Predicting Formulas of Ionic
Compounds
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Rubidium Oxide
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Rubidium [Kr]5s1 must
Oxygen [He]2s22p4
So…formula must be
This makes sense b/c rubidium is in same
family as sodium
The Covalent Bond
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A pair of electrons
Most common type of bond
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Electron orbital expands to include both
nuclei
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The Covalent Bond
The Covalent Bond
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Atoms may share more than one pair of
electrons
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Double bond –
Triple bond –
Multiple bonds are
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Covalent bonding between identical atoms
means electrons are
Covalent bonding between different atoms leads
to
Electronegativity
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The attractive force that an atom of an
element has for shared electrons
Atoms have different electronegativities
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Electrons will spend more time near atom
with stronger (larger) electronegativity
So…one atom assumes a
 The other assumes a
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Electronegativity
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Electronegativity trends and periodic table
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See table 11.5 page 237
Generally increases from left to right
Decreases down a group
Highest is fluorine (4.0)
Lowest is francium (0.7)
Electronegativity
Electronegativity
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Polarity is determined by difference in
electronegativity
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Nonpolar covalent
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Polar covalent
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Ionic compound
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Electronegativity
Electronegativity
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If the electronegativity difference is
greater than 1.7-1.9 then the bond will be
more ionic than covalent
Above 2.0 =
Below 1.5 =
See Continuum on page 239
Electronegativity
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Polar bonds form between two atoms
Molecules can also be polar or nonpolar
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Dipole
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Polar
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Nonpolar
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Lewis Structures of Compounds
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Convenient way of showing ionic or
covalent bonds
Usually the single atom in a formula is the
central atom
The Ionic Bond
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LEWIS STRUCTURES of ionic bonds
The Covalent Bond
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LEWIS STRUCTURES of covalent bonds
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Use dashes instead of dots…
The Covalent Bond
Lewis Structures of Compounds
1)
Obtain the total number of valence
electrons
1)
2)
Add the valance electrons of all atoms
Ionic – add one electron for each negative
charge and subtract one electron for each
positive charge
Lewis Structures of Compounds
2)
3)
Write the skeletal arrangement of the
atoms and connect with a single covalent
bond
Subtract two electrons for each single
bond
1)
This gives you the net number of electrons
available for completing the structure
Lewis Structures of Compounds
4)
5)
Distribute pairs of electrons around each
atom to give each atom a noble gas
structure
If there are not enough electrons then
try to form double and triple bonds
Lewis Structures of Compounds
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Write the Lewis Structure for methane
CH4
1)
2)
Total number of valence electrons is eight
Draw skeletal structure
1)
3)
4)
Dashes equal two electrons being shared
Subtract the eight electrons shown as
dashes
Check that all atoms have a noble gas
structure
Lewis Structures of Compounds
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Methane, CH4
Lewis Structures of Compounds
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Carbon Dioxide, CO2
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Total valence electrons = 16
Not Enough! Must try double bonds…
Complex Lewis Structures
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Some molecules and polyatomic ions have
strange behaviors…
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No single Lewis structure is consistent
If multiple structures are possible the
molecule shows resonance
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Resonance structures – show all possibilities
Complex Lewis Structures
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Carbonate ion, CO32-
Carbon only has 6
electrons – try double
bonds – more than
one location…..form
resonant structures…
Compounds Containing
Polyatomic Ions
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Polyatomic ion: stable group of atoms
that has a positive or negative charge
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Behaves as a single unit in many chemical
reactions
Sodium carbonate (Na2CO3)
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Carbonate ion (co3) has covalent bonds
Sodium atoms are ionically bonded to
carbonate ion
Compounds Containing
Polyatomic Ions
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Easier to dissociate ionic bond than break
covalent bond
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More in chapters 6 and 7
Molecular Shape
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Three-dimensional shape of molecule
important
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Explains
Helpful to know how to predict the
geometric shape of molecules…
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Linear?
V-shaped?
Trigonal planar?
Tetrahedral?
The VSEPR Model
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Valence Shell Electron Pair Repulsion
Model
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Make predictions about shape
Electron pairs will
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The VSEPR Model
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Linear Structure
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180o apart
The VSEPR Model
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Trigonal Planar
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120o apart
The VSEPR Model
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Tetrahedral structure
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109.50 apart
When drawing:
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Wedged line to show
atom protruding from
page; dashed line to
show atom receding from
page
The VSEPR Model
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Pyramidal shape
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Four pairs of electrons
on central atom BUT
only three shared…
Electrons are
tetrahedral but actual
shape is more of a
pyramid
The VSEPR Model
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Electron pairs determine shape BUT name
for shape is determined by position of
atoms
The VSEPR Model
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V-shaped or bent
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Four electron pairs but
only two shared
Electron arrangement is
But, molecule is
Water
 Helps explain some
properties
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The VSEPR Model
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Predict the shape for .
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Draw the Lewis Structure
Count the electron pairs and determine the
arrangement that will minimize repulsions
Determine the positions of the atoms and
name the structure