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Example of Ionic Bonding
Example of Covalent Bonding
Sharing equally
Sharing Unequally
non-polar covalent
polar covalent
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On p. 176, there is a general scale of 'ionic
character'. The greater the difference in
electronegativity, the more ionic character that
a bond has. You older brother hogs the xbox
so much, that it effectively becomes his.
If the difference in electronegativity is greater
than 1.7, the bond is considered ionic. If it is
less than 1.7, it is polar covalent. If it is less
than .3, it is non-polar covalent.
You can refer to the periodic table on p.161 for
electronegativities.
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Even though there are 'electronegativity
ranges', simply knowing this information is
impractical. For all practical purposes, ionic
bonding occurs between a metal and a nonmetal, while covalent bonding occurs between
non-metals and non-metals.
November 16, 2015
Homework:
p. 177 Section 6.1 Review # 1-6
Begin reading Ch. 6
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Section 6.2 - Covalent Bonding & Molecular
Compounds
What is a molecule?
-a group of atoms that are bonded together
covalently.
A compound whose atoms are held together
with covalent bonds is called a molecular
compound.
Covalent Bonds = Molecular Compound
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Formulas
-Chemical Formula - Shows the relative
number of atoms in a chemical compound
using atomic symbols and numerical subscripts
-Molecular Formula - shows the types and
numbers of atoms combined in a single
molecule.
Technically, there is a difference between the
two, but practically there is not. Ionic
compounds have chemical formulas, covalently
bonded compounds have molecular formulas.
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Examples of:
Chemical Formulas
Molecular Formulas
NaCl
CO
K2O
H 2O
Ca(NO3)2
C6H12O6
As you can see there is not much difference
between these two types of formulas.
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How do covalent bonds form?
Bonds form so the atoms can become more
stable. They form when the positive nucleus of
one atom interacts or attracts valence electrons
of a different atom. The atoms will get closer
and closer to each other until their nuclei begin
to repel each other. This formation represents
the lowest potential energy of the two atoms.
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How do covalent bonds form? By sharing
pair(s) of electrons.
Orbital Notation - H2, Cl2
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Orbital Notation - HCl, O2
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When bonding occurs, each atom will gain,
lose, or share electrons to become more stable,
resulting in 8 electrons in the highest occupied
energy level (valence). What is this behavior
called?
The Octet Rule
There are a few exceptions to the octet rule.
The ones that you will be responsible for
knowing are: B is stable with 6 electrons, P
with 10, and S with 12.
Why do you think this is?
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The energy required to break a chemical bond
is called 'bond energy'.
The stronger the bond, the more energy that is
required to break it. Also, the stronger the
bond, the closer the atoms will be to each
other. Bond length will be shorter in stronger
bonds.
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HW - Read about Lewis Dot Structures, p.
184-189
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Electron Dot Notation/Lewis Dot Structures
Electron dot notation & Lewis dot structures
show simplified representations of atoms
where the atomic symbol represents the
nucleus and inner-shell electrons and the
outer shell valence electrons are shown as
dots.
H
C
P
I
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There are two schools of thought on how to
draw the electrons around the symbol.
1. Electrons are negative, so they will all repel
each other and get as far away as possible.
2. The sides of the atom represent the main
group orbitals (s & p). This is my preference.
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In covalent bonding, atoms share electrons to
obtain eight valence electrons. We can
represent these bonds with Lewis Dot
structures.
HCl
H 2O
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Sometimes it is necessary to share more than
one pair of electrons.
O2
CO2
These are double bonds.
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N2
These are triple bonds.
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F2
NI3
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Tips for drawing Lewis dot structures:
1. Start from the outside.
2. Put electrons around 'their' element.
3. If you have an odd number of electrons,
the single one must be shared, so you can
place it between atoms.
4. Double check your total number of
electrons. They must equal the sum of
available electrons!
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November 16, 2015
Practice problems p. 186 1-4
p. 188 1-2
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Some molecules cannot be represented
adequately by only one Lewis dot structure.
One example is SO3. Another is ozone, O3.
Resonance refers to bonding in molecules that
cannot be correctly represented by a single
Lewis dot structure.
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Sometimes, rather than draw multiple Lewis
dot structures, you will see a hybrid structure
drawn with dotted lines representing partial
bonds. This representation is a much less
common method.
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Ionic Bonding
An ionic compound is made up of positive and
negative ions that are combined in such a
manner that the positive and negative charges
are equal, forming a neutral compound.
Since there are positive surrounded by negative
ions surrounded by positive ions an so on, a
three dimensional crystal structure is formed.
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The
crystal
structure
is called
a crystal
lattice.
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When metal and nonmetal atoms approach
one another, the loosely held valence
electron(s) of the metal is(are) transferred to
the nonmetal. This transfer results in
opposite charges, which then attract. The
end result is a crystal lattice structure.
Since there is a repeating structure of ions, it
is impossible to study an independent,
neutral unit of the compound that can be
isolated. Therefore, the chemical formula
shows the simplest ratio of ions that make
the compound neutral.
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This smallest whole number ratio of ions to
achieve an electrically neutral charge is called
a formula unit. It is the simplest collection of
atoms from which an ionic compound's
formula can be established. Unlike molecules
which are separate entities, formula units are
all together:
ballroom dancing
molecules
vs.
a mosh pit.
vs.
formula units
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Combine the following in appropriate ratios to
obtain neutrality.
Cations:
Anions:
Na+
F-
Mg2+
S2-
Al3+
N3-
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Molecular Compounds vs. Ionic Compounds
weak bonds
strong bonds
low melting points
high melting points
low boiling point
high boiling point
soft
hard
soluble in oils/nonpolar
soluble in water/polar
non-conductive
conductive if molten
or dissolved
flexible
brittle
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November 16, 2015
Polyatomic ions
are just that - ions that are made out of many
atoms, not just one. Polyatomic ions consist of
multiple atoms that are covalently bonded
together which then gain or lose electrons.
Once they have become ions, they behave like
monatomic ions.
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Examples of polyatomic ions:
Nitrate: NO3-
Ammonium: NH4+
Sulfate: SO42-
Phosphate: PO43-
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I do not make you memorize the periodic table
(or portions thereof) because it will almost
always be available, and you will learn the
common things simply by using them. I will,
however, require you to memorize the list of
polyatomic ions. It is absolutely vitally
imperative that you know these ions. For that
reason, you must memorize the table on p. 226
of your textbook. There will be a quiz weekly
until 85% of the class scores 85% or higher.
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November 16, 2015
Homework:
Section 2 Review p. 189
Section 3 Review p. 194
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November 16, 2015
Molecular Geometry
The shape that molecules physically have.
The molecular geometry that molecules have
is primarily explained by VSEPR Theory.
V alence
S hell
E lectron
P air
R epulsion
VSEPR Theory basically
says that the negative
electron pairs will repel each
other until they get as far
apart as physically possible.
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If there are two atoms bonded together, only
one shape is possible. Linear
If there are three atoms that are covalently
bonded together, they can form three possible
shapes.
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2. Bent 109.5o
3. Bent 104.9o
H 2O
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4. Trigonal Planar - BH3 or CO32-
5. Trigonal Pyramidal - NH3
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6. Tetrahedral - CH4
7. Trigonal Bipyramidal - PCl5
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10. Octahedral - SF6
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Tomorrow - Molecular Geometry Lab
You will be drawing lewis dot structures, and
then building the molecules in the correct
shapes with doughnut holes and toothpicks.
Also, don't forget to study for the Polyatomic
Ion quiz.
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Practice: Draw the Lewis dot structure and
identify the molecular geometry of the following:
HCN
PO43-
PI5
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Different shapes of molecules result from
different hybrid orbitals.
sp (linear)
sp2 (trig.planar)
sp3(tetrah.)
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What holds molecules together?
Why when you pour water, does the water stick
together to go into your cup?
This answer is fairly obvious for IONIC
compounds, but not so for molecular
compounds.
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Forces of attraction between molecules are
called INTERMOLECULAR FORCES.
There are three distinct types of intermolecular
forces, but they are all based on a dipole. A
dipole is where a charge differential exists over
a short distance - think N and S poles on a
magnet. Equal but opposite charges are
separated by a short distance in some
molecules. This charge differential leads to
dipole-dipole forces.
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Dipole-dipole forces are very similar to ionic
bonds, except they are much weaker. Look at
HCl molecules.
The positive end of one molecule is attracted to
the negative end of a different molecule.
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The next type of intermolecular force is called
'Hydrogen Bonding'. This name is a misnomer
because they are not actually bonds, but rather
attractive forces. These forces involve
hydrogen being bonded to a highly
electronegative element, such as F, Cl, O, and
N.
Hydrogen bonding is the STRONGEST
intermolecular force, and results in the highest
boiling points and melting points.
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The third type of intermolecular force is called
London Dispersion Forces, or Van der Waals
forces. These forces are caused by the random
formation of dipoles and EVERY molecular
compound experiences these forces. They are
the weakest of the intermolecular forces.
ex. people on a cruise ship
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p. 207 Section 6.5 Review
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So far, we have discussed covalent bonding
(molecular compounds) and ionic bonding
(ionic compounds). In covalent bonding,
electrons are shared between atoms. In ionic
bonding, electrons are transferred from one
atom to another, resulting in ions.
The third type of bonding that is present is
called metallic bonding.
Metallic bonding occurs only between metal
atoms.
November 16, 2015
In metallic bonding, electrons are neither
shared nor transferred. They are 'delocalized'.
This term simply means that the valence
electrons do not belong to any specific nucleus,
but are free to move between atoms.