Transcript Chemical_Bonding_

Chemical Bonds
Forming Chemical Bonds
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The force that holds two atoms together is called a
chemical bond .
The valence electrons are the electrons involved in
forming chemical bonds.
Elements tend to react to acquire eight electrons.
This is call a stable octet.
Noble gases (group VIIIA/18) have this structure
(octet) and are inert (does not form bonds).
Atoms can gain, lose, or share electrons to reach
an octet.
Forming Ions
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Positive ions (cations) are formed when atoms
lose one or more valence electrons. [Usually
these atoms are metals.]
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Reactivity of metals (cations) are based on how
easily they lose electrons. [ Ionization energy]
Negative ions (anions) are formed when atoms
gain one or more valence electrons. [These
atoms tend to be non-metals.]
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Electronegativity is the ability to attract or gain
electrons.
Forming Ionic Bonds
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Ionic bonds: Complete transfer of electrons
between atoms (difference of electronegativity of
1.6 or greater).
Two neutral atoms will form ions. The resulting
compound is called an ionic compound.
Properties of Ionic Compounds
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Form crystal lattice bonding structure.
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Example: NaCl (sodium chloride)
High melting point and boiling point due to
strong electrostatic charge between the atoms
(cation is attracted to anion after the transfer of
electrons).
Hard, rigid and brittle solids.
Conducts electricity in liquid state or
dissolved in water only.
Forming Covalent Bonds
Covalent bonds: Sharing of electrons between
atoms (difference of electronegativity of less
than 1.6).
 Occurs usually between elements close to each
other on the periodic table (mostly nonmetals).
 The resulting compound is called a molecule.
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Properties of Covalent Compounds
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Have definite and predictable shapes.
Low melting and boiling points.
Relatively soft solids.
Can exist as solids, liquids or gases.
Complete your Venn diagrams
 Requirements:
5
for ionic only
 5 for covalent only
 1 similarity
Comparison of Bonding Types
ionic
covalent
2 nonmetals
molecules
ions , Metal and nonmetal
molten salts
conductive
valence
transfer of electrons electrons
high mp
DEN > 1.6
sharing of
electrons
non-conductive
low mp
DEN < 1.6
The type of bond can usually be calculated by finding
the difference in electronegativity of the two atoms
that are going together.
Electronegativity and
Bond Character
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The character and type of a chemical bond can be
predicted using the electronegativity difference of the
elements that are bonded.
Electronegativity is a measure of how strong the atom
is pulling on electrons that it is sharing in a bond with
another atom.
For identical atoms, which have an electronegativity
difference of zero, the electrons are shared equally
and the bond is considered nonpolar covalent, which
is a pure covalent bond.
Determining Number of
Covalent Bonds
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To determine how many bonds exist in a
molecule, use the following formula
N - A = # bonds
2
-- Where N is the # of needed electrons, which is 8
for all elements but H, which is 2.
-- Where A is the # of available electrons, which is
the number of valence electrons.
Lewis Dot Structures
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A system of “drawing”
bonds
Shows how valence
electrons are arranged
Dots represent valence
electrons
Pairs of dots represent
bonding pairs of
electrons.
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Show the reaction between
sodium and chlorine:
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Show the reaction between
calcium and bromine:
Drawing Lewis Structures
1.Determine the # of bonds needed
2.Draw a skeleton structure.
3.Connect the atoms with the # of bonds. (determined in
step 1)
4.Finish by making sure all atoms in the structure have
an octet of electrons.
5.Remember, if the structure has a positive charge, it
has fewer available electrons, and if it has a
negative charge it has more available electrons.
Lewis Structure Practice
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Draw Lewis Dot Structures for the following
molecules and polyatomics:
CO2 N2 O2
Cl2
H2O SO4 -2
Exceptions to the Octet Rule
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There are two elements which don’t want an octet of electrons
and those are Be (wants 4) and B (wants 6).
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Third-row and heavier elements often satisfy the octet rule
but can exceed the octet rule using their empty valence d
orbitals, especially when surrounded by highly electronegative
atoms such as chlorine, bromine, and oxygen.
Exceptions to the Octet Rule
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The exceptions are easy to recognize-- when you
calculate the # of bonds, the formula will give you an
answer that makes no sense. If this happens, do the
following:
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Connect all of the atoms to the central element with one
bond.
Give all of the surrounding elements an octet of electrons.
Count the number of electrons you have put in the structure
so far.
- If it equals the available # of electrons you calculated, you’re done.
- If it is less than than the available # of electrons you calculates, place
the needed number of electrons around the central element.
Exceptions to the Octet Rule
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Draw Lewis Dot Structures for the following
molecules:
BF3
PCl5
SF6
XeF2
Carbon Bonding
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Carbon follows the rules previously discussed, but it
has a few unique properties.
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Carbon is always the central atom in its compounds
and it tends to bond to itself.
Carbon always has four bonds.
Anytime you have a choice for a skeleton structure,
always go for the most symmetrical option.
Carbon Bonding
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Draw Lewis Dot Structures for the following
molecules:
C2H6
C3H8
C2H2
Resonance
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Resonance occurs when you have a double bond
which can be placed in more than one location.
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More than one valid Lewis structure can be written for
a molecule. Examples: O3, NO3-, NO2-, SO22-, CO32-
Let’s Practice!
H2
 SO2
 CCl4
 SiH4
 PCl5
 XeCl6
 H 2O
 SO4-2
 NO3 C2H3O2
Molecular Shape
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Valence Share Electron-Pair Repulsion
(VSEPR) model allows us to predict the
molecular shape by assuming that the
repulsive forces of electron pairs cause them
to be as far apart as possible from each other.
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Only the valence electron pairs are considered
in determining the geometry.
Effect of the number of electron pairs
around the central atom
2 charge clouds,
linear
4 charge clouds,
tetrahedral
3 charge clouds,
trigonal planar
PREDICTING EXPECTED GEOMETRY
ACCORDING TO VSEPR THEORY
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Lewis dot structure determines the total # of electrons
around the central atom.
Multiple bonds (double and triple) count as one.
The number of bonding and nonbonding electron pairs
around the central atom determines the geometry of
electron pairs and the molecular geometry.
Lone e pairs affect geometry more than bonding pairs.
The shape is referred to as bent if there are lone pairs on
the central atom.
Molecular Shapes 2,3,4 Electron Pairs
Molecular Shape Practice
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Example:
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CO
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HCN
Molecular Shape Practice
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Example:
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CO2
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PF3
Molecular Shape Practice
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Example:
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PH3