Chem 1011 – Intersession 2011
Class #4
11-May-11
1
Class 4:
Covalent Bonding – Lewis Structures
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Sec 9.5 – Covalent Bonding: Lewis Structures
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Single Covalent Bonds
Double and Triple Covalent Bonds
Covalent Bonding: Models and Reality
2
Lewis Theory – An Overview
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Atoms bond in such a way as to obtain a stable
electron configuration (a complete valence shell)
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Group 8A are particularly stable and unreactive;
they possess an octet of electrons in their valence
shells. (Exception: He has 2 valence electrons)
Example: Ne – 10 electrons total (2 core, 8 valence)
Ne
 The aim of a Lewis Structure is to show valence electrons
arranged as pairs of dots around the atom
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More Lewis Theory
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Octet rule
the number of electrons associated with bond pairs and lone
pairs for each of the Lewis symbols in a Lewis structure will be
eight (an octet) (Exception: H will be associated with two
electrons).
Covalent bonds
are formed when electrons are shared between a pair of atoms.
Bonding representations are often simplified by omitting
the lone electron pairs and by substituting a line for each
bonding pair of electrons.
Covalent Bonding – Introduction to Lewis
Structures
1
Chem 1011 – Intersession 2011
Class #4
11-May-11
4
Electron Groups
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Bond pairs
a pair (or pairs) of electrons shared between two atoms.
Important: Bond pairs count toward the octet of both
atoms between which they are shared.
Lone pairs
a pair of electrons associated with one atom only; these
are not involved in bonding.
Single bonds
result from the sharing of one pair of electrons; may be
represented by a single line (—).
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Single Covalent Bonds
Each Cl atom has 7 valence electrons
These two electrons are
shared by each atom;
they count towards the
octet of both Cl atoms
In the diatomic molecule Cl2, each Cl is surrounded by 8
electrons (an octet).
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Single Covalent Bonds
• Another molecule with Covalent Bonds: H2O
• Tro, pg 315:
▫ But water is not a linear molecule!
• Typically, water is drawn as so:
▫ More explanation as to
why in future classes!
Covalent Bonding – Introduction to Lewis
Structures
2
Chem 1011 – Intersession 2011
Class #4
11-May-11
7
Multiple Covalent Bonds
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Sometimes, more than one pair of electrons must
be shared in order for an atom to obtain a valence
octet.
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Multiple covalent bond
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a bond in which more than two electrons are shared
between the bonded atoms.
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Multiple Covalent Bonds
• For example, Carbon Dioxide – CO2
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O C O
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•O
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• C•
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O•
• Problems:
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C O
C O
O surrounded
▫ Carbon is
by onlyO
each
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••6 electrons;
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oxygen by 7 electrons. The octets are incomplete
▫ Some unpaired electrons still remain, which is
generally forbidden in a Lewis Structure
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Multiple Covalent Bonds
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Double covalent bond
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two pairs of electrons are shared between bonded atoms. The
bond is represented by the symbol =. Each line is a distinct
bond.
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• This can•be employed
to •solve the problems
with
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• C •from• O
O C O
our CO2O
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• •structure
•before
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▫ Combine the remaining unpaired electrons between two atoms to
form a second (double) bond between those atoms
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O C O
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O C O
▫ NOTE: Each atom now has a complete octet!
Covalent Bonding – Introduction to Lewis
Structures
Proper Lewis
Structure
3
Chem 1011 – Intersession 2011
Class #4
11-May-11
10
Multiple Covalent Bonds
Triple covalent bond
Example: Nitrogen (N2)
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•N
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N N
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N N
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N•
N N
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Three pairs of electrons are shared between bonded
atoms. The bond is represented by the symbol ≡.
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11
Bond Strengths and Lengths
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In general, triple bonds are stronger, and
shorter, than double bonds.
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In general, double bonds are stronger, and
shorter, than single bonds.
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Covalent Bonding: Models and Reality
• Lewis theory of covalent bonding says that the
attractions between atoms are directional.
▫ Atoms are held together by a pair of electrons shared
between them, and so each bond links just those two
atoms
 This differs from ionic bonds, which hold together an
array of atoms
• Lewis theory predicts covalently bonded compounds will
be found as individual molecules.
▫ Compounds of nonmetals are made of individual
molecule units.
Covalent Bonding – Introduction to Lewis
Structures
4
Chem 1011 – Intersession 2011
Class #4
11-May-11
13
Covalent Bonding: Models and Reality
• Lewis theory predicts the melting and boiling points of
molecular compounds should be relatively low.
▫ Melting and boiling involves breaking the attractions between
the molecules, but not the bonds between the atoms
▫ Covalent bonds are strong, but the attractions between the
molecules are generally weak.
▫ The attractions are known as intermolecular forces; we will
describe these in depth next week
Note: Molecular compounds have low melting points and boiling
points.
▫ MP generally <300 °C
▫ At room temperature, molecular compounds will exist across all
three states, depending on the compound.
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Covalent Bonding: Models and Reality
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Covalent Bonding: Models and Reality
• Lewis theory predicts that the hardness and brittleness
of molecular compounds should vary depending on the
strength of intermolecular attractive forces.
▫ Some molecular solids are brittle and hard, but many are
soft and waxy.
• Lewis theory predicts that neither molecular solids nor
liquids should conduct electricity.
▫ There are no charged particles around to allow the material
to conduct.
▫ Molecular acids conduct electricity when dissolved in water,
but not in the solid or liquid state, due to them being
ionized by the water.
Covalent Bonding – Introduction to Lewis
Structures
5
Chem 1011 – Intersession 2011
Class #4
11-May-11
16
Friday, Friday
• Sec 9.6 – Electronegativty and Bond Polarity
▫ Electronegativity
▫ Bond Polarity, Dipole Moment and Percent Ionic
Character
• Sec 9.7 – Lewis Structures of Molecular
Compounds and Polyatomic Ions
▫ Writing Lewis Structures for Molecular
Compounds
▫ Writing Lewis Structures for Polyatomic Ions
Covalent Bonding – Introduction to Lewis
Structures
6