Chem 101
Chemical Bonds:
Forces that hold atoms together
Lectures 10 and 11
G.N.Lewis:
Not all the electrons in an atom
participate in chemical bonding.
Oxygen
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Bond Formation:
Becoming Chemically Stable
Filled shell has high chemical stability.
Filled shells can be found whenever a noble gas
element is reached in the periodic table.
O [He]2s22p4 + 2e-  O2- [He]2s22p6 = [Ne]
Na [Ne]3s1
– e-  Na+ [Ne]
Isoelectronic
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How many electrons are in a valence
shell?
Consider the s and p orbitals
Hydrogen and helium have valence shells completed
with two electrons.
Atoms of the first few periods of the periodic table
have a maximum of 8 electrons in their valence
shells.
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Octet Rule
Atoms bond in such a way that each atom acquires
eight electrons in its outer shell.
An atom may achieve an octet by two ways:.
(a) By transfer of electrons
from one atom to another.
M
Metal
(b) By sharing one or
more pairs of electrons
N
Nonmetal
Ionic bond
Covalent
bond
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Ionic Bond
Formed from the complete transfer of electrons
between atoms.
Attraction of positively and negatively charged ions.
As a result ionic compounds forms.
Electrostatic forces are effective.
N
N-
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Cations form when an atom loses valence
electrons to become positively charged.
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Anions form when an atom gains electrons and
becomes negatively charged.
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Ionic Radii
A cation radius is smaller than the radius
of its starting atom.
An anion radius is larger than the radius of
its starting atom.
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Ionic Radii
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Covalent Bonds
Covalent bonds form:
when two nonmetal atoms share electrons
and
the shared electrons in the covalent bond
belong to both atoms.
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Covalent Bonds
Example: Form HF (hydrogen fluoride):
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Bond Polarity
• Covalent bonding between unlike atoms results in unequal sharing
of the electrons.
• The result is bond polarity.
• One end of the bond has larger electron density than the other
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Electronegativity
• Measure of the ability of an atom to attract
shared electrons
• Larger electronegativity means atom
attracts shared electrons more strongly
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Dipole Moment
• Larger difference in electronegativity means
more polar bond
• Any molecule that has a center of positive
charge and a center of negative charge in
different points is said to have a
dipole moment.
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Bond Polarity
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Multiple Covalent Bonds
Single Covalent Bond the atoms share 2 electrons,
– (1 pair)
Double Covalent Bond the atoms share 4 electrons,
– (2 pairs)
Triple Covalent Bond the atoms share 6 electrons,
– (3 pairs)
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Covalent Bonds
Bond Strength = Triple > Double > Single
– For bonds between same atoms, C≡N > C=N > C—N
– Though Double not 2x the strength of Single and
Triple not 3x the strength of Single
Bond Length = Single > Double > Triple
– For bonds between same atoms, C—N > C=N > C ≡ N
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Bond Length
In the covalent bond, the valence shells of the two
atoms overlap with each other and atoms get
closer to each other.
This distance, when atoms are closer to each other
is the bond length.
Bond length is smaller than the sum of the atomic
radii.
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Bond Length
Example: In HCl, hydrogen 1s energy
sublevel overlaps with chlorine 3p energy
sublevel.
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Bond Energy is a measure of the
strength of a bond.
Energy to form a covalent bond (energy is released):
H(g) + Cl(g)
HCl(g) + heat
Energy to break a covalent bond (energy is needed):
HCl(g) + heat
H(g) + Cl(g)
The amount of energy required to break a covalent bond is
the same as the amount of energy released when the
bond is formed.
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Electron Dot Formulas of Molecules:
Lewis Structure
The number of dots around each atom is the
number of valence electrons.
Lewis structures are electron dot formulas for
molecules.
Drawing Lewis structure, bonds between atoms
and the arrangement of the atoms in a molecule
can be visualized.
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Drawing Lewis Structures
1.
Calculate the total # of valence electrons:
Add all of the valence electrons for each atom in the molecule.
2.
How many electrons do the atoms want to have to meet the octet rule?
H and He only need 2 electrons. All other atoms need 8 electrons.
3.
Subtract the total # of valence electrons from the # electrons needed to
meet the octet rule.
This # is the number of electrons that must be shared between atoms.
4.
Each bond has 2 electrons so divide the # of shared electrons by 2 to get
the # of bonds.
5.
Draw the structure.
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Electron Dot Formula for H2O
1. Count the total number of valence electrons (e-):
H
H
O
2. Electrons to meet octet rule:
H
H
O
3. Subtract valence e- from octet e4. Divide the # e- by 2 to get # of bonds
5. Draw structure.
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Electron Dot Formula for H2O
Use a single dash line to represent 1 pair of
bonding electrons.
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Practice: Draw the Lewis Structure
(electron formula) for SO2
Count the total number of valence electrons (e-):
S
O
O
Electrons to meet octet rule:
S
O
O
Subtract valence e- from octet eDivide the # e- by 2 to get # of bonds
Draw structure.
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Resonance
• When there are multiple Lewis structures for a molecule
that differ only in the position of the electrons they are
called resonance structures
– Lone pairs and multiple bonds in different positions
• The actual molecule is a combination of all the resonance
forms.
– It does not resonate between the two forms, though we
often draw it that way.
Molecular Geometry
• You will learn how to determine the exact geometry of
molecules and VSEPR in Chem 110
• You will not be tested on VSEPR in Chem 101
Linear
Trigonal planar
Tetrahedral
VSEPR Theory
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