Bonding
Dr. Ron Rusay
Fall 2001
© Copyright 2001 R.J. Rusay
Chemical Bonds
Definition:
Attractive forces which hold atoms
together and provide a particular
molecular arrangement of atoms
with new chemical properties.
Bond Model
 1.
Description of valence electron
arrangement (Lewis structure).
 2. Prediction of geometry (VSEPR
model).
 3. Description of atomic orbital types
used to share electrons or hold long
pairs.
Ionic vs. Covalent Bonding
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Electron Configurations
Noble Gases and The Rule of Eight
 REVIEW:
A
nonmetal and a metal react to form an
ionic compound: Valence electrons of
the metal are lost and the nonmetal
gains these electrons. (Ionic Bonding)
 When two nonmetals react: They share
electrons to achieve a Noble Gas
Configuration. (Covalent Bonding)
Octet Rule: General Comments
 2nd
row elements C, N, O, F observe the
octet rule.
 2nd
row elements B and Be often have fewer
than 8 electrons around themselves - they are
very reactive.
 3rd
row and heavier elements CAN exceed
the octet rule using empty valence d orbitals.
 When
writing Lewis structures, satisfy octets
first, then place electrons around elements
having available d orbitals.
Lewis Electron-Dot Symbols for
Elements in Periods 2 & 3
Isoelectronic Ions
Ions containing the the same number
of electrons are isoelectronic with a
Noble Gas

(O2, F, Na+, Mg2+, Al3+)

However the ion sizes differ
O2> F > Na+ > Mg2+ > Al3+

Ionic Bonds
Result
from electrostatic attractions of
closely packed, oppositely charged
ions.
Form when an atom which can easily
lose electrons reacts with one which
has a high electron affinity, that is, it can
easily gain electrons.
Mg and Cl; K and O
Bond Energy
It
is the energy required to break a
bond, i.e. overcome the force of
attraction.
The quantitative value provides
information about the strength and
nature of the bond.
Three Ways of Showing the Formation of
LiF, [Li+ and F -] through Electron Transfer
Covalent Bond Lengths
Interatomic distance.

It is the distance where the bond
energy is at a minimum value, and
which is the most stable atomic
form.

• IR Tutor
Covalent Bonding &
Bond Length
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Bond Lengths and Covalent Radius
The Periodic Table
Bond Lengths : Bond Strengths
Actual vs. Expected Bond Length
Actual = 0.127 nm
Expected = 0.136 nm
Electronegativity
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Electronegativity

The ability of an atom in a
molecule to attract shared electrons
to itself.
 = (H  X)actual  (H  X)expected
Periodic Trend
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Electronegativity Differences &
Polar Covalent Bonds

A molecule with a relatively high
difference in electronegativities, such as
HF, has a center of positive charge and
a center of negative charge. It is polar,
having an experimentally measureable
dipole moment.
H F
+

The spectrum of bond
differences: Ionic at one
extreme : Non-polar
covalent at the other.
Bond Shapes
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Lewis Structure
Shows
how valence electrons are
arranged among atoms in a molecule.
Reflects central idea that stability of a
compound relates to noble gas electron
configuration.
Lewis Structure of Water
Carbonate Ion CO32- :
The carbonate polyatomic ion has two coordinate bonds,
resonance forms and two electrons from a cation!
Draw the Lewis Structure for Sulfur trioxide
SO3 vs SO2 : The Coordinate Bond
Where to draw the sulfuroxygen double bond? There
are three options.
Simplified drawings without free pairs of electrons:
Experimental data shows that each of the
three sulfur-oxygen bonds are the same
length.
Resonance

Occurs when more than one valid
Lewis structure can be written for a
particular molecule.

These are resonance structures.
The actual structure is an average of
the resonance structures.
Resonance:
Delocalized Electron-Pairs
Ozone : O3
..
..
O
O
O
O
O
O
II
I
Resonance Hybrid Structure
..
O
O
O
One pair of electron’s resonates
between the two locations!!
H
H
H
C
C
H
H
Lewis Structures of Simple
Molecules
H
..
O
..
CH4
H
H
Methane
Ethyl Alcohol (Ethanol)
H
F
..
..
O
..
F
..
K+
O
H
..
..
Cl
..
C
O
KClO3
Potassium Chlorate
CF4
C
..
F
..
F
..
Carbon Tetrafluoride
Lewis Structures of Simple
Molecules
H
H
.N.
H
Ammonia
H
H
N
H
+
H
H
N
N
C
O
H
H
Ammonium Ion
Urea
H
Formal Charge

Equals the number of valence
electrons of the free atom minus [the
number of unshared valence
electrons in the molecule + 1/2 the
number of shared valence electrons
in the molecule.
Resonance and Formal Charge
O C O
(-1)
(0)
(+1)
Not as good
O C O
(0)
(0)
Better
(0)
VSEPR Model
Valence Shell Electron Pair Repulsion
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VSEPR Model
The
molecular structure which
surrounds a given atom is
determined principally by
minimizing electron pair
repulsions through maximizing
separations.
Predicting a VSEPR Structure
 1.
Draw Lewis structure.
 2. Put pairs as far apart as possible.
 3. Determine positions of atoms from
the way electron pairs are shared.
 4. Determine the name of molecular
structure from positions of the atoms.
Molecular Models
 Computer
Generated Models
Ball and stick models of ammonia, water
and methane. For many others see:
http://ep.llnl.gov/msds/pdb/
http://ep.llnl.gov/msds/orgchem/Chem226/Smell-Stereochem.html