Bonding & Molecular Shapes
Dr. Ron Rusay
Spring 2004
© Copyright 2004 R.J. Rusay
Chemical Bonds
Definition:
Attraction between atoms that hold them together.
Bonds provide a particular arrangement of atoms in a
molecule which provides it with new chemical
properties.
Q ui ck Ti m e ™ an d a
G IF de co m pr e ss or
a re ne ed ed t o s ee th i s pi c tu r e.
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Table salt: Sodium chloride
Water
Ionic vs. Covalent Bonding
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Electron Configurations
Noble Gases and The Rule of Eight
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)
Where are metals and non-metals located in the
Periodic Table?
Lewis Electron-Dot Symbols for Elements in Periods
2&3
How many valence electrons does each
element have in each of the vertical columns
(families)?
Ionic Bonds
Result
from electrostatic (positivenegative) attractions of closely packed,
oppositely charged ions.
Form when an atom which can easily
lose electrons (low electronegativity)
reacts with one which can easily gain
electrons (high electronegativity).
Na and Cl; Fe and O
Covalent Bond Lengths
The distance between atoms.

It is the distance that provides
the most stable arrangement of the
atoms where the bond energy is at
a minimum.

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
Fundamental Bonding
Patterns
 Carbon
has a total of four bonds:
• 4 single bonds
• 2 single bonds plus 1 double bond
• 1 single bond plus 1 triple bond
 Oxygen
has a total of 2 bonds plus 2
“free pairs of electrons”:
• 2 single bonds
• 1 double bond
Fundamental Bonding
Patterns
 Nitrogen
has a total of three bonds plus
1 “free pair of electrons”:
• 3 single bonds
• 1 single bonds plus 1 double bond
• 1 triple bond
 Hydrogen
only has 1 single bond.
 This is the total of almost all bonding
arrangements in organic molecules.
Lewis Structures of Simple
Molecules
H
H
.N.
H
Ammonia
H
H
H
N
N
C
O
Urea
H
Carbonate Ion CO32- :
The carbonate polyatomic ion has two coordinate bonds,
resonance forms and two electrons from a cation!
The Lewis Structure for Sulfur trioxide
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!!
Ozone and CFCs
CCl2F2
O
O
..
O
Cl
..
F
..
Freon 12
C
Cl
..
..
F
..
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.
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