Structure and Bonding – Fundamental Concepts
Table of Contents
Chapter 9: Structure and Bonding – Fundamental Concepts...........................2
Structure – ........................................................................................................................................... 2
Bonding – ............................................................................................................................................ 2
Bonding Types ......................................................................................................................... 3
Ionic Bond ........................................................................................................................................... 3
Covalent Bond ..................................................................................................................................... 3
Electronegativity ..................................................................................................................... 3
Bonding - Ionic Bonds ............................................................................................................. 4
Why do ionic bonds form? .................................................................................................................. 4
Crystalline Substances ........................................................................................................................ 5
Covalent Bonding .................................................................................................................... 8
Electron Dot Structures (a.k.a. Lewis Dot Structures) ....................................................................... 8
Lewis Dot Structures For Ions ............................................................................................................ 9
Lewis Dot structures for Molecules .................................................................................................. 10
Resonance Structures............................................................................................................ 14
Features of Resonance ...................................................................................................................... 15
Bond Order ....................................................................................................................................... 16
Formal Charge ...................................................................................................................... 17
Definition: ......................................................................................................................................... 17
Counting Rules .................................................................................................................................. 17
Electroneutrality Principle................................................................................................................ 18
Formal Charge Examples ................................................................................................................. 18
VSEPR Theory ...................................................................................................................... 20
Valence Shell Electron Pair Repulsion Theory ................................................................................. 20
VSEPR Geometries ........................................................................................................................... 21
VSEPR for Multiple central Atoms ................................................................................................... 26
Molecular Polarities .............................................................................................................. 27
Dipole Moment –............................................................................................................................... 28
Page 1 of 29
Structure and Bonding – Fundamental Concepts
Chapter 9: Structure and Bonding – Fundamental Concepts
Structure –
Bonding –
A Bond:
These are both determined based on arrangements of electrons.
Electrons are subdivided into 2 categories:
1. Core Electrons –
2. Valence Electrons –
 For main group elements, the number of valence electrons is
equal to
(The sum of the _________ electrons)
Na
K
Rb
F
Cl
Br
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Structure and Bonding – Fundamental Concepts
Bonding Types
The attractive forces that hold atoms together to form a chemical
“bond” come in 2 general types:
Ionic Bond
 One or more valence e-s are
Covalent Bond
 All covalently bonded atoms _____________
 Non-polar covalent bonds _______________________________
 Polar covalent bonds___________________________________
Picture form http://www.avonchemistry.com/chem_bond_explain.html
Electronegativity
 Is:
 It is a continuum.
 Electronegativity follows the general trends of
EN < 0.4
EN 0.5-1.6
EN >1.6 (some say 2)
(metal plus nonmetal:
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Structure and Bonding – Fundamental Concepts
____ is the most
electronegative (4.0)
Chart of electronegativities form
http://www.avonchemistry.com/chem_bond_explain.html
Bonding - Ionic Bonds
Why do ionic bonds form?
Na has a low
Cl has a high
You would expect that the formation of NaCl would be
thermodynamically favored (which it is). H =
HOWEVER, the change in energy upon formation of a salt cannot be
accounted for just by the ___ for the cation + the ___ for the anion.
I.E. for
E.A. for
Cl(g)
This would predict that energy is needed, but the Rxn is spontaneous.
The difference is the stability coming from the electrostatic
attractive forces.
The attractive force is governed by _____________________
Where e is the charge on an en+ is the charge on the (+) ion
n- is the charge on the (-) ion
d = distance between them
C = a constant
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Structure and Bonding – Fundamental Concepts
This means:
 Higher charges have _________________
 Larger ions have _______ distances between the (+) and (-)
charges leading to
For NaCl in the gas phase, the
ion pair energy is –498 kJ/mol
Crystalline Substances
So far we have assumed that all substances are in the gas phase because
that is the way that ionization energy and electron affinity are defined.
Picture from McMurry Fay – 3rd Ed.
Lattice Energy:
Na+(g) + Cl-(g) 
picture from http://www.le.ac.uk/eg/spg3/NaCl.gif
Page 5 of 29
Structure and Bonding – Fundamental Concepts
Lattice E for NaCl:
Additional Stabilization
Additional energy for the overall cycle is required to calculate the net
energy for the formation of NaCl from room temperature reactants:
 You need to get atoms of Cl in the gas phase (break Cl-Cl bond)
 You need to have Na in the gas phase (vaporize Na)
The complete cycle is called a Born-Haber Cycle
The Born-Haber Cycle for Sodium Chloride. Picture from McMurry Fay – 3rd Ed.
Page 6 of 29
Structure and Bonding – Fundamental Concepts
Picture from
McMurry Fay – 3rd
Ed.
If time Permits, work out Born-Haber Cycle for MgCl2
Example:
What is the E for the formation of magnesium chloride from
magnesium metal and chlorine gas given the following information:
The lattice energy for magnesium chloride is –2542 kJ/mol
The first ionization energy for magnesium is 737.7 kJ/mol
The second ionization energy for magnesium is 1450.7 kJ/mol
The electron affinity for chlorine is -384.6 kJ/mol (x2 = -697.2 kJ/mol)
The H of formation (sublimation) of Mg(g) is 147.7 kJ/mol
The Cl-Cl bond energy is 243 kJ/mol
Page 7 of 29
Structure and Bonding – Fundamental Concepts
Covalent Bonding
 Typically takes place when ____________
 Typical between ______________
 Involves __________________
Electron Dot Structures (a.k.a. Lewis Dot Structures)
These are a way of representing the number of electrons surrounding or
shared by each _________________________
These are cartoons, _______________________.
These were invented by
 Theory proposed in 1916
 Lewis dot structures are only relevant for ___________________
 Start with ________________________
representing the inner electrons
 Indicate atoms valence electrons _______________
on the 4 sides of the symbol.
 Remember, units place of the group number indicates
 When atoms combine to form molecules
 Shared electrons are called ________________
 Unshared electrons are called lone electrons or lone pairs
 Atoms share as many valence shell electrons as possible (until
there are no more electrons to share or until an ___________
(most stable configuration) is reached.
Page 8 of 29
Structure and Bonding – Fundamental Concepts
Generic Lewis Dot Structures
1
2
3
4
5
6
7
Examples of H plus period 2 nonmetals
(metals don’t form covalent bonds)
Lewis Dot Structures For Ions
 add or subtract electrons to account of charge
 For Ions, include symbol in bracket.
Ex: Sodium metal plus oxygen gas:
2 Na(s) + ½ O2(g)  Na2O(s)
Octet Rule: The tendency of molecules or polyatomic ions to have
structures in which eight electrons surround each atom.
Page 9 of 29
8
Structure and Bonding – Fundamental Concepts
Some molecules will _________________
in order to satisfy the octet rule.
Example of a double bond
Example of a triple bond
Two bonding pairs is called a _________________
Three bonding pairs is called a _________________
Lewis Dot structures for Molecules
Rules for Writing Lewis Dot Structures for Molecules
1. Add up valence electrons from all atoms



2. Identify the central and terminal atoms and write the skeletal
structure


H
O
C
3. Draw a bond between each pair of atoms an appropriate.

Page 10 of 29
Structure and Bonding – Fundamental Concepts
4. Assign remaining e
5. If there are leftover e-,


6. If all e- are used up and the central atoms do not have completed
octets,

Note: If there is a choice more than one arrangement of electrons will
satisfy the requirements of items 5 and/or 6, choose based on the lowest
formal charge. (We will define formal charge momentarily.)
Examples for Rows 1 + 2 (Octet rule never exceeded) (Boron less
than full octet)
H2O
NH3
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Structure and Bonding – Fundamental Concepts
CN-
Have class try
N2H4
and
CO2
Exceptions to Octet Rule and Lewis Dot Structures below 2nd Row





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Structure and Bonding – Fundamental Concepts
Examples
SeF6
BrF3
Have Class try sulfur tetrafluoride (SF4) and phosphorous
pentachloride (PCl5)
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Structure and Bonding – Fundamental Concepts
Resonance Structures
When a molecule is represented by the average of one or more possible
Lewis structures, these are called _______________
(net effect of individual resonance structures)
Example: Ozone O3
4)
Which O atom should move its e- pair?



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Structure and Bonding – Fundamental Concepts
Example: Acetate Ion CH3COO-
Which should be the O to move it's electrons?
Features of Resonance
 Contributors to a resonance hybrid must all share
 Resonance hybrids do NOT _____________________________
(This is a man-made model to try to explain nature.)
 Compounds with resonance structures do NOT ______________
 The
true
structure
is
____________________________
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Structure and Bonding – Fundamental Concepts
Bond Order
Bond type
Bond
Order
Length
When resonance structures exist, you can have
For ozone O3,
Have class try NO3- ion.
Which O forms double bond?
Bond Order =
For oxyanions,
To form the acid,
Page 16 of 29
Strength
Structure and Bonding – Fundamental Concepts
Formal Charge
 _____________________________________________________
_____________________________________________________
_____________________________________________________
 _____________________________________________________
_____________________________________________________
 _____________________________________________________
Definition:
__________________________________________________________
__________________________________________________________
FC = ________________________________________________
OR
FC = ________________________________________________
 Each atom in a molecule gets its own formal charge
(like oxidation number is assigned to each atom during redox)
Counting Rules
Lone Pairs - belong entirely to the atom ___
Shared pairs are divided equally
-single bond
-double bond
-triple bond
___
___
___
 Essentially, every dot _______________
every line _______________.
 The formal charges for all atoms must add up to
______________________ for neutral molecules and
______________________ for ions.
Page 17 of 29
Structure and Bonding – Fundamental Concepts
Electroneutrality Principle
 Structures with the _________________________ are most stable.
They are not trying to maintain ___________________________ .
Compare by adding ______________________ of formal charges.
 If charges must exist, the negative formal charges should be on
the ______________________________________ .
 Internal atoms have to ________ their electrons and turn out with a
more ____ formal charge.
Formal Charge Examples
Pick the best formula for NOCl
 Reject N –O—Cl ______________________________________
______________________________________
O—Cl—N
vs
O—N—Cl
1.
2.
3.
4.
5a1
5a2
Structure
Free atom
5a1
5a2
5b1
5b1
5b1
5b2
Formal Charge Summary
O
Cl
N
Problems with other structures:
_______________________________________________________
_______________________________________________________
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Structure and Bonding – Fundamental Concepts
What is the best resonance structure for OCN- ?
Assume that the skeleton is O—C—N.
1
2/3
4
5
6
A
Structure
B
O (__)
C (__)
C
N(__)
A
B
C
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Structure and Bonding – Fundamental Concepts
VSEPR Theory
Valence Shell Electron Pair Repulsion Theory
 Pronounced __________
 Lewis Dot Structures do NOT imply any ___________.
 VSEPR attempts to predict _________ of molecules
based on electrons will try to stay _________________________ ,
since negative charges will ______________________________ .
 We start by identifying electron ______________.
These include:
 _____________________
 _____________________
 _____________________
(triple, double and single bonds only count as _____ group)
Order of Greatest Repulsion
 _____________________
 _____________________
 _____________________
The actual structure is bases on the ____________ of electron groups
(areas of _____________________)
Like Lewis Dot Structures,
the success of this model is limited to _________________________
The arrangement of electron groups is called the
________________________________.
The arrangement of the atoms is called the
________________________________.
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Structure and Bonding – Fundamental Concepts
VSEPR Geometries
All pictures in this section are from McMurry Fay, 3 rd ed.
For 2 Electron Groups (density regions/charge clouds):
Bond Angle:
_____________
Electron Pair
Geometry:
_____________
Molecular
Geometry
_____________
For 3 Electron Groups (density regions/charge clouds):
Bond Angle:
_____________
Electron Pair
Geometry:
_____________
Molecular
Geometry
_____________
OR
_____________
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Structure and Bonding – Fundamental Concepts
For 4 Electron Groups (density regions/charge clouds):
Bond Angle:
_____________
Electron Pair
Geometry:
_____________
Molecular
Geometry
_____________
OR
_____________
OR
_____________
For 5 Electron Groups (density regions/charge clouds):
Bond Angles:
_____________ (axial) and __________ (equatorial)
Electron Pair
Geometry:
_____________
Molecular
Geometry
_____________
OR
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Structure and Bonding – Fundamental Concepts
For 5 Electron Groups (density regions/charge clouds) Continued:
OR
Molecular
Geometry
_____________
OR
_____________
OR
_____________
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Structure and Bonding – Fundamental Concepts
For 6 Electron Groups (density regions/charge clouds):
Bond Angle:
_____________
Electron Pair
Geometry:
_____________
Molecular
Geometry
_____________
OR
_____________
OR
_____________
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Structure and Bonding – Fundamental Concepts
Page 25 of 29
Structure and Bonding – Fundamental Concepts
VSEPR for Multiple central Atoms
Ethane
Each bond is _____________ and the C-C bond can spin
the minimum amount of ______________________.
to provide
Ethene (C2H4)
Each C is ___________________ ;
Overall molecule is ___________
Multiple bonds can’t _________ .
Leads to ____________________ .
Methyl Isocyanate
CH3NCO
H
HCN==C==O
H
Page 26 of 29
Structure and Bonding – Fundamental Concepts
Molecular Polarities
Bond Polarities – ________________________________________ .
If EN is < 0.4, _________________________________________ .
If EN is . 0.4, __________________________________________
Molecular Polarities – Based on relationships between all bond
polarities in a molecule.
To Determine Molecular Polarity:
1. ____________________________________________________
2. ____________________________________________________
3. ____________________________________________________

_____________________________________________________
 Draw Vectors __________________________________________
Points toward _________________________________________ .
If vectors cancel, then the molecule is non-polar.
Page 27 of 29
Structure and Bonding – Fundamental Concepts
Dipole Moment –
The net sum of the bond vectors. +Example
NH3 (8 ve)
__  bonds
__  bonds
__ lone pair
Lewis Structure:
e- geometry is ________________
molecular geometry is _______________
EN = 3.0(N) - 2.1(H) = 0.9
-

N
H
H
+
H
Vectors: ________________
Polarity: ________________
Example
CO2 (16 ve)
__  bonds
__  bonds
__ lone pair
__ centers of
electron density
e- geometry is _______________
molecular geometry is _____________
EN = _________________________
Vectors point _________________________
Vectors ____________Polarity: ________________
Page 28 of 29
__ centers of
electron density
Structure and Bonding – Fundamental Concepts
Example
Isopropyl alcohol (rubbing alcohol)
(CH3)2CHOH
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