Bonding: General Concepts
Chapter 8
Bonds
Forces that hold groups of
atoms together and make
them function as a unit.
Bonds Barf
Breaking absorbing
Release when forming
Bond Energy
- It is the energy required to break a
bond.
- It gives us information about the
strength of a bonding interaction.
Electronegativity
The ability of an atom in a molecule
to attract shared electrons to itself.
Increasing electronegativity
08_132
H
Decreasing electronegativity
2.1
Li
Be
B
C
N
O
F
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Na
Mg
Al
Si
P
S
Cl
0.9
1.2
1.5
1.8
2.1
2.5
3.0
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
0.8
1.0
1.3
1.5
1.6
1.6
1.5
1.8
1.9
1.9
1.9
1.6
1.6
1.8
2.0
2.4
2.8
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
0.8
1.0
1.2
1.4
1.6
1.8
1.9
2.2
2.2
2.2
1.9
1.7
1.7
1.8
1.9
2.1
2.5
Cs
Ba
La-Lu
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
0.7
0.9
1.0-1.2
1.3
1.5
1.7
1.9
2.2
2.2
2.2
2.4
1.9
1.8
1.9
1.9
2.0
2.2
Fr
Ra
Ac
Th
Pa
U
Np-No
0.7
0.9
1.1
1.3
1.4
1.4
1.4-1.3
(a)
Increasing electronegativity
H
Decreasing electronegativity
2.1
Li
1.0
Na
0.9
K
B
Be
2.0
1.5
Al
Mg
1.2
Ca
Sc
Ti
V
Cr
Mn
Co
Ni
Cu
1.8
1.9
1.9
1.9
1.0
1.3
1.5
1.6
1.6
0.8
Y
Zr
Nb
Mo
Tc
Rh
Pd
Sr
Ru
Ag
Rb
1.6
1.8
1.9
2.2
2.2
2.2
1.9
W
Re
Os
Ir
Pt
1.7
1.9
2.2
2.2
2.2
0.8
Cs
1.0
Ba
1.2
1.4
La-Lu
Hf
Ta
1.5
1.5
Fe
0.7
0.9
1.0-1.2
1.3
Fr
Ra
Ac
Th
Pa
U
Np-No
1.1
1.3
1.4
1.4
1.4-1.3
0.7
0.9
Au
2.4
Zn
Si
P
1.5
1.8
2.1
Ga
Ge
As
4.0
3.5
3.0
2.5
F
O
N
C
S
2.5
Se
2.4
Cl
3.0
Br
2.8
1.6
1.8
2.0
Cd
In
Sn
Sb
1.7
1.7
1.8
1.9
2.1
Hg
Tl
Pb
Bi
Po
At
1.9
1.8
1.9
1.9
2.0
2.2
1.6
Te
(b)
Pauling Electronegativity Values
I
2.5
Three Possible Types of Bonds
Nonpolar Covalent
(Electrons equally
shared.)
Polar Covalent
(Electrons shared
unequally.)
Ionic
(Electrons are
transferred.)
Achieving Noble Gas Electron
Configurations (NGEC)
Two nonmetals react: They share
electrons to achieve NGEC.
A nonmetal and a representative group
metal react (ionic compound): The
valence orbitals of the metal are emptied
to achieve NGEC. The valence electron
configuration of the nonmetal achieves
NGEC.
Isoelectronic Ions
Ions containing the same number of electrons
(O2, F, Na+, Mg2+, Al3+)
Write the electron configs to all ions above
O2> F > Na+ > Mg2+ > Al3+
largest
smallest
Ionic Bonds
-
Formed from electrostatic attractions of
closely packed, oppositely charged ions.
-
Formed when an atom that easily loses
electrons reacts with one that has a high
electron affinity.
Ionic Bonds
E = 2.31  10
19
J nm (Q1Q2 / r )
This is a statement of Coulomb’s Law where:
Q1 and Q2 = numerical ion charges
r = distance between ion centers (in nm)
When E is positive (+), repulsion is indicated.
When E is negative (-), attraction is indicated.
Lattice Energy = k(Q1Q2 / r )
Q1, Q2 = charges on the ions
r = shortest distance between centers of the
cations and anions
Which compound in each of the following
pairs of ionic substances has the most
exothermic lattice energy? Justify your
answers.
NaCl, MgCl2
LiF, LiCl
Bond Energies
Bond breaking requires energy (endothermic).
Bond formation releases energy (exothermic).
H = D(bonds broken)  D(bonds formed)
energy required
energy released
Draw the Lewis Structure for each reactant and
product before doing any calculations!
Average Bond Dissociation
Energies at 298 K
Bond Energy, kJ mol-1
C-H 414
C-C 347
C-Cl 377
Cl-Cl 243
H-Cl 431
The tables above contain information for
determining thermodynamic properties of the
reaction below.
C2H5Cl(g) + Cl2(g)  C2H4Cl2(g) + HCl(g)
Calculate the H° for the reaction above, using
the table of average bond dissociation
(a) deltaH = energy of bonds broken - energy of
bonds formed
C2H5Cl + Cl2
C2H4Cl2 + HCl
CH + Cl-Cl - C-Cl + HCl (representing the
changes)
DH = (414) + 243) - (377 + 431) = -151 kJ
Practice
+ditto
Lattice Energy
The change in energy when separated
gaseous ions are packed together to form
an ionic solid.
M+(g) + X(g)  MX(s)
Lattice energy is negative (exothermic)
from the point of view of the system.
Formation of an Ionic Solid
1. Sublimation of the solid metal
M(s)  M(g) [endothermic]
2. Ionization of the metal atoms
M(g)  M+(g) + e [endothermic]
3. Dissociation of the nonmetal
1/2X (g)  X(g)
[endothermic]
2
Formation of an Ionic Solid
(continued)
4. Formation of X ions in the gas phase:
X(g) + e  X(g) [exothermic]
5. Formation of the solid MX
M+(g) + X(g)  MX(s)
lattice energy
[quite
exothermic]
Use the following data to estimate ΔHºf for sodium chloride.
Na(s) + 1/2Cl2(g)
NaCl(s)
Lattice energy
-786 kJ/mol
Ionization energy for Na
495 kJ/mol
Electron affinity of Cl
-349 kJ/mol
Bond energy of Cl2
239 kJ/mol
Enthalpy of
sublimation for Na
109 kJ/mol
Types of Covalent Bonds
Polar covalent bond -- covalent bond in which
the electrons are not shared equally
because one atom attracts them more
strongly than the other. A dipole moment
exists.
Nonpolar covalent bond -- covalent bond in
which the electrons are shared equally
between both atoms. No dipole moment
exists.
- covalent bonds are formed
by sharing electrons between
nuclei.
.
.
H + H ----> H-H
- coordinate covalent bonds are bonds where
both shared electrons originate on the
same atom
: NH3 + H+ ----> H+ - NH3
Single, Double, & Triple Bonds
Single bonds -- one shared pair of
electrons.
Double bonds -- two shared pairs of
electrons.
Triple bonds -- three shared pairs of
electrons.
See bond energy Tables 8.4 & 8.5 on pages 373-374
in Zumdahl.
Polarity
A molecule, such as HF, that has a center
of positive charge and a center of negative
charge is said to be polar, or to have a
dipole moment.
H F
+

08_131
H
F

H




 H
F
F



 H
F

F

H
F
 H
H


F
H



 H
F
F

 H

(a)
F
(b)
The Effect of an electric field on hydrogen fluoride molecules.
08_133


+
H
O



H

(a)
(b)
Dipole Moment for the water molecule.
08_134
3
+

N
H
H

H




(a)
(b)
Dipole moment for the ammonia molecule.
08_151
Nonpolar molecule--zero dipole moment.
Localized Electron Model
A molecule is composed of atoms
that are bound together by sharing
pairs of electrons using the atomic
orbitals of the bound atoms.
Localized Electron 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 lone pairs.
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 Structures
Ionic Compounds
1
K
..
1
: Br :
..
Covalent Compounds
..
..
:F :F :
..
..
In ionic compounds, electrons are transferred
and ions are formed. In covalent compounds,
electrons are shared to form a molecule.
Covalent
Chalk
When writing Lewis structures use
(happy-have) / 2 = bonds/distribute rest
Cl2 NH3 CO2
Sulfur dioxide
Nitrite
CO3-2 CO N2
Exceptions
When writing Lewis structures, satisfy octets
first, then place electrons around elements
having available d orbitals. (Central atom)
SI4
Br3-
Xenon tetrafluoride
TeCl4
Xenon Dichloride
-
Electron Deficient Molecules
Beryllium chloride -- BeCl2 -- is electron deficient
with four electrons. It forms a linear molecule.
Boron trifluoride -- BF3 -- is electron deficient
with six electrons. It forms a trigonal planar
molecule.
See page 381 for the reaction between boron
trifluoride and ammonia.
Comments About the Octet Rule
-
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.
Exceptions for octet rules
ICl3, IF2- XeF4
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 called a resonance hybrid.
Ex. the resonance structures for the nitrate
ion
Draw the Lewis Structure for the
following
• CO2
Finding Formal Charge
Formal Charge = # valence electrons on the neutral atom
– electrons owned by the atom in the resonance structure.
1.Valence electrons of the atom (off the table)
2.Electrons owned = # of valence electrons
around the atom + the number of bonds.
(one electron per bond )
SO4-2
XeO
3
Stereochemistry
The study of the threedimensional arrangement of
atoms or groups of atoms within
molecules and the properties
which follow such arrangement.
VSEPR Model
Valence Shell Electron Pair
Repulsion -- The structure
around a given atom is
determined principally by
minimizing electron pair
repulsions.
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.(Parent
Geometry)
4. Determine the name of molecular structure
from positions of the atoms.(Actual
Geometry)
Molecular Geometry
Parent Geometry is
Actual Geometry is the
electron pair arrangement
about the central atom.
arrangement of atoms about
the central atom.
•linear
•linear
•trigonal planar
•bent
•tetrahedral
•trigonal pyramid
•trigonal bipyramidal
•seesaw
•octahedral
•T-shaped
•square pyramid
•square planar
http://www.wwnorton.com/COLLEGE/chemi
stry/gilbert/tutorials/chapter_07/vsepr/inde
x.html
08_142
Lone
pair
N
N
H
H
H
(a)
(b)
Lone pair of electrons on the ammonia molecule.
08_143
Lone pair
Bonding
pair
O
Bonding
pair
O
H
(a)
H
Lone pair
(b)
O
H
(c)
H
Lone pairs on the water molecule.
08_144

Cl
Cl
P
P
Cl
Cl
Cl
Cl
Octahedral structure for phosphorus hexachloride.
08_145
Xe
Octahedral structure for xenon.
08_150
F
F
F
90 °
Xe
F
Xe
leads to the
structure
F
F
F
F
(a)
F
F
F
Xe
F
F
180°
F
leads to the
structure
Xe
F
F
(b)
Parent and actual geometry for xenon tetrafluoride.
08_152
I
I
I
I
I
I
I
I
I
(a)
(b)
(c)
Three possible arrangements of the electron pairs in triiodide ion.
08_06T
Table
8.6 Arrangements of Electron Pairs Around an Atom Yielding Minimum Repulsion
Number of
Electron Pairs
Arrangement of Electron Pairs
2
Linear
A
3
Trigonal
planar
A
4
Tetrahedral
A
90°
5
Trigonal
bipyramidal
6
Octahedral
120° A
A
Example
VSEPR Model Summary
•
Determine the Lewis structure(s) for the
molecule.
•
For molecules with resonance structures, use any
of the structures to predict the molecular
structure.
•
Sum the electron pairs around the central atom to
determine the parent geometry.
•
The arrangement of the pairs is determined by
minimizing electron-pair repulsions.(Actual
Geometry)
VSEPR Model Summary
(Continued)
•
Lone pairs require more
space than bonding pairs
since they are tightly
attracted to only one
nucleus. Lone pairs
produce slight distortions
of bond angles less than
120o.