Does nitrogen trichloride exhibit a dipole moment?
Bond Polarity vs Molecular Polarity
 Bond polarity depends upon electronegativity difference (% Ionic Character)

Molecular polarity depends up the symmetry of the molecule.
Nonpolar molecule--zero dipole moment.
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Table 8.2 on page 357 in Zumdahl.
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.
See Table 8.3 on page 361 in Zumdahl.
Sizes of ion related to position on the periodic table:
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Li
Be
(0.60) (0.31)
60
31
Na
Mg
(0.95) (0.65)
95
65
Al
(0.50)
50
O
F
(1.40)
140
(1.36)
136
S
Cl
(1.84)
184
(1.81)
181
Ca
Ga
Se
Br
(1.33) (0.99)
133
99
(0.62)
62
(1.98)
198
(1.95)
195
Rb
Sr
In Sn Sb
Te
I
(1.48) (1.13)
148
113
(0.81) (0.71) (0.62)
81
71
62
(2.21)
221
(2.16)
216
K
Isoelectric Ions:
Ions containing the same number of electrons
(O2-, F-, Na+ , Mg2+ , Al3+)
O2- > F- > Na+ > Mg2+ > Al3+
largest
→
smallest
Lattice Energy: The change in energy when separated gaseous ions are packed together to form an
ionic solid (“salt”).
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
X2 (g) → X(g)
[endothermic]
2
-
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)
[quite exothermic]
QQ 
Lattice Energy = k  1 2 
 r 
Q1, Q2 = charges on the ions
r = shortest distance between centers of the cations and anions
Comparison of the energy changes in the formation of sodium fluoride and magnesium oxide:
Mg2+(g) + O2-(g)
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Electron affinity
737
Mg2+(g) + O(g)
Mg2+(g) +
-3916
1
2
O2(g)
247
Lattice
energy
2180
Ionization energy
Na(g) + F(g)
Na+(g) +
Mg(g) +
Mg(s) +
-602
MgO(s)
1
2
1
2
495
O2(g)
O2(g)
150
Overall
energy
change
109
1
2
F2(g)
77
Ionization
energy
Na(g) +
Na(s) +
1
2
1
2
-328
Electron
affinity
-923
Lattice
energy
Na+(g) + F-(g)
F2(g)
F2(g)
-570
NaF(s)
Bond Energies: Bond breaking requires energy (endothermic). Bond formation releases energy
(exothermic).
ΔH = S(bonds broken) S(bonds formed)
Draw the Lewis Structure for each reactant and product before doing any calculations!
Multiple 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.
Models: Models are attempts to explain how nature operates on the microscopic level based on
experiences in the macroscopic world.
Fundamental Properties of Models
- A model does not equal reality.
- Models are oversimplifications, and are therefore often wrong.
- Models become more complicated as they age.
- We must understand the underlying assumptions in a model so that we don’t misuse it.
Example: The Bohr Model of an atom, although incorrect can still be a useful tool
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.
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 Dot 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.
Ionic compounds
1

  
K
 F 
  
Covalent Compounds
 
 F  F 

 
In ionic compounds, electrons are transferred and ions are formed. In covalent compounds,
electrons are shared to form a molecule. Potassium gains the stability of argon,
bromine of krypton, and fluorine of neon (NGEC).
Lone Pair and Bonding Pair:
 
 F  F 

 
Electrons shared between atoms are bonding pairs. Electrons that are not involved in bonding are
called lone pairs. Each fluorine has three lone pairs and one bonding pair shared between them.
Electron Deficient Molecules (“weird valences”):
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.
Rules for Writing Lewis Structures:
• Sum the valence electrons from all the atoms.
• Use a pair of electrons to from a bond between each pair of bound atoms.
Arrange remaining electrons to satisfy the duet rule for hydrogen and the octet rule for the
second-row elements.
Practice:
NO+
• 5 e- + 6 e- - 1 e- = 10 e• Each atom has an octet and is satisfied.
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.
See the resonance structures for the nitrate ion on page 385 in Zumdahl.
Odd-Electron Molecules
NO2
•
•
•
contains 17 electrons.
cannot satisfy the octet rule.
a more sophisticated model is needed-the molecular orbital model.
Stereochemistry: The study of the three-dimensional 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
1.
2.
3.
4.
a VSEPR Structure
Draw Lewis structure.
Put pairs as far apart as possible.
Determine positions of atoms from the way electron pairs are shared.(Parent Geometry)
Determine the name of molecular structure from positions of the atoms.(Actual
Geometry)
Molecular Geometry:
Parent Geometry is electron pair arrangement about the central atom.
• linear
• trigonal planar
• tetrahedral
• trigonal bipyramidal
• octahedral
Actual
•
•
•
•
•
•
•
Geometry is the arrangement of atoms about the central atom.
linear
bent
trigonal pyramid
seesaw
T-shaped
square pyramid
square planar
Lone pair of electrons on the ammonia molecule.
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Lone
pair
N
N
H
H
H
(a)
(b)
Lone pairs on the water molecule:
Octahedral structure for phosphorus hexachloride.
08_144

Cl
Cl
P
P
Cl
Cl
Cl
Octahedral structure for xenon.
08_145
Xe
Cl
Parent and actual geometry for xenon tetrafluoride.
08_150
F
F
F
F
90 °
Xe
Xe
leads to the
structure
F
F
F
F
(a)
F
F
F
Xe
F
F
180°
leads to the
structure
F
Xe
F
F
(b)
Three possible arrangements of the electron pairs in triiodide ion.
08_152
I
I
I
I
I
I
I
I
I
(a)
(b)
(c)
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
5
Trigonal
bipyramidal
120° A
6
Octahedral
90°
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)
•
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 120°.