Chapter 2: Chemical
Bonds
What we are going to learn
Quick reminder on ionic and covalent bonding.
Lewis dot symbols.
More Lewis dot symbols.
And more lewis dot symbols.
Bond strength and length.
Bonding: Finding that special
element(s) that completes you
Share or take electrons to be isoelectronic with
noble gas. aka have a full octet.
I have 6 valence electrons.
Me too!
How about me and you go
back to my place and form a
covalent bond or two?
Types of Bonds
Ionic:
•“trades” electrons
• Metal and a non-metal
Covalent: •“shares” electrons
•non-metals
Metallic: •delocalized electrons
NaCl
CO
Ag
•metals
•electrically conductive
•collective description of many bonds
Energy of Ionic Bond Formation
All chemical processes consume (___________) or
release (______________) energy.
We use this for many many things that you’ll see in
Chem 1B
Here we will look at the energy of forming an ionic
bond.
Energy of Ionic Bond Formation
Break it into three parts.
Na ________ an electron (_______________)
Cl _______ an electron (________________)
e-
+ and – attract and form a bond.
Now add all the energies
Energy is lower, so it is ____________.
Ionization energy must be low enough to make this ___________ (aka
__________________), typically only happens with metallic elements.
Lewis Dot Symbols
Represents Elements with
their valence electrons.
Element symbol goes in
middle
Valence electrons placed
around
Bonds formed by line
representing two electrons
Examples:
Ionic Bonds
K
+
+
K
I
I
-
K + I
F
+ Mg +
F
Mg + 2F
-
2+
F Mg F
-
Making Ionic Compounds:
Criss-Cross Trick
3+
Al
2O
Al
O
Al O
Chapter 9
Combine to make
neutral compound
Making Ionic Compounds
2+
Mg
Chapter 9
2O
Mg
O
Mg O
MgO
Combine to make
neutral compound
Covalent bonding: Molecular Compounds
In a covalent bond electrons are shared
Cl
+ Cl
Lewis Structure
Covalent bonding: Molecular Compounds
In a covalent bond electrons are shared
S
+2
O
O S O
General Lewis Structure
Guidelines
Step 1: Add up all valence electrons.
Step 2: Make a skeletal structure by connecting each
element with a bond (which is 1 pair of electrons)
Step 3: Distribute the remaining electrons to satisfy
octet rule
Check for common exceptions to the octet rule
S, P, Xe, Be, B, Al
Step 4: Check formal charges, if you can minimize
them by moving electrons, do so.
Non-Octet Breaking Examples
Here are the ones we will do in class:
N2
CH2O
BH3NH3
XeF44N2O
Breaking the Octet Rule
Second Period Elements can’t have more than 8 electrons.
Seriously- don’t do it!!!!!!
Third Period elements can
This is because they have _____________that can be
used for bonding.
Some elements also commonly have less than 8 atoms.
Examples that Break the Octet
Rule
Here are the ones we will do in class:
XeF4
SF6
BH3
AlH3
H2SO4
POCl3
ClF4-
Resonance Structures
Molecule has resonance if there are are more than one
allowed arrangements of electrons.
Can’t move _________________________
In reality structure is a mix of the resonance structures.
Examples we’ll do in class
NO3
[HCO2]Back through N2O
Delocalized Electrons
Look a the structures we drew.
How are the electrons really placed
around the molecule, which one is
“right”?
Equivalent structures, or structures
with the same energy
___________________________.
Low energy structure contribute
more than high energy structures.
Electrons in these are called
“_________________________”
Line Structures
Short-hand notation for molecules
Typically used for organic molecules
Rules
Each Carbon is represented by a corner or end of a line
All hydrogens attached to carbons are not drawn
Other atoms, all heteroatoms, and hydrogens attached
to heteroatoms need to be drawn
Examples
What is the formula for the
following:
Benzene
Kekule Structure.
6 membered ring
Shown in many forms
H
H
H
H
H
H
p orbitals become
__________ over entire
molecule
Each bond is about the
length of
__________________
Electronegativity
Ability of an atom to __________________________
__________________________________________
Same trend as electron affinity and ionization energy.
Not as many important exceptions, mostly involving
the D block. We won’t worry about them.
Electronegativity and Polarity
A difference in electronegativity between two elements of >2 is ionic
If there is little or no difference in electronegativity it is non-polar.
If there is a difference between the two that is <2, it is a polar covalent
bond.
I3-
HLi
HF
An odd example O3
O
O
O
O
O
+
O
-
+
O O
O
-
Microwaves: Quantum Mechanics at work.
http://phet.colorado.edu/en/simulation/microwaves
Polar water molecules are excited rotationally by microwaves
Friction heats up food
Why can’t you put metal in a microwave?
Going Back to Lewis
Structures we did previously:
Which bonds are polar?
Polarizing Power and Covalent Character
More electrons are further
from the _________
MgI2
NaCl
The more electrons the
more “______________”
The more polarizable a
molecule, the more covalent
character it has.
Which has more ionic
character, MgI2 or NaCl?
Bond Strengths and Lengths
Bond Strength is Measured in
dissociation energy (D)
The stronger the bond the
________________
E.G. A C-C single bond is always
____________ and ___________
than a C-C triple bond.
Resonance structure bonds are
________________________
structures.
Bonds are ________________
SSbetween larger atoms