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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.
How about me and you go back to my place and form a covalent bond or two?
I have 6 valence electrons.
Me too!
Types of Bonds Ionic:
Covalent:
Metallic:
•“trades” electrons • Metal and a non-metal
•“shares” electrons •non-metals
•delocalized electrons •metals •electrically conductive •collective description of many bonds
NaCl
CO
Ag
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 (________________)
+ 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.
e-
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 I K I
Mg F F Mg F
K + I
Mg + 2F
F
+
+ +
+ -
- - 2+
Making Ionic Compounds: Criss-Cross Trick
Al3+ O2-
Al O
Al O
Combine to make neutral compound
Chapter 9
Making Ionic Compounds
Mg2+ O2-
Mg O
Mg O
MgO
Chapter 9
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
XeF44-
N2O
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
6 membered ring
Shown in many forms
p orbitals become __________ over entire molecule
Each bond is about the length of __________________
H
H
H
H
H
H
Kekule Structure.
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 _________
The more electrons the more “______________”
The more polarizable a molecule, the more covalent character it has.
Which has more ionic character, MgI2 or NaCl?
NaCl MgI2
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
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