Chapter 2: Chemical Bonds - UCI Sitessites.uci.edu/brindley1an/files/2013/06/Chapter2SV.pdf · Bond...
Transcript of Chapter 2: Chemical Bonds - UCI Sitessites.uci.edu/brindley1an/files/2013/06/Chapter2SV.pdf · Bond...
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Chapter 2: Chemical Bonds
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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.
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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!
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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
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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.
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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-
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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:
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Ionic Bonds K I K I
Mg F F Mg F
K + I
Mg + 2F
F
+
+ +
+ -
- - 2+
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Making Ionic Compounds: Criss-Cross Trick
Al3+ O2-
Al O
Al O
Combine to make neutral compound
Chapter 9
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Making Ionic Compounds
Mg2+ O2-
Mg O
Mg O
MgO
Chapter 9
Combine to make neutral compound
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Covalent bonding: Molecular Compounds
In a covalent bond electrons are shared
Cl Cl +
Lewis Structure
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Covalent bonding: Molecular Compounds
In a covalent bond electrons are shared
S 2 O + O S O
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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.
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Non-Octet Breaking Examples
Here are the ones we will do in class: N2
CH2O
BH3NH3
XeF44-
N2O
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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.
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Examples that Break the Octet Rule
Here are the ones we will do in class: XeF4
SF6
BH3
AlH3
H2SO4
POCl3
ClF4-
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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
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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 “_________________________”
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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
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Examples
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What is the formula for the following:
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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.
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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.
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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
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An odd example O3
O
O O
O
O+
O-
O-
O+
O
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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?
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Going Back to Lewis Structures we did previously:
Which bonds are polar?
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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
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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