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Chapter 1 Electronic Structure and Bonding Acids and Bases
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Transcript of Chapter 1 Electronic Structure and Bonding Acids and Bases
Chapter 1
Electronic Structure and
Bonding
Acids and Bases
• 1 nm = 10 Å• An atom vs. a nucleus ~10,000 x larger
~ 0.1 nm
Nucleus =1/10,000of the atom
Anders Jöns Ångström(1814-1874)
1 Å = 10 picometers = 0.1 nanometers = 10-4 microns = 10-8 centimeters
Question 1
• What is the electronic configuration of carbon?
• A) 1s2 2s2 2px2
• B) 1s2 2s2 2px1 2py
12pz0
• C) 1s2 2s2 2px12py
12pz1
• D) 1s2 1px1 1py
12s2
Electron Configurations Noble Gases and The Rule of Eight
• When two nonmetals react to form a covalent bond: They share electrons to achieve a Noble gas electron configuration.
• When a nonmetal and a metal react to form an ionic compound: Valence electrons of the metal are lost and the nonmetal gains these electrons.
G.N. LewisPhoto Bancroft Library, University of California/LBNL Image Library
Notes from Lewis’s notebook and his “Lewis” structure.Notes from Lewis’s notebook and his “Lewis” structure.
Footnote:G.N. Lewis, despite his insight and contributions to chemistry, was never awarded the Nobel prize.
http://chemconnections.org/organic/Movies%20Org%20Flash/LewisDotStructures.swf
• Ionic compounds are formed when electron(s) are transferred. • Electrons go from less electronegative element to the more electronegative forming ionic bonds.
Ionic Compounds
Covalent Compounds•Share electrons. •1 pair = 1 bond.•Octet rule (“duet” for hydrogen)•Lewis structures:
Notice the charges: In one case they balance, can you name the compound?In the other they do not, can you name the polyatomic ion?
More about “formal” charge to come.
Question 2
• Select the correct Lewis structure for methyl fluoride (CH3F).
• A) B)
•
• C) D)
Important Bond Numbers(Neutral Atoms / Normal electron distribution)
H F ICl Brone bond
Otwo bonds
Nthree bonds
Cfour bonds
Question 3
• What is the correct Lewis structure of formaldehyde (H2CO)?
• A) B)
• C) D)
Question 4
• Which of the following contains a triple bond?
• A) SO2
• B) HCN
• C) C2H4
• D) NH3
Formal Charge
Formal charge is the charge of an atom in a Lewis structure which has a different than normal distribution of electrons.
Important Bond Numbers(Neutral Atoms / Normal electron distribution)
H F ICl Brone bond
Otwo bonds
Nthree bonds
Cfour bonds
Important Bond Numbers(Neutral Atoms / Normal electron distribution)
Organic Chemistry
C H O N
# of Valence e- s
4
1
6
5
Total # of Bonds (neutral atom)
4
1
2
3
Combinations of bonds
(neutral atom):
# of single bonds
4
2
1
1
2
0
3
1
0
# of double bonds
0
1
0
0
0
1
0
1
0
# of triple bonds
0
0
1
0
0
0
0
0
1
Total Bonds
4
4
4
1
2
2
3
3
3
# of Free Pairs of
electrons
0
0
0
0
2
2
1
1
1
Formal Charge
• Equals the number of valence electrons (Group Number of the free atom) minus [the number of unshared valence electrons in the molecule + 1/2 the number of shared valence electrons in the molecule].
• Moving/Adding/Subtracting atoms and electrons.
Formal charge = number of valence electrons –(number of lone pair electrons +1/2 number of bonding electrons)
HNO3 Nitric Acid
Complete the following table. It summarizes the formal charge on a (“central”) atom for the most important species in organic chemistry.
Question 5
• What is the formal charge of the carbon atom in the Lewis structure?
• A) -1
• B) 0
• C) +1
• D) +2 C
Question 6
• What is the formal charge of the oxygen atom in the Lewis structure?
• A) -1
• B) 0
• C) +1
• D) +2
Resonance
Resonance
Eg. SO2
Bond order 1.5
Bond length > double bond; < single bond
Resonance is a very important intellectual concept that was introduced by Linus Pauling in 1928 to explain experimental observations.
TUTORIAL
Resonance
•Two or more Lewis structures may be legitimately written for certain compounds (or ions) that have double bonds and/or free pairs of non-bonded electrons
•It is a mental exercise in “pushing” or moving electrons.
•Refer to Table 1.6
• Step 1: The atoms must stay in the same position. Atom connectivity is the same in all resonance structures. Only electrons move.
• NON-Example: The Lewis formulas below are not resonance forms. A hydrogen atom has changed position.
Rules of Resonance
N
H
H
C
O
H
N
H
C
OH
H
• Step 2: Each contributing structure must have the same total number of electrons and the same net charge.
• Example:All structures have 18 electrons and a net charge of 0.
Rules of ResonanceRules of Resonance
N
H
H
C
O
H
N
H
H
C
O
H
N
H
H
C
O
H
• Step 3: Calculate formal charges for each atom in each structure.
• Example:None of the atoms possess a formal charge in this Lewis structure.
Rules of ResonanceRules of Resonance
N
H
H
C
O
H
• Step 4: Calculate formal charges for the second and third structures.
• Example:These structures have formal charges.
Rules of ResonanceRules of Resonance
N
H
H
C
O
H
N
H
H
C
O
H
NOTE: They are less favorable Lewis structures.
•same atomic positions
•differ in electron positions
more stable more stable Lewis Lewis
structurestructure
less stable less stable Lewis Lewis
structurestructure
........
CC OO NN OOHH
HH
HH
.... ::....++ ––
........
CC OO NN OOHH
HH
HH
....::....
“Pushing” Electrons
•same atomic positions
•differ in electron positions only
more stable more stable Lewis Lewis
structurestructure
less stable less stable Lewis Lewis
structurestructure
........
CC OO NN OOHH
HH
HH
.... ::....++ ––
........
CC OO NN OOHH
HH
HH
....::....
“Pushing” Electrons
Why use Resonance Structures?
•Delocalization of electrons and charges between two or more atoms helps explain energetic stability and chemical reactivity.
•Electrons in a single Lewis structure are insufficient to show electron delocalization.
•A composite of all resonance forms more accurately depicts electron distribution. (HYBRID) NOTE: Resonance forms are not always evenly weighted. Some forms are better than others.
•Ozone (O3)
–Lewis structure of ozone shows one double bond and one single bond
Expect: one short bond and one Expect: one short bond and one long bondlong bond
Reality: bonds are of equal length Reality: bonds are of equal length (128 pm)(128 pm)
Resonance Example
OO OO••••
OO••••
••••••••••••••••––++
•Ozone (O3)
–Lewis structure of ozone shows one double bond and one single bond
Resonance:Resonance:
OO OO••••
OO••••
••••••••••••••••––++
OO OO••••
OO••••
••••••••••••••••––++
OO OOOO••••
••••••••••••••••
–– ++
••••
Resonance Example
•Ozone (O3)
–Electrostatic potential
map shows both end
carbons are equivalent
with respect to negative
charge. Middle carbon
is positive.
OO OO••••
OO••••
••••••••••••••••––++
OO OOOO••••
••••••••••••••••
–– ++
••••
Resonance Example
Detailed Resonance Examples
Question 7
• Which resonance structure contributes more to the hybrid?
• A) B)
VSEPR ModelValence Shell Electron Pair Repulsion
VSEPR Model
The molecular structure of a given atom is determined principally by minimizing electron pair (bonded &free) repulsions through maximizing separations.
Some examples of minimizing interactions.
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.
• 4. Determine the name of molecular structure from positions of the atoms.
Linear Linear LinearLinear
Trigonal PlanarTrigonal Planar Trigonal PlanarTrigonal Planar
Trigonal Planar Trigonal Planar BentBent
TetrahedralTetrahedral TetrahedralTetrahedral
TetrahedralTetrahedral Trigonal PyramidalTrigonal Pyramidal
TetrahedralTetrahedral BentBent
Trigonal BipyramidalTrigonal Bipyramidal Trigonal BipyramidalTrigonal Bipyramidal
Trigonal BipyramidalTrigonal Bipyramidal SeesawSeesaw
Trigonal BipyramidalTrigonal Bipyramidal T-shapeT-shape
Trigonal BipyramidalTrigonal Bipyramidal LinearLinear
OctahedralOctahedral OctahedralOctahedral
OctahedralOctahedral Square PyramidalSquare Pyramidal
OctahedralOctahedral Square PlanarSquare Planar
Orbital Orbital GeometryGeometry
Molecular Molecular GeometryGeometry Bond AngleBond Angle
00
00
11
00
11
22
00
11
22
33
00
11
22
# of lone pairs# of lone pairs
Chem 226
Lewis Structures / VSEPR / Molecular Models
• Computer Generated Models
Ball and stick models of ammonia, water and methane.
http://chemconnections.org/organic/chem226/Labs/VSEPR/
Worksheet 1: Bonds, Formulas, Structures & Shapeshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
Covalent Compounds•Equal sharing of electrons: nonpolar covalent bond, same electronegativity (e.g., H2)• Unequal sharing of electrons between atoms of different electronegativities: polar covalent bond (e.g., HF)
Question 8
• Which of the following bonds is the most polar?
•
• A) B)
•
• C) D)
• Dipole moments are experimentally measured.• Polar bonds have dipole moments.
dipole moment (D) = = e x d(e) : magnitude of the charge on the atom(d) : distance between the two charges
Bond Dipole & Dipole Moment
Question 9
• Which of the following bonds have the greatest dipole moment ()?
• A) B)
• C) D)
Bond Polarity
A molecule, such as HF, that has a center of positive charge and a center of negative charge is polar, and has a dipole moment. The partial charge is represented by and the polarity with a vector arrow.+ δ−FH
Question 10
• In which of the compounds below is the + for H the greatest?
• A) CH4
• B) NH3
• C) SiH4
• D) H2O
Question 11
• In which of the following is oxygen the positive end of the bond dipole?
• A) O-F
• B) O-N
• C) O-S
• D) O-H