bonding: general concepts -...
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Week 6 CHEM 1310 - Sections L and M 1
Bonding: General Concepts
More on Lewis Structures– Formal Charges– Resonance– Breakdown of the Octet Rule
VSEPR Theory– Steric Number
Chapter Review
Week 6 CHEM 1310 - Sections L and M 2
Formal Charges
For some compounds, there are differentLewis structures possible.
How can we tell which is the correct Lewisstructure? Consider the formal charge…
Image from chemprofessor.com
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Week 6 CHEM 1310 - Sections L and M 3
Formal Charge
FormalCharge = # valence e-
on free atom -# valence e- assigned tothe atom in the molecule
# valence e-
assigned to theatom in themolecule
= # lone pair e- + 0.5 x (# shared e-)
Week 6 CHEM 1310 - Sections L and M 4
Formal Charge
FormalCharge = # valence e-
on free atom -# valence e- assigned tothe atom in the molecule
# valence e-
assigned to theatom in themolecule
= # lone pair e- + 0.5 x (# shared e-)
What is the formal charge onNitrogen in this compound?
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Week 6 CHEM 1310 - Sections L and M 5
Formal Charge
FormalCharge = # valence e-
on free atom -# valence e- assigned tothe atom in the molecule
# valence e-
assigned to theatom in themolecule
= # lone pair e- + 0.5 x (# shared e-)
# valence e- on free N = 5
1 lone pair e-
0.5 x (6 shared e-) = 3
5 - (1+3) = 5-4 = +1
Week 6 CHEM 1310 - Sections L and M 6
Resonance
Resonance is a way to depict average ofmultiple Lewis structures
O O O OOO
Electrons are not moving…representation means realstructure is average of these extremes
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Week 6 CHEM 1310 - Sections L and M 7
Other Examples of Resonance
C
O
OO
-2
C
O
OO
-2
C
O
OO
-2
Carbonate, CO3-2
N C S-1
N C S-1
Thiocyanate ion, NCS-1
N N O N N O N N O
N2O, laughing gas
Week 6 CHEM 1310 - Sections L and M 8
Breakdown of Octet Rule
Case 1Odd electron molecules (stable free radicals)
such as NO
# of valence e- = 5 (N) + 6 (O) = 11 e-
Lewis structure does not satisfy the octet rule butthe molecule is stable, although somewhat reactive
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Week 6 CHEM 1310 - Sections L and M 9
Breakdown of Octet Rule
Case 2Octet deficient molecules (mainly B compounds)
such as BF3
Lewis structure doesnot satisfy the octetrule for B, but themolecule is stable,although somewhat
reactive
B
F
FF
Week 6 CHEM 1310 - Sections L and M 10
Breakdown of Octet RuleCase 3
Valence Shell ExpansionThird and higher period elements can exhibit bonding
where an octet on the central atom is exceeded -can expand up 12 e-!
S
F
F
F
F
F
F
S has 12 e-
SF4 S
F
F
F
F
34 total valence e-
8 e- in bonds24 e- in lone pairs2 left overS gets extra lone pair
SF6
If electrons remainafter satisfying theoctet rule, add lonepairs to the central
atom!
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Week 6 CHEM 1310 - Sections L and M 11
Molecular Shapes
Molecular shapes are built from 5 basicarrangements:– Linear
– Planar Tetragonal
– Tetrahedral
– Trigonal Bipyramidal
– Octahedral
What do these shapeslook like?
How can we predictthe shape in which a compound is found?
Week 6 CHEM 1310 - Sections L and M 12
Linear Molecules
In linear molecules, all atoms lie in a straightline.
All diatomic molecules are linear.When 3 atoms are present in the molecule,
the atoms have a bond angle of 180°.
Example: CO2
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Week 6 CHEM 1310 - Sections L and M 13
Trigonal Planar Molecules
Central atom lies at thecenter of a triangleformed by the other 3atoms
All atoms in the sameplane
Bond angles = 120° Example: BF3
Week 6 CHEM 1310 - Sections L and M 14
Tetrahedral Molecules
Central atom at the center of the tetrahedronThree-dimensional, pyramid shape
– not planar like linear and planar trigonal molecules
Bond angles = 109.5°
Example: CH4
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Week 6 CHEM 1310 - Sections L and M 15
Trigonal Bipyramidal Molecules
Has 2 trigonal pyramids w/triangular facesCentral atom lies at middle of triangular plane
shared by upper and lower pyramidsBond angles = 90° and 120°
Example: PCl5
Week 6 CHEM 1310 - Sections L and M 16
Octahedral Molecules
2 square pyramids sharing a square baseCentral atom lies at middle of square plane
shared by upper and lower pyramidsBond angles = 90°
Example: SF6
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Week 6 CHEM 1310 - Sections L and M 17
Predicting Molecular Shapes
How can we predict the shape of aparticular molecule?
Valence Shell Electron Pair Repulsion(VSEPR) Theory
Electron pairs in the valence shell of an atom repel eachother on a spherical surface formed by the underlying
core of the atom.
How can we apply VSEPR theory to helppredict the shape of a molecule?
Week 6 CHEM 1310 - Sections L and M 18
Steric Number and VSEPR Theory
StericNumber
(SN)
# atoms bonded
to a centralatom
= +
# lone pairs oncentral
atom
O HHSN = 2 + 2 = 4 when SN = 4, shape = tetrahedral
This is why water is bent and not linear!
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Week 6 CHEM 1310 - Sections L and M 19
Steric Number and VSEPR Theory
StericNumber
(SN)
# atoms bonded
to a centralatom
= +
# lone pairs oncentral
atom
Steric Number
2
3
4
5
6
Shape (Geometry)
Linear
Trigonal Planar
Tetragonal
Trigonal Bipyramidal
Octahedral
Week 6 CHEM 1310 - Sections L and M 20
What To Study - Ch. 12
Characteristics of WavesQuantum NumbersAufbau Principle
– Electron Configuration
Trends in Periodic Table– Ionization Energy– Electron Affinity– Atomic Radius
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Week 6 CHEM 1310 - Sections L and M 21
What To Study - Ch. 13
Types of Chemical Bonds– Ionic– Covalent: Polar and Nonpolar
ElectronegativityBond Polarity and Dipole MomentsCalculation of Bond Energy for ReactionLewis StructuresVSEPR Theory to deduce molecular shape
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