Download - Chapter 9
Dr. S. M. Condren
Chapter 9
Bonding and Molecular Structure:
Fundamental Concepts
Dr. S. M. Condren
Chemical BondingProblemsProblems and questions — and questions —
How is a molecule or How is a molecule or polyatomic ion held polyatomic ion held together?together?
Why are atoms distributed at Why are atoms distributed at strange angles?strange angles?
Why are molecules not flat?Why are molecules not flat?
Can we predict the structure?Can we predict the structure?
How is structure related to How is structure related to chemical and physical chemical and physical properties?properties?
Dr. S. M. Condren
Structure & Bonding
NN triple bond. Molecule is NN triple bond. Molecule is unreactiveunreactive
White phosphorus White phosphorus is a tetrahedron of is a tetrahedron of P atoms. P atoms. Very reactive!
Red phosphorus, Red phosphorus, a polymer. Used a polymer. Used in matches. in matches. Less reactive!
Dr. S. M. Condren
Forms of Chemical Bonds
There are 2 extreme forms of connecting or bonding atoms:There are 2 extreme forms of connecting or bonding atoms:
Ionic—complete transfer of —complete transfer of 1 or more electrons from 1 or more electrons from one atom to anotherone atom to another
Covalent——some valence some valence electrons shared between electrons shared between atomsatoms
Most bonds are somewhere in between.
Dr. S. M. Condren
Ionic Compounds
Metal of low IEMetal of low IE
Nonmetal Nonmetal of high EAof high EA
2 Na(s) + Cl2 Na(s) + Cl22(g) ---> (g) ---> 2 Na2 Na++ + 2 Cl + 2 Cl--
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Covalent BondingThe bond arises from the mutual The bond arises from the mutual
attraction of 2 nuclei for the same attraction of 2 nuclei for the same
electrons.electrons. Electron sharing results.results.
Bond is a balance of attractive and repulsive forces.Bond is a balance of attractive and repulsive forces.
Dr. S. M. Condren
A bond can result from a “head-to-head”A bond can result from a “head-to-head”
overlap of atomic orbitals on of atomic orbitals on
neighboring atoms.neighboring atoms.
ClH H Cl••
••
••
••
••
••
+
Overlap of H (1s) and Cl (3p)
Note that each atom has a single, unpaired electron.
Bond Formation
Dr. S. M. Condren
Chemical Bonding: Objectives
Objectives are to understand:are to understand:
1. 1. valence e- distribution in in molecules and ions.molecules and ions.
2. 2. molecular structures
3. 3. bond properties and their and their effect on molecular properties.effect on molecular properties.
Dr. S. M. Condren
• Electron distribution is Electron distribution is depicted withdepicted with
Lewis electron dot structures
• Valence electrons are Valence electrons are distributed as shared ordistributed as shared or BOND PAIRS
and unshared and unshared oror LONE PAIRS.
G. N. Lewis G. N. Lewis 1875 - 19461875 - 1946
Electron Distribution in Molecules
Dr. S. M. Condren
• Valence electrons are distributed as Valence electrons are distributed as
shared orshared or BOND PAIRS and and
unshared orunshared or LONE PAIRS.
•
••
•
••
H Cl
shared orbond pair
This is called a This is called a LEWIS ELECTRON DOT structure.structure.
Bond and Lone Pairs
lone pairs
Dr. S. M. Condren
Electrons are divided between Electrons are divided between core and and valence electrons
B 1sB 1s22 2s 2s22 2p 2p11
Core = [He]Core = [He] , , valence = 2s2 2p1
Br [Ar] 3dBr [Ar] 3d1010 4s 4s22 4p 4p55
Core = [Ar] 3dCore = [Ar] 3d1010 , , valence = 4s2 4p5
Valence Electrons
Dr. S. M. Condren
No. of valence electrons of a main group atom = Group number
••For Groups 1A-4A, no. of bond pairs = For Groups 1A-4A, no. of bond pairs = group number.group number.
• For Groups 5A -7A, BP’s = 8 - Grp. No.Group 3A Group 5A
Rules of the Game
Dr. S. M. Condren
Rules of the Game
No. of valence electrons of an atom = Group numberNo. of valence electrons of an atom = Group number
For Groups 1A-4A, no. of bond pairs = group numberFor Groups 1A-4A, no. of bond pairs = group number
For Groups 5A -7A, BP’s = 8 - Grp. No. For Groups 5A -7A, BP’s = 8 - Grp. No.
••Except for H (and sometimes atoms of 3rd and higher periods),
BP’s + LP’s = 4This observation is called theThis observation is called the
OCTET RULE
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Hydrophobic vs. Hydrophilic
• Hydrophobic - translation of Greek – water fear
• Hydrophilic – translation of Greek – water friendship
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Ammonia, NH3
1. Decide on the central atom; never H.1. Decide on the central atom; never H.
Central atom is atom of lowest affinity Central atom is atom of lowest affinity for electrons.for electrons.
Therefore, N is centralTherefore, N is central
2. Count valence electrons2. Count valence electrons
H = 1 and N = 5H = 1 and N = 5
Total = (3 x 1) + 5 Total = (3 x 1) + 5
= 8 electrons / 4 pairs= 8 electrons / 4 pairs
Building a Dot Structure
Dr. S. M. Condren
3.3. Form a single bond between Form a single bond between the central atom and each the central atom and each surrounding atomsurrounding atom
H H
H
N
Building a Dot Structure
H••
H
H
N4. 4. Remaining electrons form Remaining electrons form LONE PAIRS to complete octet as needed. to complete octet as needed.
3 BOND PAIRS and 1 LONE PAIR.
Note that N has a share in 4 pairs (8 Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair.electrons), while H shares 1 pair.
Dr. S. M. Condren
Lewis StructuresCH4 methane
Step 1. Central atom = C
Step 2. Count valence electronsC = 44 x H = 4 x 1 = 4
TOTAL = 8 e- or 4 pairsStep 3. Form bonds
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Lewis Structures
C2H6 ethane
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Multiple Covalent Bonds
double bond => 2 pairs shared
triple bond => 3 pairs shared
normally occurs between:
C atoms; N atoms; O atoms;
a C atom and a N, O or S atom
a N atom and a O or S atom
a S atom and an O atom
Dr. S. M. Condren
Double and even triple bonds are commonly observed for C, N, P, O, and S
HH22COCO
SOSO33
CC22FF44
Dr. S. M. Condren
1. Central atom = _______1. Central atom = _______
O OC This leaves 6 pairs.
Carbon Dioxide, CO2
2. Valence electrons = __ or __ pairs2. Valence electrons = __ or __ pairs
3. Form bonds.3. Form bonds.
C4 2
Dr. S. M. Condren
4. Place lone pairs on outer atoms.4. Place lone pairs on outer atoms.
••O OC
•• ••
••••••
••O OC
•• ••
••••••
••O OC
•• ••
••
••O OC
•• ••
••
The second bonding pair forms aThe second bonding pair forms a pi (π) bond.bond.
5. So that C has an octet, we shall form 5. So that C has an octet, we shall form DOUBLE BONDS between C and O.DOUBLE BONDS between C and O.
Carbon Dioxide, CO2
••O OC
•• ••
••••••
Dr. S. M. Condren
Steps to form Lewis Electron Dot Structure
1. Central atom = _______1. Central atom = _______2. Valence electrons = __ or __ pairs2. Valence electrons = __ or __ pairs3.3. Form bonds.Form bonds.4.4. Place lone pairs on outer atoms.Place lone pairs on outer atoms.5.5. Form multiple bonds as necessary to Form multiple bonds as necessary to
obey Lewis’ obey Lewis’ “Octet Rule”.6.6. Remember that there are MANY Remember that there are MANY
compounds that do not obey the Octet compounds that do not obey the Octet Rule.Rule.
Dr. S. M. Condren
Lewis Structures
CO carbon monoxide
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Exceptions to Octet Rule
NO nitric oxide
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Exceptions to Octet Rule
NO2 nitrogen dioxide
resonance
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Exceptions to Octet Rule
PF5
expanded octet
Dr. S. M. Condren
Exceptions to Octet Rule
expanded octet
SF4 F
F
F
F
•• ••
••••
••
••
••
••
••
••
••••
••
S
F
F
F
F
•• ••
••••
••
••
••
••
••
••
••••
••
S
Dr. S. M. Condren
Exceptions to Octet Rule
SF6 expanded octet
Dr. S. M. Condren
• Atoms in molecules often bear a charge (+ or -).
• The predominant resonance structure of a molecule is the one with charges as close to 0 as possible.
• Formal charge = Group number – 1/2 (no. of bonding electrons)
- (no. of LP electrons)
Formal Atom Charges
Dr. S. M. Condren
••
••O OC
••••
+4 - (1/2)(8) - 0 = 0
+6 - (1/2)(4) - 4 = 0
Carbon Dioxide, CO2
Dr. S. M. Condren
6 - (1/2)(2) - 6 = -1 5 - (1/2)(6) - 2 = 0
4 - (1/2)(8) - 0 = 0
••
•
••S NC
•••
Thiocyanate Ion, SCN-
Dr. S. M. Condren
••
•
••S NC
•••
••
•
••S NC
•••
•••
••S NC
•••
Which is the most important resonance form?
Thiocyanate Ion, SCN-
Dr. S. M. Condren
MOLECULAR GEOMETRY
Dr. S. M. Condren
VSEPR Valence alence Shell hell Electron lectron Pair air Repulsion theory.epulsion theory.
Most important factor in determining geometry is Most important factor in determining geometry is relative repulsion between electron pairs.relative repulsion between electron pairs.
MOLECULAR GEOMETRY
Molecule adopts the shape that minimizes the electron pair repulsions.
Dr. S. M. Condren
Electron Pair Geometries
Dr. S. M. Condren
1. Central atom = S1. Central atom = S
2. Valence electrons = 18 or 9 pairs2. Valence electrons = 18 or 9 pairs
••O OS
••
••
••
••••••
bring inleft pair
OR bring inright pair
3. Form double bond so that S has an 3. Form double bond so that S has an octet — but note that there are two ways of octet — but note that there are two ways of doing this.doing this.
•• OS
••
••
••
••••••O
Sulfur Dioxide, SO2
Dr. S. M. Condren
This leads to the following structures.This leads to the following structures.
••O OS
••
••
••
••••••
bring inleft pair
OR bring inright pair
••O OS
••
••
••
••••••
bring inleft pair
OR bring inright pair
••O OS
••
••
••
••••
••O OS••
••
••
••
••
••O OS
••
••
••
••••
••O OS••
••
••
••
••
These equivalent structures are calledThese equivalent structures are called
RESONANCE STRUCTURES. The true . The true
electronic structure is aelectronic structure is a HYBRID of the two.of the two.
Sulfur Dioxide, SO2
Dr. S. M. Condren
Geometries for Four Electron Pairs
Dr. S. M. Condren
Structure Determination by VSEPR
Ammonia, NHAmmonia, NH33
1. Draw electron dot structure
2. Count BP’s and LP’s = 4
H
H
H
lone pair of electronsin tetrahedral position
N
H••
H
H
N
3. The 4 electron pairs are at the 3. The 4 electron pairs are at the corners of a corners of a tetrahedron..
Dr. S. M. Condren
Ammonia, NHAmmonia, NH33
The electron pair geometry is tetrahedral.The electron pair geometry is tetrahedral.
Structure Determination by VSEPR
H
H
H
lone pair of electronsin tetrahedral position
N
The MOLECULAR GEOMETRY, the positions of the atoms, is PYRAMIDAL.
Dr. S. M. Condren
Structure Determination by VSEPRWater, HWater, H22OO1. Draw electron dot structure1. Draw electron dot structure
H
HO
H
HO
The electron pair geometry is The electron pair geometry is TETRAHEDRAL..
The electron pair geometry is The electron pair geometry is TETRAHEDRAL..
H O H••
••
H O H••
••
2. Count BP’s and LP’s = 42. Count BP’s and LP’s = 42. Count BP’s and LP’s = 42. Count BP’s and LP’s = 43. The 4 electron pairs are at the 3. The 4 electron pairs are at the corners of a tetrahedron.corners of a tetrahedron.3. The 4 electron pairs are at the 3. The 4 electron pairs are at the corners of a tetrahedron.corners of a tetrahedron.
The molecular The molecular geometry is geometry is BENT.
The molecular The molecular geometry is geometry is BENT.
Dr. S. M. Condren
Consequences of H2O Polarity
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Structures with Central Atoms
with More Than or less Than
4 Electron Pairs
Often occurs with Group 3A elements and with those of 3rd period and higher.
Dr. S. M. Condren
Molecular Geometries for
Five Electron PairsAll based on trigonal bipyramid
Dr. S. M. Condren
All are based on the 8-sided octahedron
Molecular Geometries for Six
Electron Pairs
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• What is the effect of bonding and structure on molecular properties?
Free rotation Free rotation around C–C single around C–C single bondbond
No rotation No rotation around C=C around C=C double bonddouble bond
Bond Properties
Dr. S. M. Condren
Double bondDouble bondDouble bondDouble bondSingle bondSingle bondSingle bondSingle bond
Triple Triple bondbondTriple Triple bondbond
AcrylonitrileAcrylonitrileAcrylonitrileAcrylonitrile
Bond Order # of bonds between a pair of atoms
Dr. S. M. Condren
Fractional bond orders occur in molecules with resonance structures.
Consider NO2-
Bond order = 3 e - pairs in N— O bonds
2 N — O bondsBond order =
3 e - pairs in N— O bonds2 N — O bonds
Bond order = Total # of e - pairs used for a type of bond
Total # of bonds of that typeBond order =
Total # of e - pairs used for a type of bondTotal # of bonds of that type
O O O O
N••
••••
••••
••••••••••
••••
••N
Bond Order
1 1
The N—O bond order = 1.5
Dr. S. M. Condren
Bond order is proportional to two important bond properties:
(a) bond strength
(b) bond length
745 kJ745 kJ
414 kJ414 kJ
110 pm110 pm
123 pm123 pm
Bond Order
Dr. S. M. Condren
Dr. S. M. Condren
Compare O-O and O=O. Is O=O expected to be
stronger, weaker, or the same strength?
Dr. S. M. Condren
Is O=O expected to be
longer, shorter, or the same length?
Dr. S. M. Condren
Estimate the energy of the reaction
H—H + Cl—Cl ----> 2 H—Cl H—H = 436 kJ/mol H—H = 436 kJ/mol Cl—Cl = 242 kJ/molCl—Cl = 242 kJ/molH—Cl = 432 kJ/molH—Cl = 432 kJ/mol
H—H = 436 kJ/mol H—H = 436 kJ/mol Cl—Cl = 242 kJ/molCl—Cl = 242 kJ/molH—Cl = 432 kJ/molH—Cl = 432 kJ/mol
Sum of H-H + Cl-Cl bond energies = Sum of H-H + Cl-Cl bond energies = 436 kJ + 242 kJ = +678 kJ436 kJ + 242 kJ = +678 kJ
Using Bond Energies
Net = ∆H = [Net = ∆H = [bondsbondsbrokenbroken] – [] – [ bonds bondsformedformed]]
2 mol H-Cl bond energies = 864 kJNet = ∆H = +678 kJ - 864 kJ = -186 kJNet = ∆H = +678 kJ - 864 kJ = -186 kJ∆Hf
o (HCl(g)) = -92.31 kJ/mol or -184 kJ
Dr. S. M. Condren
Molecular Polarity
Why do ionic compounds dissolve in Why do ionic compounds dissolve in water?water?
WaterWaterBoiling point Boiling point
= 100 ˚C= 100 ˚C
MethaneMethaneBoiling point Boiling point
= -161 ˚C= -161 ˚C
Why do water and methane Why do water and methane differ so much in their differ so much in their
boiling points?boiling points?
Dr. S. M. Condren
HCl is HCl is POLAR because it has a because it has a positive end and a positive end and a negative end.negative end.
Cl has a greater share Cl has a greater share in bonding electrons in bonding electrons than does H.than does H.
Cl has a greater share Cl has a greater share in bonding electrons in bonding electrons than does H.than does H.
Cl has slight negative charge Cl has slight negative charge (-(-)) and H and H has slight positive charge has slight positive charge (+ (+ ))
H Cl••
••
+ -••H Cl
••
••
+ -••
Bond Polarity
Dr. S. M. Condren
• Three molecules with Three molecules with polar, covalent bonds.polar, covalent bonds.
• Each bond has one Each bond has one atom with a slight atom with a slight negative charge negative charge (-(-)) and another with a and another with a slight slight positivepositive charge charge (+ (+ ))
Bond Polarity
Dr. S. M. Condren
Linus Pauling, 1901-1994
The only person to receive two unshared The only person to receive two unshared Nobel prizes (for Peace and Chemistry). Nobel prizes (for Peace and Chemistry).
Chemistry areas: bonding, electronegativity, Chemistry areas: bonding, electronegativity, protein structureprotein structure
Dr. S. M. Condren
Electronegativity,
is a measure of the ability of an atom in a is a measure of the ability of an atom in a molecule to attract electrons to itself.molecule to attract electrons to itself.
Dr. S. M. Condren
Due to the bond polarity, the H—Cl Due to the bond polarity, the H—Cl bond energy is GREATER than bond energy is GREATER than expected for a “pure” covalent expected for a “pure” covalent bond.bond.
BOND ENERGY
““pure” bondpure” bond 339 kJ/mol calc’d339 kJ/mol calc’d
real bondreal bond 432 kJ/mol measured432 kJ/mol measured
BOND ENERGY
““pure” bondpure” bond 339 kJ/mol calc’d339 kJ/mol calc’d
real bondreal bond 432 kJ/mol measured432 kJ/mol measured
Difference = 92 kJ. This difference is Difference = 92 kJ. This difference is proportional to the difference in proportional to the difference in
ELECTRONEGATIVITY, , ..
Difference = 92 kJ. This difference is Difference = 92 kJ. This difference is proportional to the difference in proportional to the difference in
ELECTRONEGATIVITY, , ..
H Cl••
••
+ -••
Bond Polarity
Dr. S. M. Condren
Electronegativity
Pauling Scale
• relative attraction of an atom for electrons, its own and those of other atoms
• same trends as ionization energy, increases from lower left corner to the upper right corner
• fluorine: E.N. = 4.0
Dr. S. M. Condren
Electronegativities of the Elements
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Which bond is more polar (or DIPOLAR)?Which bond is more polar (or DIPOLAR)?
O—HO—H O—FO—F
3.5 - 2.13.5 - 2.1 3.5 - 4.03.5 - 4.0
1.41.4 0.50.5
OH is more polar than OFOH is more polar than OF
and polarity is “reversed.”and polarity is “reversed.”and polarity is “reversed.”and polarity is “reversed.”
O H+-
O F+ -
O H+-
O F+ -
Bond Polarity
Dr. S. M. Condren
Molecules will be polar ifMolecules will be polar ifa)a) bonds are polarbonds are polar ANDANDb)b) the molecule is NOT “symmetric”the molecule is NOT “symmetric”
All above are All above are NOTNOT polar polar
Molecular Polarity
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Polar or Nonpolar?Compare CO2 and H2O. Which one is polar?
Dr. S. M. Condren
Covalent Bond Properties
electronegativity
nonpolar bonds => diff. EN = 0
polar bonds => diff. EN > 0
ionic bonds => diff. EN > 1.5
Dr. S. M. Condren
CH4 … CCl4 Polar or Not?
• Only CH4 and CCl4 are NOT polar. These are the only two molecules that are “symmetrical.”