Transcript
Copyright © 2010 Pearson Prentice Hall, Inc.
John E. McMurry • Robert C. Fay
Lecture NotesAlan D. Earhart
Southeast Community College • Lincoln, NE
General Chemistry: Atoms First
Chapter 5Covalent Bonds and Molecular Structure
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/2
Molecules and the Covalent Bond
Covalent Bond: A bond that results from the sharing of electrons between atoms.
Molecule: The unit of matter held together by covalent bonds.
Chapter 5/3
Molecules and the Covalent Bond
Molecules and the Covalent Bond
Strengths of Covalent Bonds
Chapter 5/6
Chapter 5/7
A Comparison of Ionic and Covalent Bonds
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/8
Polar Covalent Bonds: Electronegativity
Electronegativity: The ability of an atom in a molecule to attract the shared electrons in a covalent bond.
Chapter 5/9
Polar Covalent Bonds: Electronegativity
Chapter 5/10
Polar Covalent Bonds: Electronegativity
Chapter 5/11
Polar Covalent Bonds: Electronegativity
Chapter 5/12
Polar Covalent Bonds: Electronegativity
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Naming Molecular Compounds
Because nonmetals often combine with one another in different proportions to form different compounds, numerical prefixes are usually included in the names of binary molecular compounds.
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Naming Molecular Compounds
N2O4
The second element listed is more anionlike and takes the name of the element with an “ide” modification to the ending.
The first element listed is more cationlike and takes the name of the element.
The prefix is added to the front of each to indicate the number of each atom.
dinitrogen tetraoxide
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/15
Electron-Dot Structures
Electron-Dot Structures (Lewis Structures): A representation of an atom’s valence electrons by using dots and indicates by the placement of dots how the valence electrons are distributed in the molecule.
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Electron-Dot Structures
Chapter 5/17
Electron-Dot Structures
Chapter 5/18
Electron-Dot Structures
Chapter 5/19
Electron-Dot Structures of Polyatomic Molecules
Step 1: Valence Electrons• Count the total number of valence electrons for
all atoms in the molecule.• Add one additional electron for each negative
charge in an anion or subtract one for each positive charge in a cation.
Chapter 5/20
Electron-Dot Structures of Polyatomic Molecules
Step 2: Connect Atoms• Draw lines to represent bonds between atoms.• For hydrogen and second row atoms, use the
number of bonds listed below.• For third row and greater atoms, they may have
more bonds than predicted by the octet rule.• The least electronegative atom is usually the
central atom.
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Electron-Dot Structures of Polyatomic Molecules
Step 4: Assign Electrons to the Central Atom• If unassigned electrons remain after step 3,
place them on the central atom.
Step 3: Assign Electrons to the Terminal Atoms• Subtract the number of electrons used for
bonding in the previous step from the total number determined in step 1.
• Complete each terminal atom’s octet (except for hydrogen).
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Electron-Dot Structures of Polyatomic Molecules
Step 5: Multiple Bonds• If no unassigned electrons remain after step 3
but the central atom does not yet have an octet, use one or more lone pairs of electrons from a neighboring atom to form a multiple bond (either a double or a triple).
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/23
Electron-Dot Structures of Polyatomic Molecules
2(1) + 6 = 8 valence electrons
Step 4:
Step 1:
Step 2:
bonding pair of electrons
lone pair of electrons
HO
H
Draw an electron-dot structure for H2O.
HO
H
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Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for CCl4.
4 + 4(7) = 32 valence electrons
Step 3:
Step 1:
Step 2: ClC
Cl
Cl
Cl
ClC
Cl
Cl
Cl
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Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for H3O1+.
3(1) + 6 - 1 = 8 valence electrons
Step 4:
Step 1:
Step 2: HO
H
HHO
H
H
1+
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/26
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for CH2O.
4 + 2(1) + 6 = 12 valence electrons
Step 3:
Step 1:
Step 2: HC
O
H
HC
O
H
Step 5: HC
O
H
HC
O
H
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/27
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for SF6.
6 + 4(7) = 34 valence electronsStep 1:
Step 2: Step 3:
F
F
S
FF
F F
F
F
S
FF
F F
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/28
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for ICl3.
7 + 3(7) = 28 valence electronsStep 1:
Step 2: Step 4:
Cl
I
ClCl
Cl
I
ClCl
Step 3:
Cl
I
ClCl
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/29
Electron-Dot Structures and Resonance
Draw an electron-dot structure for O3.
Step 1:
Step 2:
3(6) = 18 valence electrons
Step 4:
Step 5:
OOO
OOOStep 3:
OOO
OOO
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Electron-Dot Structures and Resonance
Step 4: OOO
Or, move a lone pair from this oxygen?
Move a lone pair from this oxygen?
OOO OOO
Resonance
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Formal Charges
FormalCharge
# ofvalence e-
in free atom-
21
-# of
nonbondinge-
# ofbonding
e-=
Calculate the formal charge on each atom in O3.
OOO
6 - (2) - 6 = -1126 - (4) - 4 = 0
12 6 - (6) - 2 = +1
12
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/32
Molecular Shapes: The VSEPR Model
VSEPR: Valence-Shell Electron-Pair Repulsion model
Electrons in bonds and in lone pairs can be thought of as “charge clouds” that repel one another and stay as far apart as possible, this causing molecules to assume specific shapes.
Working from an electron-dot structure, count the number of “charge clouds,” and then determine the molecular shape.
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Molecular Shapes: The VSEPR Model
Two Charge Clouds
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Molecular Shapes: The VSEPR Model
Three Charge Clouds
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Molecular Shapes: The VSEPR Model
Four Charge Clouds
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Molecular Shapes: The VSEPR Model
Four Charge Clouds
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Molecular Shapes: The VSEPR Model
Five Charge Clouds
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Molecular Shapes: The VSEPR Model
Five Charge Clouds
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Molecular Shapes: The VSEPR Model
Five Charge Clouds
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Molecular Shapes: The VSEPR Model
Five Charge Clouds
Molecular Shapes: The VSEPR Model
Five Charge Clouds
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Molecular Shapes: The VSEPR Model
Six Charge Clouds
Chapter 5/43
Molecular Shapes: The VSEPR Model
Six Charge Clouds
Chapter 5/44
Molecular Shapes: The VSEPR Model
Six Charge Clouds
Chapter 5/45
Molecular Shapes: The VSEPR Model
Six Charge Clouds
Chapter 5/46
Chapter 5/47
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/48
Valence Bond Theory
sigma () bonds
Valence Bond Theory: A quantum mechanical model which shows how electron pairs are shared in a covalent bond.
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Valence Bond Theory
Valence Bond Theory: A quantum mechanical model which shows how electron pairs are shared in a covalent bond.
• Covalent bonds are formed by overlap of atomic orbitals, each of which contains one electron of opposite spin.
• Each of the bonded atoms maintains its own atomic orbitals, but the electron pair in the overlapping orbitals is shared by both atoms.
• The greater the amount of overlap, the stronger the bond.
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Hybridization and sp3 Hybrid Orbitals
How can the bonding in CH4 be explained?
4 valence electrons2 unpaired electrons
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/51
Hybridization and sp3 Hybrid Orbitals
How can the bonding in CH4 be explained?
4 valence electrons4 unpaired electrons
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Hybridization and sp3 Hybrid Orbitals
4 nonequivalent orbitals
How can the bonding in CH4 be explained?
Copyright © 2010 Pearson Prentice Hall, Inc. Chapter 5/53
Hybridization and sp3 Hybrid Orbitals
4 nonequivalent orbitals
How can the bonding in CH4 be explained?
4 equivalent orbitals
Hybridization and sp3 Hybrid Orbitals
Hybridization and sp3 Hybrid Orbitals
Other Kinds of Hybrid Orbitals
Chapter 5/57
Other Kinds of Hybrid Orbitals
Other Kinds of Hybrid Orbitals
Chapter 5/59
Other Kinds of Hybrid Orbitals
Chapter 5/60
Other Kinds of Hybrid Orbitals
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Molecular Orbital Theory: The Hydrogen Molecule
Atomic Orbital: A wave function whose square gives the probability of finding an electron within a given region of space in an atom.
Molecular Orbital: A wave function whose square gives the probability of finding an electron within a given region of space in a molecule.
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Molecular Orbital Theory: The Hydrogen Molecule
* antibonding orbitalhigher in energy
bonding orbitallower in energy
Bond Order =(# Bonding e- - # Antibonding e-)
2
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Molecular Orbital Theory: The Hydrogen Molecule
= 12
2 - 0Bond Order =
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Molecular Orbital Theory: The Hydrogen Molecule
= 02
2 - 2Bond Order: =
21
22 - 1
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Molecular Orbital Theory: Other Diatomic Molecules
Oxygen, O2, is predicted to be diamagnetic by electron-dot structures and valence bond theory.
However, it is known to be paramagnetic.
O2 OO
Diamagnetic: All electrons are spin-paired. It is weekly repelled by magnetic fields.
Paramagnetic: There is at least one unpaired electron. It is weakly attracted by magnetic fields.
Molecular Orbital Theory: Other Diatomic Molecules
Chapter 5/68
Molecular Orbital Theory: Other Diatomic Molecules
Chapter 5/69
Molecular Orbital Theory: Other Diatomic Molecules
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