Download Chapter-2

Molecular structure and bonding
Chemistry 481(01) Spring 2017
Lewis structures
Instructor: Dr. Upali Siriwardane
e-mail: [email protected]
Office: CTH 311 Phone 257-4941
Office Hours:
2.1 The octet rule
2.2 Structure and bond properties
2.3 The VSEPR model
M,W 8:00-9:00 & 11:00-12:00 am;
Tu,Th, F 9:30 - 11:30 a.m.
Valence-bond theory
2.4 The hydrogen molecule
2.5 Homonuclear diatomic molecules
2.6 Polyatomic molecules
April 4 , 2017: Test 1 (Chapters 1, 2, 3, 4)
April 27, 2017: Test 2 (Chapters (6 & 7)
May 16, 2016: Test 3 (Chapters. 19 & 20)
May 17, Make Up: Comprehensive covering all Chapters
Chemistry 481, Spring 2017, LA Tech
Molecular orbital theory
2.7 An introduction to the theory
2.8 Homonuclear diatomic molecules
2.9 Heteronuclear diatomic
2.10 Bond properties
Chapter-2-1
What changes take place during this
process of achieving closed shells?
Chemistry 481, Spring 2017, LA Tech
Chapter-2-2
How do you get the Lewis Structure from
Molecular formula?
• Add all valence electrons and get valence
a) sharing leads to covalent bonds and molecules
Covalent Bond: each atom gives one electron
Coordinative bond: two electron comes from one atom
b) gain/loss of electrons lead to ionic bond
electron pairs
• Pick the central atom: Largest atom normally or
atom forming most bonds
• Connect central atom to terminal atoms
Cations and anions: Electrostatic attractions
c) Sharing with many atoms lead to
• Fill octet to all atoms (duet to hydrogen)
metallic bonds: delocalization of electrons
Chemistry 481, Spring 2017, LA Tech
Chapter-2-3
Chemistry 481, Spring 2017, LA Tech
Chapter-2-4
What is VSEPR Theory
1. Draw Lewis structure for SbF5, ClF3, and IF6+:
Chemistry 481, Spring 2017, LA Tech
Chapter-2-5
Valence Shell Electron Pair Repulsion
This theory assumes that the molecular structure is
determined by the lone pair and bond pair
electron repulsion around the central atom
Chemistry 481, Spring 2017, LA Tech
Chapter-2-6
1
What Geometry is Possible around
Central Atom?
• What is Electronic or Basic Structure?
• Arrangement of electron pairs around the central
atom is called the electronic or basic structure
• What is Molecular Structure?
• Arrangement of atoms around the central atom is
called the molecular structure
Possible Molecular Geometry
• Linear (180)
• Trigonal Planar (120)
• T-shape (90, 180)
• Tetrahedral (109)
• Square palnar ( 90, 180)
• Sea-saw (90, 120, 180)
• Trigonal bipyramid (90, 120, 180)
• Octahedral (90, 180)
Chemistry 481, Spring 2017, LA Tech
Chapter-2-7
2. Predict geometry of central atom using VSEPR
and the hybridization in problem 1.
SbF5, ClF3, and IF6+:
Chemistry 481, Spring 2017, LA Tech
Chapter-2-8
Formal Charges
Formal charge =
valence electrons - assigned electrons
If there are two possible Lewis structures for a molecule,
each has the same number of bonds, we can determine
which is better by determining which has the least formal
charge. It takes energy to get a separation of charge in the
molecule
•(as indicated by the formal charge) so the structure with
the least formal charge should be lower in energy and
thereby be the better Lewis structure
Chemistry 481, Spring 2017, LA Tech
Chapter-2-9
Formal Charge Calculation
Chemistry 481, Spring 2017, LA Tech
Chapter-2-10
Electron counts" and" formal
charges in NH4+ and BF4-
An arithmetic formula for calculating formal charge.
Formal charge =
group number
in periodic table
Chemistry 481, Spring 2017, LA Tech
–
number of
bonds
–
number of
unshared electrons
Chapter-2-11
Chemistry 481, Spring 2017, LA Tech
Chapter-2-12
2
Resonance structures of CO32- ion
Resonance structures of SO2
They both are!
O -S=O
O
O =S- O
S
O
This results in an average of 1.5 bonds between
each S and O.
Ave. Bond order= total pairs shared/ # bonds= 3/2=1.5
Chemistry 481, Spring 2017, LA Tech
Chapter-2-13
Resonance structures of C6H6
Chemistry 481, Spring 2017, LA Tech
Chapter-2-14
Exceptions to the octet rule
• Benzene, C6H6, is another example of a
compound for which resonance structure
must be written.
• All of the bonds are the same length.
Not all compounds obey the octet rule.
• Three types of exceptions
• Species with more than eight electrons around
an atom.
• Species with fewer than eight electrons around
an atom.
or
• Species with an odd total number of electrons.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-15
Chemistry 481, Spring 2017, LA Tech
Chapter-2-16
Linear Combination of Atomic Orbitals
Valence-bond (VB) theory
VB theory combines the concepts of atomic orbitals,
hybrid orbitals, VSEPR, resonance structures, Lewis
structures and octet rule to describe the shapes and
structures of some common molecules.
It uses the overlap of atomic orbitals or hybrid orbitals of the
Symmetry Adapted
Linear Combination of Atomic Orbitals –LCAO
Atomic orbitals on single atom:
Hybridization
Atomic orbitals in a molecule with more than one
atom:
Molecular Orbital (MO) formation
General rule
Number of Hybrid Orbital produced = # hybridized
Number of MO produced = # orbitals combined
to from sigma (s) , pi (p) bonds and (d) bonds
Chemistry 481, Spring 2017, LA Tech
Chapter-2-17
Chemistry 481, Spring 2017, LA Tech
Chapter-2-18
3
How do you tell the hybridization of a central
atom?
What is hybridization?
Mixing of atomic orbitals on the central atom
•Get the Lewis structure of the molecule
Bonding
•Look at the number of electron pairs on the central
atom. Note: double, triple bonds are counted as
a hybrid orbital could over lap with another ()atomic orbital
or () hybrid orbital of another atom to make a covalent
single electron pairs.
•Follow the following chart
bond.
possible hybridizations: sp, sp2, sp3, sp3d, sp3d2
Chemistry 481, Spring 2017, LA Tech
Chapter-2-19
Chemistry 481, Spring 2017, LA Tech
Chapter-2-20
What is hybridization?
Kinds of hybrid orbitals
Mixing of atomic orbitals on the central atoms
Hybrid
sp
sp2
geometry
# of orbital
linear
2
trigonal planar
3
sp3
sp3d
sp3d2
tetrahedral
trigonal bipyramid
octahedral
valence shell (highest n orbitals)
Bonding: s
4
5
6
Px
p
Py
Pz
d
dx2-
dz2
2
y
sp,
sp2,
sp3,
sp3d,
sp3d2
Chemistry 481, Spring 2017, LA Tech
Chapter-2-21
Chemistry 481, Spring 2017, LA Tech
Chapter-2-22
Possible hybridizations of s and p
Possible hybridizations of s and p
sp-hybridization:
sp-hybridization:
1 = 1/2s - 1/2p
2 = 1/2s + 1/2p
sp2-hybridization:
1 = 1/3s + 1/6px + 1/2py
2 = 1/3s + 1/6px - 1/2py
3 = 1/3s - 2/6px
sp3-hybridization:
1 = 1/4s + 1/4px + 1/4py + 1/4pz
2 = 1/4s - 1/4px - 1/4py + 1/4pz
3 = 1/4s + 1/4px - 1/4py - 1/4pz
4 = 1/4s - 1/4px + 1/4py -1/4pz
Chemistry 481, Spring 2017, LA Tech
Chapter-2-23
Chemistry 481, Spring 2017, LA Tech
Chapter-2-24
4
What are p and s bonds
s bonds
Atoms with more than eight electrons
single bond resulting from head to head overlap of
atomic orbital
• Except for species that contain hydrogen, this is
the most common type of exception.
• For elements in the third period and beyond, the
p bond
double and triple bond resulting from lateral or side
way overlap of p atomic orbitals
d orbitals can become involved in bonding.
Examples
• 5 electron pairs around P in PF5
• 5 electron pairs around S in SF4
d bond
double and triple bond resulting from lateral or side
way overlap of d atomic orbitals
Chemistry 481, Spring 2017, LA Tech
Chapter-2-25
• 6 electron pairs around S in SF6
Chemistry 481, Spring 2017, LA Tech
Chapter-2-26
An example: SO423. Why hypervalent compounds are formed by
elements such as Si, P and S, but not by C,N and
O?
O
1. Write a possible
arrangement.
O
S
O
O
2. Total the electrons.
6 from S, 4 x 6 from O
add 2 for charge
total = 32
O
||
3. Spread the electrons
Chapter-2-27
||
O
around.
Chemistry 481, Spring 2017, LA Tech
O - S- O
Chemistry 481, Spring 2017, LA Tech
Chapter-2-28
Atoms with fewer than eight electrons
Atoms with fewer than eight electrons
Beryllium and boron will both form
compounds where they have less
than 8 electrons around them.
Electron deficient. Species other than
hydrogen and helium that have fewer than 8
valence electrons.
:
:F:B:F:
:F:
F-
:
:
:
:Cl:Be:Cl:
:
:
:
:
:
:
They are typically very reactive species.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-29
F
|
B
|
F
+
Chemistry 481, Spring 2017, LA Tech
H
|
:N – H
|
H
F H
| |
F - B <- N - H
| |
F H
Chapter-2-30
5
What is a Polar Molecule?
How do you a Pick Polar Molecule?
• Molecules with unbalanced electrical charges
a) Get the molecular structure from VSEPR theory
• Molecules with a dipole moment
b) From c (electronegativity) difference of bonds
• Molecules without a dipole moment are called
non-polar molecules
see whether they are polar-covalent.
c) If the molecule have polar-covalent bond, check
whether they cancel from a symmetric
arrangement.
d) If not molecule is polar
Predicting symmetry of molecule and the polarity
will be discussed in detail in Chapter 7.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-31
Chemistry 481, Spring 2017, LA Tech
Chapter-2-32
Linear Combination of Atomic Orbitals
Symmetry Adapted
Linear Combination of Atomic Orbitals –LCAO
Atomic orbitals on single atom:
Hybridization
Atomic orbitals in a molecule with more than one
atom:
Molecular Orbital (MO) formation
General rule
Number of Hybrid Orbital produced = # hybridized
Number of MO produced = # orbitals combined
Chemistry 481, Spring 2017, LA Tech
Chapter-2-33
What are p and s bonds
s bonds
Chemistry 481, Spring 2017, LA Tech
Chapter-2-34
What are d bonds
d bond
double and triple bond resulting from lateral or side
way overlap of d atomic orbitals
p bond
Chemistry 481, Spring 2017, LA Tech
6. Draw a diagram to illustrate each described
overlap:
a) s bonding overlap of two p orbitals
b) d bonding overlap of two d orbitals
c) p bonding overlap of a p orbital and a d orbital
d) s antibonding overlap of a p and a d orbital
e) d antibonding overlap of two d orbitals.
Chapter-2-35
Chemistry 481, Spring 2017, LA Tech
Chapter-2-36
6
Kinds of hybrid orbitals
Hybrid
sp
sp2
geometry
# of orbital
linear
2
trigonal planar
3
sp3
sp3d
sp3d2
tetrahedral
trigonal bipyramid
octahedral
5. Using valence-bond (VB) theory to explain the
bonding in the coordination complex ion,
Co(NH3)63+.
4
5
6
Chemistry 481, Spring 2017, LA Tech
Chapter-2-37
Hybridization involving d orbitals
•Co(NH3)6
3+
ion
Co3+:
[Ar]
3d6
Chemistry 481, Spring 2017, LA Tech
Chapter-2-38
5. What is the oxidation state of metal in (a)
Co(NH3)63+ ion (b) PtCl42- ion.
a) [Co(NH3)6] 3+
•Co3+: [Ar] 3d6 4s0 4p0
Co3+ and NH3 is neutral
•Concentrating the 3d electrons in the dxy, dxz, and
dyz orbitals in this subshell gives the following
electron configuration hybridization is sp3d2
Oxidation Sate of Co3+ is +3 and NH3 is 0
Therefore sum of the oxidation should be equal to
+3
+3= Co(NH3)6 = (Co)3+6((NH3)0)= +3
Co is +3 in [Co(NH3)6]3+
b) Pt is +2 in [PtCl4]2- because Cl- is -1
Chemistry 481, Spring 2017, LA Tech
Chapter-2-39
Linear Combination of Atomic Orbitals
Symmetry Adapted
Linear Combination of Atomic Orbitals –LCAO
Atomic orbitals on single atom:
Hybridization
Atomic orbitals in a molecule with more than one
atom:
Molecular Orbital (MO) formation
General rule
Number of Hybrid Orbital produced = # hybridized
Number of MO produced = # orbitals combined
Chemistry 481, Spring 2017, LA Tech
Chapter-2-41
Chemistry 481, Spring 2017, LA Tech
Chapter-2-40
Basic Rules of Molecular Orbital Theory
The MO Theory has five basic rules:
• The number of molecular orbitals = the number of atomic
orbitals combined
• Of the two MO's, one is a bonding orbital (lower energy)
and one is an anti-bonding orbital (higher energy)
• Electrons enter the lowest orbital available
• The maximum # of electrons in an orbital is 2 (Pauli
Exclusion Principle)
• Electrons spread out before pairing up (Hund's Rule)
Chemistry 481, Spring 2017, LA Tech
Chapter-2-42
7
Molecular Orbital Theory
Bonding and Anti-bobding Molecular Orbital
• Molecular orbitals are obtained by combining the
atomic orbitals on the atoms in the molecule.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-43
Bond Order
Chemistry 481, Spring 2017, LA Tech
Chapter-2-44
Homo Nuclear Diatomic Molecules
• Calculating Bond Order
Period 1 Diatomic Molecules: H2 and He2
Chemistry 481, Spring 2017, LA Tech
Chapter-2-45
Homo Nuclear Diatomic Molecules
Chemistry 481, Spring 2017, LA Tech
Chapter-2-46
Homo Nuclear Diatomic Molecules
Period 2 Diatomic Molecules and Li2 and Be2
Chemistry 481, Spring 2017, LA Tech
Chapter-2-47
Chemistry 481, Spring 2017, LA Tech
Chapter-2-48
8
Molecualr Orbital diagram for
Molecualr Orbital diagram for
B2, C2 and N2
O2, F2 and Ne2
Chemistry 481, Spring 2017, LA Tech
Chapter-2-49
7. Using molecular orbital theory and diagrams,
explain why, O2 is a paramagnetic whereas N2 is
diamagnetic.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-50
Electronic Configuration of molecules
When writing the electron configuration of an
atom, we usually list the orbitals in the order in
which they fill.
Pb: [Xe] 6s2 4f14 5d10 6p2
We can write the electron configuration of a
molecule by doing the same thing.
Concentrating only on the valence orbitals, we
write the electron configuration of O2 as
follows.
O2: (2s) 2(2s*) 2 (2p) 4 (2p*) 2
Chemistry 481, Spring 2017, LA Tech
Chapter-2-51
Electronic Configuration and bond order
Chemistry 481, Spring 2017, LA Tech
Chapter-2-53
Chemistry 481, Spring 2017, LA Tech
Chapter-2-52
Electronic Configuration and bond order
Chemistry 481, Spring 2017, LA Tech
Chapter-2-54
9
Hetero Nuclear Diatomic Molecules
Carbon monoxide CO
Chemistry 481, Spring 2017, LA Tech
Chapter-2-55
MO Correlation Diagrams ( Walsh Diagrams)
8. Draw molecular orbital diagrams for HF, CO,
NO, NO+. Calculate their bond order and predict
magnetic properties.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-56
Walsh Diagram for H3:
• The correlation diagram clearly indicates that the
molecular orbital energy levels changes as the H3
changes from linear to cyclic (equilateral triangle)
structure. In the case of
• linear H3 the overlap between two terminal H is
minimal, where as in the case of cyclic H3 the overlap
is substantial. This will bring the lowest MO (bonding)
and the highest MO (antibonding) down in energy. At
the same time, the non-bonding MO (middle one) will
• go up in energy, leading to a degenerate set of
levels. Thus H3+ (two electrons) will be triangular.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-57
9. Draw a molecular orbital diagram for
triangular H3+ and describe the bonding.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-59
Chemistry 481, Spring 2017, LA Tech
Chapter-2-58
10. Draw a Walsh diagram (orbital correlation
diagram) and show that triangular H3+ is more
stable than linear H3+.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-60
10
Conjugated and aromatic molecules
trans-1,3-Butadiene
• trans-1,3-Butadiene
• Allyl radical
• Cyclopropenium ion: C3H3+
• Cyclobutadiene
• Cyclopentadiene
• Benzene
• C7H7+ (tropyllium) and C8H82+
Chemistry 481, Spring 2017, LA Tech
Chapter-2-61
Allyl radical
Chemistry 481, Spring 2017, LA Tech
Chapter-2-62
Cyclopropenium ion: C3H3+
Chapter-2-63
Chemistry 481, Spring 2017, LA Tech
Chapter-2-64
Benzene
Cyclopentadiene
Chemistry 481, Spring 2017, LA Tech
Chemistry 481, Spring 2017, LA Tech
Chapter-2-65
Chemistry 481, Spring 2017, LA Tech
Chapter-2-66
11
Aromatic Rings
11. Using molecular orbital diagrams for pi (p)
orbitals explain the relative stabilities of the
following:
(a) C3H3 and C3H3+
(b) C4H4 and C4H4+
(c) C5H5 and C5H5(d) C6H6 and C6H6+
(e) C7H7 and C7H7+
Chemistry 481, Spring 2017, LA Tech
Chapter-2-67
Chemistry 481, Spring 2017, LA Tech
Chapter-2-68
Chemistry 481, Spring 2017, LA Tech
Chapter-2-70
The Isolobal Analogy
• Different groups of atoms can give rise to
similar shaped fragments.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-69
12. Pick the isolobal fragments among the
following:
a) Co3(CO)9Co(CO) 3, Co3(CO)9PR,
Metallic Bonding
Co3(CO)9CH
• Metals are held together by delocalized
b) H3CCl,
Mn(CO)5H,
bonds formed from the atomic orbitals of all
the atoms in the lattice.
Re(CO) 5Cl
• The idea that the molecular orbitals of the
c) R2SiH2,
Fe(CO)4H2,
Chemistry 481, Spring 2017, LA Tech
band of energy levels are spread or
delocalized over the atoms of the piece of
metal accounts for bonding in metallic
solids.
H2CH2
Chapter-2-71
Chemistry 481, Spring 2017, LA Tech
Chapter-2-72
12
Linear Combination of Atomic Orbitals
Bonding Models for Metals
•Band Theory of Bonding in Solids
•Bonding in solids such as metals,
insulators and semiconductors may be
understood most effectively by an
expansion of simple MO theory to
assemblages of scores of atoms
Chemistry 481, Spring 2017, LA Tech
Chapter-2-73
Chemistry 481, Spring 2017, LA Tech
Chapter-2-74
Chemistry 481, Spring 2017, LA Tech
Chapter-2-76
Linear Combination of Atomic Orbitals
Chemistry 481, Spring 2017, LA Tech
Chapter-2-75
Band Theory of Metals
13. Describe metallic bonding and properties in
terms of:
a) Electron-sea model of bonding:
b) Band Theory:
Chemistry 481, Spring 2017, LA Tech
Chapter-2-77
Chemistry 481, Spring 2017, LA Tech
Chapter-2-78
13
14. Draw the s band (molecular orbitals) for ten
Na on a line (one dimensional) and show bonding
and anti-bonding molecular orbitals and fill
electrons.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-79
15. Describe the metallic properties of sodium in
terms of band theory.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-80
Types of Materials
16. Using a band diagram, explain how magnesium
can exhibit metallic behavior even though its 3s
band is completely full.
• A conductor (which is usually a metal) is a
solid with a partially full band
• An insulator is a solid with a full band and
a large band gap
• A semiconductor is a solid with a full band
and a small band gap
• Element
C
Si
Ge
Sn
Chemistry 481, Spring 2017, LA Tech
Chapter-2-81
Chemistry 481, Spring 2017, LA Tech
Band Gap
5.47 eV
1.12 eV
0.66 eV
0
eV
Chapter-2-82
17. Draw a Band diagram for
carbon/silicon/germanium/tin, and label valence
band, conduction band and band gap?
Chemistry 481, Spring 2017, LA Tech
Chapter-2-83
Chemistry 481, Spring 2017, LA Tech
Chapter-2-84
14
18. Draw a band diagrams to show the difference
between(Band gaps: C = 5.47, Si = 1.12, Ge = 0.66, Sn = 0)
Conductor (Sn):
19. Draw a band diagram for thermal/photo
(Intrinsic) and doped (Extrinsic) semiconductors
and explain the origin of semicondictivity?
Thermal/photo (Intrinsic) (Ge):
Insulator (C):
Doped (Extrinsic) (Si/As):
Semiconductor (Ge):
Chemistry 481, Spring 2017, LA Tech
Chapter-2-85
20. Draw a band diagram for a p-type (Si/Ga) and ntype (Si/As) semiconductors and show holes and
electrons that is responsible for semiconductivity.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-86
22. What the difference between a transistor
(semiconductor device) and vacuum tube?
p-type(Si/Ga):
n-type(Si/As):
Chemistry 481, Spring 2017, LA Tech
Chapter-2-87
What is a transistor?
Chemistry 481, Spring 2017, LA Tech
Chemistry 481, Spring 2017, LA Tech
Chapter-2-88
21. What is a transistor with emitter (E), collector(C)
and base (B), and how it works?
Chapter-2-89
Chemistry 481, Spring 2017, LA Tech
Chapter-2-90
15
23. Using the diagram explain how a diode work.
Superconductors
• When Onnes cooled mercury to 4.15K, the
resistivity suddenly dropped to zero
Chemistry 481, Spring 2017, LA Tech
Chapter-2-91
The Meissner Effect
Chemistry 481, Spring 2017, LA Tech
Chapter-2-92
Theory of Superconduction
•BCS theory was proposed by J. Bardeen, L. Cooper
•Superconductors show perfect diamagnetism.
•Meissner and Oschenfeld discovered that a
and J. R. Schrieffer. BCS suggests the formation of
so-called 'Cooper pairs'
superconducting material cooled below its critical
temperature in a magnetic field excluded the
magnetic flux. Results in levitation of the magnet in
a magnetic field.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-93
Cooper pair formation - electronphonon interaction: the electron
is attracted to the positive charge
density (red glow) created by the
first electron distorting the lattice
around itself.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-94
High Temperature Superconduction
•BCS theory predicted a theoretical maximum to Tc of around
30-40K. Above this, thermal energy would cause electronphonon interactions of an energy too high to allow
formation of or sustain Cooper pairs.
• 1986 saw the discovery of high temperature
superconductors which broke this limit (the highest known
today is in excess of 150K) - it is in debate as to what
mechanism prevails at higher temperatures, as BCS cannot
account for this.
Chemistry 481, Spring 2017, LA Tech
Chapter-2-95
16