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CH107 – Physical Chemistry
• Atomic and molecular structure• Intermolecular forces & dynamics• Driving forces for equilibrium
Contents of PartI (14 Lectures ) • Origin of Quantization: Lecture 14
Need of a new theory for electrons, atoms and moleculesPostulates of Quantum MechanicsEnergy Quantization: Particle in a Potential Well
•Electronic Structure in Atoms: Lecture 58Hydrogen Atom and Quantum NumbersAtomic Orbitals and Electron DensitiesMultielectronic atoms and the implications of “Spin”
• Chemical Bonding: Lectures 912Molecular Orbital Theory – Linear Combination of Atomic OrbitalsEnergetic and electronic structure of diatomic molecules
• MolecularElectronic structure Lecture 1314 Bonding in polyatomics using hybridization
Classical EM theory can not explain Blackbody Radiation
Theories based on classical physics unable to explain temperaturedependence of emitted radiation (radiant energy density)
Sun, stars…hot iron rod
All classical theories led to the so called “Ultraviolet Catastrophe”
23
8( ) ; bk T cT d d
cνπρ ν ν ν ν
λ= =
Max Planck assumed energies of osci l lators are discontinuous
3
/( )
1v bv taT d
eνρ ν =
−
Assumption: Energy of electronic oscillators were discrete;Assumption: Energy of electronic oscillators were discrete;Proportional to integral multiple of frequenciesProportional to integral multiple of frequencies
E = Energy of electronic oscillatorsv = frequency of electronic oscillatorsh = Planck’s constant = 6.626 x e34 joulesecNote: h came in as a fitting parameter
OscE nhν=
1858-1947
Planck never believed his theory was right, since he was a classical physicist
3
3 /
8( )1B
vh k T
h dT dc e νπ ν νρ ν
÷
=−
Photoelectr ic Effect
1. Increasing intensity of light increases number of photoelectrons, but not their max. kinetic energy (KEMAX)!
2. Light below a certain wavelength will not cause ejection of electrons, no matter how high it’s intensity!
3. Extremely weak violet light ejects few electrons! But their KEMax >> KEMax of electrons ejected by intense light of longer wavelength
0
20
( )
Wave Energy related
to
of
E E Sin kx t
Intensity E
Independent
ω
ω
= −
µ
Photodetectors, Photovoltaics, Elevator sensor, smoke detectors
Experimental Observations
Einstein: l ight behaves l ike partic les
2
0
Energy to remove e' from surface
1
2
0
P M
M
E hv KE mv
KE hv
φ φ
φφ
=
= = + = +
= − ≥
Borrowing Planck’s idea that ∆E=hv, Einstein further proposed radiation itself existed as small packets of energy (Quanta), known as PHOTONS
P
P
E
E h
νν
µ=
1879-1955; Nobel prizeFor explanation of Photoelectric effect
Energy of photon isfrequency dependent(self contradictory! )
• Electrons rotate in circular orbits around a central massive nucleus (+ve), and obey laws of classical
mechanics.
• Allowed orbits are those for which the electron’s angular momentum mevr = n h/2π, n=1,2,3,4,…… • Only certain discrete energy values: “Stationary States”
Atom in such a state does not emit EM radiation (light)
• Transition from a stationary state (Ea) to another (Eb), atom emits or absorbs