Atomic Physics

1
Chapter 23

• Atomic Physics

2
Chapter 23 section 1

• Objectives
• Explain how Planck resolved the ultraviolet
  catastrophe in blackbody radiation.
• Calculate energy of quanta using Plancks
  equation.

3
Electromagnetic Radiation

• all objects emit EM radiation
• d/o temperature and other properties
• Usually from infrared, visible light, and UV
• ? varies w/temperature
• Low temperature are usually infrared
• Cannot be seen
• As temperature increases the ? is in the visible
  region of the EM spectrum
• Most objects absorb some radiation and reflect
  the rest
• A system that absorbs all incoming radiation is
  called a blackbody

4
Blackbody

• trap radiation
• black body radiation
• EM radiation emitted by a black body
• Based only on its temperature
• As temperature increases the total E given off
  increases
• As temperature increases the ? decreases
• This contradicts classical physics
• Classical physics states that as ? approaches
  zero the E should become infinte

5
Ultraviolet Catastrophe

• Disagreement is b/c the difference happens at the
  UV end of the spectrum

6
Demo.

• thermal sources
• Match, candle, burner, nail, light bulb
• Compare heat of each
• Test
• List color of flame
• Relist order

7
Max Planck

• In 1900, Planck developed a formula for
  blackbody radiation
• The formula agreed w/experimental data at all
  wavelengths
• He proposed that blackbody radiation was produced
  by submicroscopic electric oscillators
  (resonators)
• Resonators could only absorb and then give off
  certain discrete amounts of E

8
Plancks Equation

• E of a resonator w/frequency f is a multiple of
  hf
• E nhf
• n quantum
• h Plancks constant 6.63 x 10-34 Js
• E is considered quantized
• Can only have certain levels (E levels)
• Resonators absorb or give off E in discrete units
  called quanta (photons)
• Quantum mechanics
• E difference btwn 2 E levels is represented by
  Ehf
• resonator will radiate or absorb E only when it
  changes quantum states

9
Quantum Energy

• E can be expressed in units of eV (electron
  volt)
• E that an e- or proton gains when it is
  accelerated through a potential difference of 1 V
• 1 eV 1.60 x 10-19 CV or J

10
Review and Assignment

• Explain how Planck resolved the ultraviolet
  catastrophe in blackbody radiation.
• Calculate energy of quanta using Plancks
  equation.
• Page 833 1 4

11
Chapter 23 section 1 day 2

• Objectives
• Solve problems involving maximum kinetic energy,
  work function, and threshold frequency in the
  photoelectric effect.

12
Photoelectric Effect

• In 1873 Maxwell affirmed that light was a form
  of EM waves
• Heinrich Hertz provided experimental evidence of
  Maxwells theories
• One experiment could not be explained by the wave
  theory of light
• Photoelectric effect

13
Photoelectric Effect

• when light strikes a metal surface the surface
  gives off e-
• e- emitted are photoelectrons
• according to classical physics, light waves of
  any frequency should be able to eject an e- from
  metal if the intensity is high enough
• but this did NOT happen
• No e- was emitted if the frequency fell below a
  certain frequency even if intensity was high
• If frequency increases enough the photoelectric
  effect was seen

14
Einstein

• resolved this conflict by writing a paper that
  extended Plancks concept of quantization
• Each photon has an E and a photon can be absorbed
  by an e- and the E acquired hf
• E for the e- to escape the metal surface is
  called the work function (hft)
• ft is the threshold frequency

15
Maximum Kinetic Energy

• KEmax hf hft
• There is a direct relationship btwn KEmax and
  frequency

16
Review and Assignment

• Solve problems involving maximum kinetic energy,
  work function, and threshold frequency in the
  photoelectric effect.
• Page 836 1 4

17
Chapter 23 section 1 day 3

• Objectives
• Solve problems involving maximum kinetic energy,
  work function, and threshold frequency in the
  photoelectric effect.

18
Photon Theory of Light

• explains the features of the photoelectric
  effect
• If light intensity doubles the of photons also
  doubles and of e- ejected doubles
• e- are emitted instantaneously (b/c E is in
  photons)

19
Arthur Compton

• realized that if light behaves like a particle a
  collision btwn an e- and photon should be just
  like two balls hitting each other
• photons should have momentum and E and they
  should be conserved in collisions
• when collisions take place, E and frequency of
  photons are decreased and wavelength increases
• this change in wavelength is called the Compton
  shift
• This supports Einsteins photon theory of light
• Shift in wavelength d/o angle through which the
  photon is scattered
• Shift is difficult to see in visible light, but
  can be seen in x-rays and shorter wavelengths

20
Applications of PE Effect

• phototube contains a cathode and anode
• When light is shone on it e- are ejected and
  causes a current
• Used to produce sound on a movie film
• burgular alarm UV light triggers phototube and
  causes current
• When beam is broken, no current and alarm
  activated

21
Review and Assignment

• Solve problems involving maximum kinetic energy,
  work function, and threshold frequency in the
  photoelectric effect.
• Page 839 1 5

22
Chapter 23 section 2

• Objectives
• Explain the strengths and weaknesses of
  Rutherfords model of the atom.
• Recognize that each element has a unique emission
  and absorption spectrum.
• Explain atomic spectra using Bohrs model of the
  atom.

23
Atomic Models

• atoms models have changed a lot
• Newtons day atoms where a solid spheres
• 1897 Thompson suggested the plum pudding model
• e- embedded in a sphere
• 1911 Geiger and Marsden and Rutherford proved
  Thompson wrong
• Gold foil experiment
• Used alpha particles
• A few were deflected
• Nucleus is in the middle and had a charge
• E- are in moving orbitals
• E- movement did not agree w/Maxwells theory of
  electromagnetism

24
Atomic Models

• 1913 Niels Bohr proposed a new model of the H
  atom
• Explained atomic spectra
• Each element has a unique spectrum of colored
  light that is given off when current is applied
• e- moves in circular paths around the nucleus
• F btwn protons and e- is what holds the e- in
  orbit
• Only certain orbitals are stable
• e- can jump from one orbit to another
• Total E of atom remains constant
• e- radiates E only when it jumps
• E can be found w/Plancks equation

25
Ground State

• lowest E state and the radius is the shortest
  (Bohr radius)
• when E is added e- jump to higher E state
  (excited state)
• when the e- returns to lower E level it is
  called spontaneous emission (bright lines)
• size of jump and E correlate
• this model only worked w/H atom
• Not a complete picture

26
Review and Assignment

• Explain the strengths and weaknesses of
  Rutherfords model of the atom.
• Recognize that each element has a unique emission
  and absorption spectrum.
• Explain atomic spectra using Bohrs model of the
  atom.
• Page 847 1 7

27
Chapter 23 section 3

• Objectives
• Recognize the dual nature of light.
• Calculate the de Broglie wavelength of matter
  waves.

28
Dual Nature of Light

• light is both a wave and particle
• some experiments are explained by photon concept
  and others by the wave model
• both are accepted
• wave interference wave model
• photoelectrons particle theory
• Smaller the wavelengths the more like a particle

29
Louis de Broglie

• 1924 de Broglie extended the wave particle
  duality
• All forms of matter have both wave and particle
  properties
• First was questioned and later confirmed by
  experiments of e- diffraction
• Wavelength of a photon Plancks contant (h)
  divided by photons momentum (p)
• Increase in momentum equals decrease in
  wavelength
• ? h/p or h/mv
• Frequency can be found w/ f E/h

30
Louis de Broglie

• also saw a connection btwn his theory of wave
  character and the stable orbits of Bohrs model
• e- orbitals could be stable only if it contained
  an integral (whole) of e- wavelengths
• 1st orbit 1 wavelength
• 2nd orbit 2 wavelengths, etc.

31
Review and Assignment

• Recognize the dual nature of light.
• Calculate the de Broglie wavelength of matter
  waves.
• Page 851 1 5

32
Chapter 23 section 3 day 2

• Objectives
• Distinguish between classical ideas of
  measurement and Heisenbergs uncertainty
  principles.
• Describe the quantum-mechanical picture of the
  atom, including the electron cloud and
  probability waves.

33
Werner Heisenberg

• quantum mechanics does not have unlimited
  precision b/o wave nature of particles
• 1927 Heisenberg stated it is fundamentally
  impossible to make simultaneous measurements of a
  particles position and momentum w/infinite
  accuracy
• Heisenbergs uncertainty principle

34
Erwin Schrodinger

• 1926 Schrodinger proposed a wave equation that
  described how waves change in space and time
• wave function (?)
• ? d/o position of object and time
• ?2 is proportional to the probability of finding
  an e- at a given position
• Proposed by Max Born
• Uses a curve that show the distance from the
  nucleus and e- is
• Most probable location is 5.3 x 10-11 m from the
  nucleus is ground state

35
Electron Cloud

• quantum mechanisms also predicts wave function
  for H atoms at ground state is spherical
  (electron cloud)
• denser regions are where e- are more likely to be
  found

36
Review and Assignment

• Distinguish between classical ideas of
  measurement and Heisenbergs uncertainty
  principles.
• Describe the quantum-mechanical picture of the
  atom, including the electron cloud and
  probability waves.
• Page 854 1 6