1. Chemistry 141 Monday, October 30, 2017 Lecture 23 Light and Matter
Chemistry 11 - Lecture 11
9/30/2009
Chemistry 141
Monday, October 30, 2017
Lecture 23
Light and Matter

2. The photoelectric effect
The experiment:
When light is shined on a metal surface under certain conditions, free electrons are ejected
frequency (s-1) KE
Light source
threshold frequency

3. The photoelectric effect
Einstein (1905) proposed that light is quantized – that is, it behaves like particles
 = wavelength (m)
n = Frequency (Hz, s-1)
c = ×108 m/s (in vac.)
h = 6.626×10-34 J·s
hν1< hν2 < hν3
hν1
hν1
hν1
hν2
hν2
hν2
hν3

4. Quantization of energy

5. Photon energies
 = wavelength (m)
n = Frequency (Hz, s-1)
c = ×108 m/s (in vac.)
h = 6.626×10-34 J·s
Earlier, we calculated the wavelength of red light (700 nm). What is the energy of a photon with this wavelength?

6. The electromagnetic spectrum
400 nm
700 nm
Examples of electromagnetic radiation:
visible light x-rays
microwave ovens radio and TV signals

8. Light and Matter

9. Continuous vs. line spectra
When white light is dispersed through a prism, the resulting spectrum is continuous
When light from a hydrogen lamp is dispersed through a prism, it is observed to consist of discrete lines
What wavelengths of light are emitted?
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10. The Balmer Series n n (Hz) l (nm) 3 4.5711014 656.3
Johann Balmer worked out that the frequencies of the light emitted by excited H-atoms satisfy the equation below
n n (Hz) l (nm)    
For n = 3, 4, 5, …

11. The Balmer Series n n (Hz) l (nm) 3 4.5711014 656.3
Johann Balmer worked out that the frequencies of the light emitted by excited H-atoms satisfy the equation below
n n (Hz) l (nm)    
For n = 3, 4, 5, …
where n2 > n1
RH = ×107 m-1 (Rydberg’s constant)

12. The Bohr atom
Bohr postulated that electrons traveled in circular orbits
Only orbits with certain radii were allowed
n can only be an integer!!
The energy of an electron in a given orbit is:
E depends on n
Energy is also quantized
Note that E is negative!

13. Atomic energy levels Energy levels are like a staircase
A change from lower to higher energy level corresponds to absorption of energy
A change from higher to lower energy level corresponds to emission of energy
E=0
energy of level
n=4
energy to move between levels
energy
n=3
“excited states”
n=2
n=1
“ground state”

14. Bohr atom energies Brackett Paschen Energy Balmer Lyman Energy n = 
-13.6 eV
n = 5
n = 6
Lyman
-3.40 eV
-1.51 eV
-0.85 eV
-0.54 eV
-0.38 eV
0 eV
Balmer
Paschen
Brackett
Energy
Bohr atom energies
Energy

15. Example
What is the wavelength of the photon absorbed when going from the n=2 energy level to the n=4 energy level?

16. The Balmer Series n n (Hz) l (nm) 3 4.5711014 656.3
Johann Balmer worked out that the frequencies of the light emitted by excited H-atoms satisfy the equation below
n n (Hz) l (nm)    
For n = 3, 4, 5, …

17. Bohr atom: does it work? What next?
Made accurate predictions for H and He+ line spectra
Theoretical basis of Bohr’s model is questionable
There is no physical explanation for why the electron “orbits” are stable
Bohr simply threw in idea of “allowed orbits” to classical physics to get agreement with experiments
Only works for one-electron atoms
Predictions for multielectron atoms were incorrect
What next?