1. Chemistry 141 Friday, October 27, 2017 Lecture 22 Light and Matter
Chemistry 11 - Lecture 11
9/30/2009
Chemistry 141
Friday, October 27, 2017
Lecture 22
Light and Matter

2. Simple model of the atom
~10-8 cm
10-13 cm
Particle
Mass
(amu)
Charge
(eV)
Proton +1
Neutron
Electron
1/1822.9 -1
Do electrons, protons, neutrons, and atoms behave the same as larger objects (like a baseball)? (Recall results of Rutherford’s experiments)
Can we apply the laws of classical physics to these small particles?
Can we write equations to describe the forces on and motion of an electron?

3. Light and Matter

4. Properties of waves Animations of Waves What is a wave?
“A disturbance from an equilibrium condition which travels, or propagates, from one region of space to another”
-- Young, H. D., University Physics, 8th ed., Addison-Wesley, Reading, MA, 1992.
Animations of Waves

5. Properties of waves
Wavelength: distance between adjacent peaks/troughs
units – length: m, cm, nm
symbol – lambda: λ
Frequency: number of peaks/troughs of a wave that cross a fixed point in space in one second
units – cycles/second: s-1, Hertz (Hz)
symbol – nu: ν (not v!)
Amplitude: maximum displacement from equilibrium
Speed: rate at which a peak/trough moves through a medium

6. Wavelength – frequency relationship

7. Electromagnetic waves (light)
Consist of oscillating electric and magnetic fields
All electromagnetic waves travel at the speed of light
 = wavelength (m)
n = frequency (Hz, s-1)
c = speed of light
= ×108 m/s (in vacuum)
Copyright © 2002 by Houghton Mifflin Company. All rights reserved.

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

9. Color and Light  = wavelength (m) n = Frequency (Hz, s-1)
A traffic light emits light of frequency 4.28×1014 Hz. What is the wavelength of this radiation? Should you go or stop? How does the wavelength of red light compare to the wavelength of blue light? What about the frequency?
 = wavelength (m)
n = Frequency (Hz, s-1)
c = ×108 m/s (in vac.)

10. Blackbody radiation
Classical theory cannot explain the wavelengths of light emitted from a blackbody

11. Quantization of energy

12. The photoelectric effect
The experiment:
When light is shined on a metal surface under certain conditions, free electrons are ejected
Light source
Ejected electrons
Metal surface

13. The photoelectric effect
Einstein (1905) proposed that light is quantized – that is, it behaves like particles
hν1< hν2 < hν3
hν1
hν1
hν1
hν2
hν2
hν2
hν3

14. 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?

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

17. The failures of classical physics
In the early part of the 20th century, several experiments were performed that could not be explained in terms of classical physics:
The photoelectric effect
Light acts like particles in ejecting electrons from metal surface
Blackbody radiation & the ultraviolet catastrophe
The amount of energy emitted comes in packets
The energy of each packet is related to its frequency
Emission of the highest frequency packets is limited due to finite total energy
Light has both wave-like and particle-like properties!
Hydrogen atom emission spectrum

18. The spectrum of white light
When white light is dispersed through a prism, the resulting spectrum is continuous
White light is composed of every possible color
Copyright © 2002 by Houghton Mifflin Company. All rights reserved.

19. The hydrogen atom emission spectrum
When a light source containing hydrogen atoms is dispersed through a prism, it is observed to consist of discrete lines
What wavelengths of light are emitted?
Copyright © 2002 by Houghton Mifflin Company. All rights reserved.

20. 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
The table shows the four hydrogen atom emission lines that appear in the visible region
n n (Hz) l (nm)    
For n = 3, 4, 5, …

21. The Rydberg equation Balmer: Rydberg
Johannes Rydberg extended Balmer’s formula to account for all hydrogen atom emission lines, which extend from the far infrared to the ultraviolet region
For emission lines, n2 > n1
The constant RH is referred to as Rydberg’s constant
Why do these equations work and why are line spectra observed at all?
Balmer:
Rydberg