Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2013

Lecture Notes 20: 11 March

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Atomic Structure & Electromagnetic Radiation (Light), cont.

Electromagnetic Radiation comprises the various types or forms of radiation which propagate through space thar are not associated with mass.

Electromagnetic radiation behaves in most circumstances as waves [Figure 7.1 p 276] and can thus be characterized as waves.

Labeled sine wave diagram

 

Three parameters determine a wave:

These parameters are related by the the expression:

v = f•lambda

For electromagnetic radiation (light) the speed is defined in a vacuum: v = c = 2.9979 x 108m/s

c = f• lambda in vacuo

 

Quantum Reality

It turns out that the energy associated with light is not continuous like an ocean wave, but rather is "packaged' as we would expect with a particle, and the energy/package is proportional to the frequency. Thus the energy per packet, or photon is:

Ephoton = hf = hc/ lambda

where E is the energy per photon in J when lambda is expressed in meters and

h = Plank's constant = 6.626 x 10-34J*s

Example: What is the energy of a 400.0 nm photon?

Ephoton= hc/lambda

Ephoton= (6.626 x 10-34J*s)(2.9979 x 108m/s) / (4.000 x 10-7m) = 4.966 x 10-19 J

As particles photons also exhibit momentum (mv), that is, in reflecting off a surface they act as if they were particles with a certain mass and velocity. Though photons themselves don't have "mass" per se, their energy can be interconverted to mass via Einstein's famous equation:

E= mc2

Example: What is the energy of 1 g of matter?

E= mc2

E = (0.001 kg)(2.9979 x 108m/s)2

E = 9 x 1013kJ

(Note a Joule = kg m2s-2, this corresponds to about 22 megatons of TNT, or 2,000 times more powerful than the Hiroshima atom bomb!)

Since light has matter-like properties, perhaps matter has light-like (wave) properties. In the 1920's de Broglie noted that one could set the energy relationships above equal to each other and postulate a wavelength for matter particles:

E= mc2= hc/lambda

dividing both sides by c2,

m = hc/lambdac2 = h/lambdac for photons, but since matter moves at less than c,

m = h/lambdav, and

lambda= h/mv

It turns out that small particles such as electrons do indeed behave as if they are waves under certain circumstances.

As an example, for an electron with mass = 9.11 x 10-31kg traveling at 4 x 104m/s:

lambda= h/mv

lambda= (6.626 x 10-34J*s) / (9.11 x 10-31kg)(4 x 104m/s)

= (6.626 x 10-34kg m2s-2*s) / (9.11 x 10-31kg)(4 x 104m/s)

lambda = 1.8 x 10-8m = 2 x 10-8m = 2 x 10 nm = 0.002 mum

Compare this to the wavelength of visible light (400 - 700 nm). This is why electron microscopes are so valuable - they have much greater resolution since the wavelengths are much shorter. Other particles, such as neutrons have higher masses and thus shorter wavelengths so they should have even greater potential resolution. Today folks have even created systems where atoms may be used as probes using their wave-like properties.

Waves vs. Particles: which is true? General discussion of particle-wave duality. (Double slit experiment - If you would like a nice low key cartoon introduction to this aspect of "quantum ierdness" try this link: http://www.youtube.com/watch?v=DfPeprQ7oGc)

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© R A Paselk

Last modified 11 March 2013