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Honors Chemistry

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Quantum Intro

Quantum 1

Everything you know is wrong. Well, not everything. Just most of it. If there is one take-home lesson from this is that the assumption we all make that two opposite conditions cannot both be true at the same time is not valid, or at least not useful, when examining the world of subatomic particles and EM radiation. Think of it this way: our interaction with the BIG world averages out all the little events and we don’t notice them. Think about atmospheric pressure. The collisions of air molecules with your body average out and you don’t feel like you are being pummeled. But, if you were a spec of might have a different experience. Matter and energy, as we experience them, seem to be two different things with different properties. Actually, they are manifestations of the same thing.

Nature of light:

  1. All electromagnetic (EM) radiation travels a the same speed in a vacuum (2.9979x108 m/sec. 3.00x108 m/sec will do )
  2. We characterize EM radiation in terms of two parameters other than speed, wavelength, which is given the Greek symbol λ, (lower case lambda) and frequency, which is given the Greek symbol ν (lower case nu)
  3. Wavelength is exactly that, the distance from the peak of one wave to the peak of the next and is given in meters (visible light has wavelengths in the 4-7x10-7 range, while UV light is shorter and X-rays much shorter. FM radio has values in the 3-10 meter range.
  4. Frequency is “cycles per second;” just how many waves will pass a given point per second. It has units of 1/seconds or sec-1.
  5. Logically, if I multiplied λ in meters by ν in 1/s I would get m/s, which is speed. Therefore νλ=c, the speed of light. Also λ=1/ν That is, as wavelength goes up, frequency decreases.
  6. Light has a dual nature. In some ways, it behaves like a wave (shows diffraction patterns and refraction), in others it behaves like a particle (can travel in a vacuum, and has a measurable momentum).

Some More annoying things about light:

Max Planck did an experiment in which he heated objects until they glowed. He then increased the energy he put in (by electricity) and expected the total energy he got out to go up as a straight line. Instead, it went up as a step function (that's not quite true…I'm presenting a clarifying view). More properly, the total energy was always some multiple of the same value for any frequency. That is, the height of the step was always the same for any wavelength.
He found that the value was equal to hν where “h” is Planck’s constant and ν is the frequency of light emitted. The value of Planck’s constant is 6.63e-34 J*s. The total energy emitted is given by nhν where “n” is some integer. Note, since λν=c, hν=hc/λ.

Energy is "quantized"

Energy, therefore, was quantized. There seemed to be “packets” of energy of discreet amounts give by E=hν. These packets are known as “quanta,” which just means “amount.” Energy is said to be “quantized.” Notice that if I multiply frequency (1/s) times Planck’s constant (J*s), seconds cancel and the answer is in Joules. The interesting thing here is that our whole definition of atoms from Dalton and the discovery of an electron all depended on the idea that if there is some fundamental indivisible thing...that’s the particle. So...these quanta of energy come really close to our definition of “particle.”
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