Download
1 / 17
170 likes | 328 Views
History of Atomic Theory. … continued…. Problem with Rutherford. If electrons are orbiting around a nucleus, they are continuously accelerating . 19th century physics - if the continuously accelerating, continuously emitting energy Result: would eventually collapse into the nucleus.
E N D
History of Atomic Theory … continued…
Problem with Rutherford • If electrons are orbiting around a nucleus, they are continuously accelerating. • 19th century physics - if the continuously accelerating, continuously emitting energy • Result: would eventually collapse into the nucleus. • Matter is fairly stable, Rutherford’s model is not.
This would take 0.000000000001 seconds… And you would not exist for very long.
The Nature of Light: • Study of light is related to the development of atomic theory… • Debate over the nature of light – wave or particle? • Propagates as a wave, interacts with matter as a particle (wave/particle duality) • (does not mean it is 2 things – we just don’t have a unified model (yet))….
The Photoelectric Effect • Electrons are ejected from metals when exposed to light... • Classical theory of light says that greater intensity = greater Ek of ejected e-s…. • In REALITY, higher FREQUENCY light gave more Ek to e-s, and below a “threshold frequency”, no e-s would be ejected no matter how intense the light was. http://phet.colorado.edu/en/simulation/photoelectric
Creating Quantum Atomic Theory • 19th century physicists could not explain the wavelength distribution of light emitted from heated objects.. (fig 2/169)
At higher temp, intensity increases, and max intensity shifts to shorter wavelengths
1900 – Max Planck- could explain this distribution only by assuming that there was a “smallest” unit of energy that matter could absorb or emit (a QUANTUM). • Thus, energy is quantized (not continuous
Energy of a quantum is proportional to the frequency of emitted light. (E=hf) • Matter, at the atomic level, can absorb or emit only discrete quantities of energy; a whole # multiple of hf h is Planck's constant… f = frequency
Here comes Einstein! • Albert Einstein took this idea further in 1905 to conclude that the light is also quantized… • Explains the photoelectric effect • It comes in “bursts,” not a continuous steam of energy as previously thought. (Nobel prize!) “The quanta really are a hopeless mess.”(Albert Einstein, On doing Quantum Theory calculations with Pauli)
In Brief: • A quantum of LIGHT energy is called a photon. • The energy a photon has depends on the frequency of the light. (Fig 5/170 and Fig 7/171) • Ejection of electrons from a metal explained in terms of a photon-electron collision.
Photon-electron collision • Energy from the photon is transferred to the electron and used by the electron to; • Break free from the atom • Remainder of energy is the kinetic energy of the ejected electron • A minimum quantity of energy is required for the electron to break free (Fig 8/172) • (increase intensity = increase # of photons, not the energy of individual photons)
Quantum Theory is one of the major scientific achievements of the 20th century… • “All these fifty years of conscious brooding have brought me no nearer to the answer to the question, 'What are light quanta?' Nowadays every Tom, Dick and Harry thinks he knows it, but he is mistaken.” (Albert Einstein, 1954)
Homework: • R 169-172 • 173 #1-4
