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ENERGY & LIGHT THE QUANTUM MECHANICAL MODEL

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Atomic Models What was Rutherford’s model of the atom like? What is the significance of the proton? What is the significance of the electron? To understand the quantum mechanical model of the atom, we need an understanding of light and how it transmits energy.

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Energy is transmitted in waves by radiation What are different kinds of radiation? Visible light, microwaves, x-rays What is the electromagnetic spectrum? The whole range of types of radiation. What is meant by the “dual nature” of light? Light behaves as waves. Light behaves as packets of energy called photons.

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Anatomy of a Wave Crest Trough Amplitude Frequency

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Electromagnetic Spectrum All waves of EMS travel at the speed of light in a vacuum Speed of light = 3.00 X 10 8 m/s C = λν λ = wavelength v = frequency in m or cm in Hz or s -1

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Problem 1 What is the frequency of a wave if the wavelength is 5.00 X 10 -6 cm? Look on page 362. What part of the EMS is it in?

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Problems 2 & 3 What is the wavelength of radiation if the frequency is 1.5 X 10 13 Hz? Is it longer or shorter than red light waves? What is the wavelength of yellow light emitted by sodium vapor lamp if the frequency is 5.10 X 10 14 Hz

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Waves & Energies Max Planck Different wavelengths have different energies Energy of a body changes in small discrete units. These small units or blocks of energy are called quanta. Planck explains why metals change colors when heated with the mathematical expression E q = h ν – E q = energy of a quantum – h = 6.626 X 10 -34 Joule seconds (Js) = Planck’s constant – v = frequency (nu) Quantum – amount of energy to move an electron up one energy level

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Waves & Energies Albert Einstein Proposed that light could be described as quanta of energy called photons (bundles of energy) This explained the photoelectric effect – certain energy levels will excite e - of metals and others will not. Revised equation – E p = h ν What does E p stand for?

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Problems 4 & 5 Calculate the energy of a quantum of radiant energy with a frequency of 5.00 X 10 15 Hz? What is the energy of a photon of microwave radiation with a frequency of 3.20 X 10 11 s -1 ?

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Atomic Emission Spectra Bohr applied quantum theory to explain AES of hydrogen. Ground state – lowest possible energy level of an electron If electron moves up energy level(s) it is quantized. If falls back down emits same amount of energy in the form of visible light (photons)

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Atomic Emission Spectra Color of light emitted is unique to element How astronomers determine composition of stars. Is a discontinuous spectrum as opposed to continuous (light through a prism)

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Atomic Emission Spectra Bohr’s theory didn’t explain for all atoms New ideas – deBroglie’ s mathematical model λ = h/mv where m = mass and v = velocity Visible objects produce wavelengths too small to be seen or easily measured – 200g baseball traveling at 30 m/s has λ of 1X 10 -32 cm Electrons by contrast have λ large enough to be measured – 2 X 10 -3 cm

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