Chapter 12 Quantum Mechanics and Atomic Theory

Chapter 12 Quantum Mechanics and Atomic Theory - Chapter 12...

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Chapter 12 Quantum Mechanics and Atomic Theory 12.1 Electromagnetic Radiation A. Electromagnetic Radiation: radiant energy that exhibits wavelike behavior and travels through space at the speed of light in a vacuum. Has electric and magnetic fields that simultaneously oscillate in planes mutually perpendicular to each other and to the direction of propagation through space. B. Waves are characterized by wavelength, frequency, and speed. Wavelength: the distance between two consecutive peals or troughs in a wave. Frequency: the number of waves (cycles) per second that pass a given point in space. Short- wavelength radiation must have a high frequency. (Waves with the shortest wavelength have the highest frequency and waves with the longest wavelength have the lowest frequency.) There is an inverse relationship between wavelength and frequency. ( c = νλ) Where c is the speed of light (constant 2.99792458 X10 8 ), v is the frequency in cycles per second, and λ is the wavelength in meters. (Hertz, Hz, is cycles per second.) 12.2 The Nature of Matter A. Blackbody radiation: radiation that originates from the thermal energy of the body only. Ultraviolet catastrophe: predicts a radiation profile that has no maximum and goes to infinite intensity at very short wavelengths. B. Planck’s constant: 6.626 X10 -34 J s. The change in energy for a system ∆ E can be represented by the equation ∆ E = nhv , where n is and integer, h is Plank’s constant, and v is the frequency of the electromagnetic radiation absorbed or emitted. C. Energy is quantized and can only be transferred in discrete units of size hv . Each of these small “packets” of energy is called a quantum. A system can transfer energy only in whole quanta. Thus energy seems to have particulate properties. D. Photon: a quantum of electromagnetic radiation. The energy of a photon is calculated by: E photon = hv = hc /λ, where h is Planck’s constant, v is the frequency of the radiation and λ is the wavelength of the radiation. E. Photoelectric effect: the phenomenon in which electrons are emitted from the suface of a metal when light strikes it. The following observations characterize the photoelectric effect. 1. Studies in which the frequency of the light is carried show that no electrons are emitted by a given metal below a specific threshold frequency v o . 2. For light with frequency lower than the threshold frequency, no electrons are emitted regardless of the intensity of the light. 3. For light with frequency greater than the threshold frequency, the number of electrons emitted increases with the intensity of the light. 4. For light with frequency greater than the threshold frequency, the kinetic energy of the emitted electrons increases linearly with the frequency of the light.
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These observations can be explained by assuming that electromagnetic radiation in quantized (consists of photons), and that the threshold frequency represents the minimum energy required to remove the electron from the metal’s surface.
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Chapter 12 Quantum Mechanics and Atomic Theory - Chapter 12...

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