Atomic Spectra

Atomic Spectra - 7.1 From Classical Physics to Quantum...

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CHAPTER 7 Quantum Theory and the Electronic Structure of Atoms 7.1 From Classical Physics to Quantum Theory • Classical Physics viewed energy as continuous; i.e., any quantity of energy could be released. • Max Planck found that atoms and molecules (under some conditions) emit energy only in discrete quantities called quanta , founding quantum theory Waves • Wave – a vibrating disturbance by which energy is transmitted • Familiar examples – water waves – radio waves – microwaves •Wavelength, λ – distance between identical points on successive waves Unit = length (m) Top wave has 3 λ of bottom. •Frequency, ν – # waves that pass through a point per s Unit = cycles/s, Hz, s -1 Bottom wave has 3 ν of top. •Amplitude – distance from midpoint of wave to peak or trough Top and bottom waves have same amplitude. Speed of Waves • Speed (u) = λν • Units of λν are length/time, usually m/s • Speed of wave depends on type and nature of medium in which it travels Electromagnetic Radiation • Energy is transmitted in the form of electromagnetic waves. • Can be emitted or absorbed by atoms. • Electric and magnetic field components w/ same λ , ν and u, but traveling in mutually perpendicular planes • Speed of light in vacuum (c) = 3.0 x 10 8 m/s
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Magnetic field component electric field component c = λν Electromagnetic radiation speed of light wavelength frequency Quantum Theory • Problem –Heated solids emit electromagnetic radiation. –Amount of radiant energy is related to its wavelength. –Classical physics cannot explain. • Planck’s solution = quantum theory Planck’s Quantum Theory • Planck’s assumption – atoms and molecules emit or absorb energy only in discrete quantities. • “Bundles” of energy are quanta –smallest quantity of energy that can be emitted or absorbed
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Planck’s Constant = 6.63 x 10 -34 J . s Frequency of light Energy of a photon E = h ν What is the frequency of green light with wavelength 500 nm? c = λν ν = c/ λ ν = 3.00 x 10 8 m•s -1 /500 x 10 -9 m ν = 6.00 x 10 14 s -1 What is the energy of a photon of this light? E = h ν = (6.63 x 10 -34 J . s)(6.00 x 10 s ) E = 3.80 x 10 -19 J • Light strikes the metal, which ejects electrons If light were continuous waves , sufficiently intense light of any frequency would eject electrons. 7.2 Photoelectric Effect • Emission of electrons from certain metals exposed to light • Number , not energy, of photoelectrons depends on intensity of light • Light must be above a certain threshold frequency to eject an electron Einstein’s Explanation • Using Planck’s quantum theory –Light is made up of particles called photons –Each photon has energy = h ν –Electrons are held in metal with certain binding energy, BE –Photon must have energy > BE –Photon energy above BE shows up as kinetic energy (KE) of photoelectron h ν = KE + BE Energy of incident photon Kinetic energy of emitted electron Binding energy of emitted electron Photoelectric Effect
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Wave-Particle Duality Light can behave as 1. Wave – e.g., diffraction by grating 2. Particle – e.g., photoelectric effect
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This note was uploaded on 12/10/2009 for the course CHEM 1111 at Colorado.

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Atomic Spectra - 7.1 From Classical Physics to Quantum...

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