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Physics and the Quantum Mechanical Model l OBJECTIVES: - Calculate the wavelength, frequency, or energy of light, given two of these values.

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Section 13.3 Physics and the Quantum Mechanical Model l OBJECTIVES: - Explain the origin of the atomic emission spectrum of an element.

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Light l The study of light led to the development of the quantum mechanical model. l Light is a kind of electromagnetic radiation. l Electromagnetic radiation includes many kinds of waves l All move at 3.00 x 10 8 m/s = c

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Parts of a wave Wavelength Amplitude Origin Crest Trough

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Parts of Wave - p.372 l Origin - the base line of the energy. l Crest - high point on a wave l Trough - Low point on a wave l Amplitude - distance from origin to crest l Wavelength - distance from crest to crest Wavelength is abbreviated by the Greek letter lambda =

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Frequency l The number of waves that pass a given point per second. l Units: cycles/sec or hertz (hz or sec -1 ) Abbreviated by Greek letter nu = c =

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Frequency and wavelength l Are inversely related l As one goes up the other goes down. l Different frequencies of light are different colors of light. l There is a wide variety of frequencies l The whole range is called a spectrum, Fig. 13.10, page 373

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Radio waves Micro waves Infrared. Ultra- violet X- Rays Gamma Rays Low energy High energy Low Frequency High Frequency Long Wavelength Short Wavelength Visible Light

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Prism l White light is made up of all the colors of the visible spectrum. l Passing it through a prism separates it.

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If the light is not white l By heating a gas with electricity we can get it to give off colors. l Passing this light through a prism does something different.

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Atomic Spectrum l Each element gives off its own characteristic colors. l Can be used to identify the atom. l How we know what stars are made of.

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These are called discontinuous spectra, or line spectra unique to each element. These are emission spectra The light is emitted given off Sample 13-2 p.375

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Light is a Particle l Energy is quantized. l Light is energy l Light must be quantized l These smallest pieces of light are called photons. l Photoelectric effect? l Energy & frequency: directly related.

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Energy and frequency E = h x l E is the energy of the photon is the frequency l h is Planck’s constant l h = 6.6262 x 10 -34 Joules x sec. l joule is the metric unit of Energy

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The Math in Chapter 11 l 2 equations so far: c = E = h l Know these!

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Examples l What is the wavelength of blue light with a frequency of 8.3 x 10 15 hz? l What is the frequency of red light with a wavelength of 4.2 x 10 -5 m? l What is the energy of a photon of each of the above?

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Explanation of atomic spectra l When we write electron configurations, we are writing the lowest energy. l The energy level, and where the electron starts from, is called it’s ground state- the lowest energy level.

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Changing the energy l Let’s look at a hydrogen atom

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Changing the energy l Heat or electricity or light can move the electron up energy levels (“excited”)

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Changing the energy l As the electron falls back to ground state, it gives the energy back as light

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l May fall down in steps l Each with a different energy Changing the energy

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l Further they fall, more energy, higher frequency. l This is simplified l the orbitals also have different energies inside energy levels l All the electrons can move around. Ultraviolet Visible Infrared

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What is light? l Light is a particle - it comes in chunks. l Light is a wave- we can measure its wavelength and it behaves as a wave If we combine E=mc 2, c=, E = 1/2 mv 2 and E = h We can get: = h/mv l called de Broglie’s equation l Calculates the wavelength of a particle.

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Sample problem l What is the approximate mass of a particle having a wavelength of 10 -7 meters, and a speed of 1 m/s? Use = h/mv = 6.6 x 10 -27 (Note: 1 J = N x m; 1 N = 1 kg x m/s 2

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Matter is a Wave l Does not apply to large objects l Things bigger than an atom l A baseball has a wavelength of about 10 -32 m when moving 30 m/s l An electron at the same speed has a wavelength of 10 -3 cm l Big enough to measure.

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The physics of the very small l Quantum mechanics explains how the very small behaves. l Classic physics is what you get when you add up the effects of millions of packages. l Quantum mechanics is based on probability

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Heisenberg Uncertainty Principle l -It is impossible to know exactly the location and velocity of a particle. l The better we know one, the less we know the other. l Measuring changes the properties. l Instead, analyze interactions with other particles

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More obvious with the very small l To measure where a electron is, we use light. l But the light moves the electron l And hitting the electron changes the frequency of the light.

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Moving Electron Photon Before Electron Changes velocity Photon changes wavelength After Fig. 13.19, p. 382

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