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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 1 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 5 Electrons In Atoms 5.1 Revising the Atomic Model 5.2 Electron Arrangement in Atoms 5.3 Atomic Emission Spectra and the Quantum Mechanical Model

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 2 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Objectives 1.Explain light and atomic emission spectra 2.Calculate wavelength or frequency 3.Describe Einstein’s explanation of the photoelectric effect 4.Explain how the frequencies of light emitted by an atom are related to changes of electron energies 5.Distinguish between quantum mechanics and classical mechanics

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 3 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What gives gas-filled lights their colors? An electric current passing through the gas in each glass tube makes the gas glow with its own characteristic color. CHEMISTRY & YOU

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 4 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Light and Atomic Emission Spectra What causes atomic emission spectra? spectrum: wavelengths of visible light that are separated when a beam of light passes through a prism; range of wavelengths of electromagnetic radiation. atomic emission spectrum: the pattern formed when light passes through a prism or diffraction grating to separate it into the different frequencies of light it contains.

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 5 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The Nature of Light By the year 1900, there was enough experimental evidence to convince scientists that light consisted of waves (a disturbance or variation that transfers energy progressively from point to point, Merriam-Webster’s dictionary). The amplitude of a wave is the wave’s height from zero to the crest. The wavelength, represented by (the Greek letter lambda), is the distance between the crests. Light and Atomic Emission Spectra

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 6 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The frequency, represented by (the Greek letter nu), is the number of wave cycles to pass a given point per unit of time. The SI unit of cycles per second is called the hertz (Hz). Light and Atomic Emission Spectra The Nature of Light

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 7 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The product of frequency and wavelength equals a constant (c), the speed of light. c = Light and Atomic Emission Spectra The Nature of Light

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 8 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The frequency ( ) and wavelength ( ) of light are inversely proportional to each other. As the wavelength increases, the frequency decreases. Light and Atomic Emission Spectra

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 9 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. According to the wave model, light consists of electromagnetic waves. Electromagnetic radiation includes radio waves, microwaves, infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays. All electromagnetic waves travel in a vacuum at a speed of 10 8 m/s. Light and Atomic Emission Spectra The Nature of Light

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 10 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The sun and incandescent light bulbs emit white light, which consists of light with a continuous range of wavelengths and frequencies. Light and Atomic Emission Spectra The Nature of Light When sunlight passes through a prism, the different wavelengths separate into a spectrum of colors. In the visible spectrum, red light has the longest wavelength and the lowest frequency.

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 11 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The electromagnetic spectrum consists of radiation over a broad range of wavelengths. Light and Atomic Emission Spectra Wavelength (m) Low energy ( = 700 nm) High energy ( = 380 nm) Frequency (s -1 ) 3 x x x

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 12 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. When atoms absorb energy, their electrons move to higher energy levels. These electrons lose energy by emitting light when they return to lower energy levels. Light and Atomic Emission Spectra Atomic Emission Spectra

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 13 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A prism separates light into the colors it contains. White light produces a rainbow of colors. Light and Atomic Emission Spectra Light bulb SlitPrism Screen Atomic Emission Spectra

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 14 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Light from a helium lamp produces discrete lines. Light and Atomic Emission Spectra SlitPrism Screen Helium lamp Atomic Emission Spectra

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 15 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The energy absorbed by an electron for it to move from its current energy level to a higher energy level is _______to the energy of the light emitted by the electron as it drops back to its original energy level. The wavelengths of the spectral lines are _____________ of the element, and they make up the atomic emission spectrum of the element. No two elements have the _______ emission spectrum. Light and Atomic Emission Spectra Atomic Emission Spectra identical characteristic same

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 16 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 5.2 Calculating the Wavelength of Light Calculate the wavelength of the yellow light emitted by a sodium lamp if the frequency of the radiation is 5.09 × Hz (5.09 × /s).

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 17 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 5.2 Use the equation c = ____ to solve for the unknown wavelength. KNOWNS frequency ( ) = 5.09 × /s c = × 10 8 m/s UNKNOWN wavelength ( ) = ? m Analyze List the knowns and the unknown. 1

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 18 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 5.2 Rearrange the equation to solve for. == c Solve for by dividing both sides by : = c Calculate Solve for the unknown. 2 c =

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 19 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 5.2 Substitute the known values for and c into the equation and solve. = = = 5.89 10 –7 m c 10 8 m/s 5.09 /s Calculate Solve for the unknown. 2

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 20 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 5.2 The magnitude of the frequency is much larger than the numerical value of the speed of light, so the answer should be much less than 1. The answer should have 3 significant figures. Evaluate Does the answer make sense? 3

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 21 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What is the frequency of a red laser that has a wavelength of 676 nm?

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 22 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What is the frequency of a red laser that has a wavelength of 676 nm? c = = = = 4.43 m c 10 8 m/s 6.76 10 –7 /s c =

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 23 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. How did Einstein explain the photoelectric effect? The Quantum Concept and Photons

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 24 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. German physicist Max Planck (1858– 1947) showed mathematically that the amount of radiant energy (E) of a single quantum absorbed or emitted by a body is proportional to the frequency of radiation ( ). The Quantization of Energy E or E = h The Quantum Concept and Photons

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 25 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The constant (h), which has a value of 10 –34 J·s (J is the joule, the SI unit of energy), is called Planck’s constant. The Quantization of Energy The Quantum Concept and Photons E or E = h

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 26 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Albert Einstein used Planck’s quantum theory to explain the photoelectric effect. The Photoelectric Effect In the photoelectric effect, electrons are ejected when light shines on a metal. The Quantum Concept and Photons

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 27 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Not just any frequency of light will cause the photoelectric effect. The Photoelectric Effect Red light will not cause potassium to eject electrons, no matter how intense the light. Yet a very weak yellow light shining on potassium begins the effect. The Quantum Concept and Photons

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 28 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The photoelectric effect could not be explained by classical physics. Classical physics correctly described light as a form of energy. But, it assumed that under weak light of any wavelength, an electron in a metal should eventually collect enough energy to be ejected. The Photoelectric Effect The Quantum Concept and Photons

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 29 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. To explain the photoelectric effect, Einstein proposed that light could be described as quanta of energy that behave as if they were particles. The Photoelectric Effect The Quantum Concept and Photons

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 30 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The Quantum Concept and Photons The Photoelectric Effect These light quanta are called photons. Einstein’s theory that light behaves as a stream of particles explains the photoelectric effect and many other observations. Light behaves as waves in other situations; we must consider that light possesses both wave-like and particle-like properties.

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 31 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The Quantum Concept and Photons No electrons are ejected because the frequency of the light is below the threshold frequency. If the light is at or above the threshold frequency, electrons are ejected. If the frequency is increased, the ejected electrons will travel faster. The Photoelectric Effect

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 32 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculating the Energy of a Photon What is the energy of a photon of microwave radiation with a frequency of 3.20 × /s? Sample Problem 5.3

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 33 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 5.3 Use the equation E = _____ to calculate the energy of the photon. KNOWNS frequency ( ) = 3.20 × /s h = × 10 –34 J·s UNKNOWN energy (E) = ? J Analyze List the knowns and the unknown. 1 h

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 34 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Substitute the known values for and h into the equation and solve. E = h = (6.626 10 –34 J·s) (3.20 /s) = 2.12 10 –22 J Sample Problem 5.3 Calculate Solve for the unknown. 2

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 35 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 5.3 Individual photons have very small energies, so the answer seems reasonable. Evaluate Does the result make sense? 3

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 36 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What is the frequency of a photon whose energy is 10 –17 J? E = ______ = _____ = ________________________________

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 37 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What is the frequency of a photon whose energy is 10 –17 J? E = h == h E = = = Hz E 10 –34 J h 10 –17 J·s

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 38 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. An Explanation of Atomic Spectra How are the frequencies of light emitted by an atom related to changes of electron energies?

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 39 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. An Explanation of Atomic Spectra When an electron has its lowest possible energy, the atom is in its ground state. In the ground state, the principal quantum number (n) is 1. Excitation of the electron by absorbing energy raises the atom to an excited state with n = 2, 3, 4, 5, or 6, and so forth. A quantum of energy in the form of light is emitted when the electron drops back to a lower energy level.

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 40 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. An Explanation of Atomic Spectra The light emitted by an electron moving from a higher to a lower energy level has a frequency directly proportional to the energy change of the electron.

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 41 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. An Explanation of Atomic Spectra The three groups of lines in the hydrogen spectrum correspond to the transition of electrons from higher energy levels to lower energy levels.

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 42 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The glass tubes in lighted signs contain helium, neon, argon, krypton, or xenon gas, or a mixture of these gases. Why do the colors of the light depend on the gases that are used? Each different gas has its own characteristic emission spectrum, creating different colors of light when excited electrons return to the ground state. CHEMISTRY & YOU

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 43 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. In the hydrogen spectrum, which of the following transitions produces a spectral line of the greatest energy? A. n = 2 to n = 1 B. n = 3 to n = 2 C. n = 4 to n = 3

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 44 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Quantum Mechanics How does quantum mechanics differ from classical mechanics?

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 45 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Given that light behaves as waves and particles, can particles of matter behave as waves? Louis de Broglie referred to the wavelike behavior of particles as matter waves. His reasoning led him to a mathematical expression for the wavelength of a moving particle. Quantum Mechanics

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 46 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The Wavelike Nature of Matter Today, the wavelike properties of beams of electrons are useful in viewing objects that cannot be viewed with an optical microscope. The electrons in an electron microscope have much smaller wavelengths than visible light. These smaller wavelengths allow a much clearer enlarged image of a very small object, such as this pollen grain, than is possible with an ordinary microscope. Quantum Mechanics

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 47 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Classical mechanics adequately describes the motions of bodies much larger than atoms, while quantum mechanics describes the motions of subatomic particles and atoms as waves. Quantum Mechanics

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 48 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The Heisenberg Uncertainty Principle The Heisenberg uncertainty principle states that it is impossible to know both the velocity and the position of a particle at the same time. This limitation is critical when dealing with small particles such as electrons. But it does not matter for ordinary-sized objects such as cars or airplanes. Quantum Mechanics

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 49 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. To locate an electron, you might strike it with a photon. The electron has such a small mass that striking it with a photon affects its motion in a way that cannot be predicted accurately. The very act of measuring the position of the electron changes its velocity, making its velocity uncertain. Quantum Mechanics Before collision: A photon strikes an electron during an attempt to observe the electron’s position. After collision: The impact changes the electron’s velocity, making it uncertain.

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 50 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The Heisenberg uncertainty principle states that it is impossible to simultaneously know which two attributes of a particle? __________________________ velocity and position

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 51 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. When atoms absorb energy, their electrons move to higher energy levels. These electrons lose energy by emitting light when they return to lower energy levels. To explain the photoelectric effect, Einstein proposed that light could be described as quanta of energy that behave as if they were particles. The light emitted by an electron moving from a higher to a lower energy level has a frequency directly proportional to the energy change of the electron. Key Concepts and Key Equations

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 52 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Classical mechanics adequately describes the motions of bodies much larger than atoms, while quantum mechanics describes the motions of subatomic particles and atoms as waves. C = E = h Key Concepts and Key Equations

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 53 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms amplitude: the height of a wave’s crest wavelength: the distance between adjacent crests of a wave frequency: the number of wave cycles that pass a given point per unit of time; frequency and wavelength are inversely proportional to each other hertz: the unit of frequency, equal to one cycle per second

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 54 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. electromagnetic radiation: energy waves that travel in a vacuum at a speed of 10 8 m/s; includes radio waves, microwaves, infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays spectrum: wavelengths of visible light that are separated when a beam of light passes through a prism; range of wavelengths of electromagnetic radiation Glossary Terms

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 55 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. atomic emission spectrum: the pattern formed when light passes through a prism or diffraction grating to separate it into the different frequencies of light it contains Planck’s constant: the constant (h) by which the amount of radiant energy (E) is proportional to the frequency of the radiation ( ) photoelectric effect: the phenomenon in which electrons are ejected when light shines on a metal Glossary Terms

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 56 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. photon: a quantum of light; a discrete bundle of electromagnetic energy that interacts with matter similarly to particles ground state: the lowest possible energy of an atom described by quantum mechanics Heisenberg uncertainty principle: it is impossible to know both the velocity and the position of a particle at the same time Glossary Terms

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 57 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrons and the Structure of Atoms BIG IDEA Electrons can absorb energy to move from one energy level to a higher energy level. When an electron moves from a higher energy level back down to a lower energy level, light is emitted.

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5.3 Atomic Emission Spectra and the Quantum Mechanical Model 58 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. END OF 5.3

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