Chapter 5 Electrons In Atoms

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1 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 1 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. CHEMISTRY & YOU 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. 2 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

2 Light and Atomic Emission Spectra Light and Atomic Emission Spectra What causes atomic emission spectra? 3 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. The amplitude of a wave is the wave s height from zero to the crest. The wavelength, represented by λ (the Greek letter lambda), is the distance between the crests. Light and Atomic Emission Spectra 4 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

3 The Nature of Light Light and Atomic Emission Spectra The frequency, represented by ν (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). 5 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. The Nature of Light Light and Atomic Emission Spectra The product of frequency and wavelength equals a constant (c), the speed of light. c = λν 6 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

4 Light and Atomic Emission Spectra The frequency (ν) and wavelength (λ) of light are inversely proportional to each other. As the wavelength increases, the frequency decreases. 7 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Light and Atomic Emission Spectra The Nature of Light 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 m/s. 8 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

5 The Nature of Light Light and Atomic Emission Spectra The sun and incandescent light bulbs emit white light, which consists of light with a continuous range of wavelengths and frequencies. 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. 9 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Light and Atomic Emission Spectra The electromagnetic spectrum consists of radiation over a broad range of wavelengths. Low energy (λ = 700 nm) High energy (λ = 380 nm) Frequency ν (s -1 ) 3 x x x Wavelength λ (m) 10 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

6 Light and Atomic Emission Spectra Atomic Emission Spectra 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. 11 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Light and Atomic Emission Spectra Atomic Emission Spectra A prism separates light into the colors it contains. White light produces a rainbow of colors. Screen Light bulb Slit Prism 12 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

7 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 Hz. 13 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem Analyze List the knowns and the unknown. Use the equation c = λν to solve for the unknown wavelength. KNOWNS frequency (ν) = /s c = m/s UNKNOWN wavelength (λ) =? m 14 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

8 Sample Problem Calculate Solve for the unknown. Write the expression that relates the frequency and wavelength of light. c = λν 15 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem Calculate Solve for the unknown. Rearrange the equation to solve for λ. c = λν λ = ν c Solve for λ by dividing both sides by ν: c ν = λν ν 16 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

9 Sample Problem Calculate Solve for the unknown. Substitute the known values for ν and c into the equation and solve. c λ = = 8 m/s = m ν /s 17 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? 18 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

10 What is the frequency of a red laser that has a wavelength of 676 nm? c = λν ν = c ν = = 8 m/s λ /s = m c λ 19 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. The Quantum Concept and Photons The Quantum Concept and Photons How did Einstein explain the photoelectric effect? 20 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

11 The Quantum Concept and Photons The Quantization of Energy German physicist Max Planck ( ) 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 (ν). E ν or E = hν 21 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. The Quantum Concept and Photons The Quantization of Energy The constant (h), which has a value of J s (J is the joule, the SI unit of energy), is called Planck s constant. E ν E or E ν = or hνe = hν 22 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

12 The Quantum Concept and Photons The Photoelectric Effect Albert Einstein used Planck s quantum theory to explain the photoelectric effect. In the photoelectric effect, electrons are ejected when light shines on a metal. 23 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. The Quantum Concept and Photons The Photoelectric Effect To explain the photoelectric effect, Einstein proposed that light could be described as quanta of energy that behave as if they were particles. 24 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

13 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. 25 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 wavelike and particle-like properties. 26 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

14 The Quantum Concept and Photons The Photoelectric Effect 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. 27 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 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

15 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. 29 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Quantum Mechanics 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. 30 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

16 Quantum Mechanics 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. 31 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Quantum Mechanics 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. 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. 32 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

17 The Heisenberg uncertainty principle states that it is impossible to simultaneously know which two attributes of a particle? 33 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 34 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

18 Key Concepts and Key Equations 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. 35 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts and Key Equations 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 = λν E = h ν 36 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

19 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 37 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms electromagnetic radiation: energy waves that travel in a vacuum at a speed of 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 38 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

20 Glossary Terms 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 (ν) photoelectric effect: the phenomenon in which electrons are ejected when light shines on a metal 39 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms 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 40 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

21 BIG IDEA Electrons and the Structure of Atoms 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. 41 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. END OF Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.