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SGCh07

# SGCh07 - Chapter Seven QUANTUM THEORY AND THE ELECTRONIC...

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126 Chapter Seven QUANTUM THEORY AND THE ELECTRONIC STRUCTURE OF ATOMS Electromagnetic Radiation Bohr's Model of the Hydrogen Atom The Dual Nature of the Electron Quantum Mechanics and Quantum Numbers Electron Configurations and the Aufbau Principle ELECTROMAGNETIC RADIATION STUDY OBJECTIVES 1. Use the equations relating wavelength, frequency, and the speed of light. 2. Describe a quantum and be able to calculate the energy of a quantum emitted by an atom. 3. Explain how Einstein accounted for the frequency dependence of the photoelectric effect. Electromagnetic Radiation. To understand some of the experiments that illuminated the behavior of electrons in atoms requires some appreciation of the nature of electromagnetic radiation. Radiation is energy that is transmitted through space in the form of waves. Electromagnetic radiation consists of an electric field component and a magnetic field component that are perpendicular to each other and to their direction of propagation (see Figure 7.3 of the text). The distance between identical points on successive waves, such as the wave crests, is called the wavelength , λ (lambda). All electromagnetic waves travel at the same speed through a vacuum. This speed, known as the speed of light , is 3.00 × 10 8 m/s and has the symbol c . The frequency of a wave is the number of waves that pass a given point per second. Its symbol is ν (nu), and the unit of frequency is cycles per second which is written as /s or s –1 . The SI unit for one cycle per second is called a hertz (Hz). These three quantities, c, λ , and ν , are related by the equation: c = λ ν Figure 7.4 in the textbook shows that the wavelengths of electromagnetic waves have been observed to vary from as short as 5 × 10 –4 nm for some γ rays to more than 100 m for radio waves. The visible spectrum ranges from 400 to 700 nm and is only a tiny fraction of the complete span of electromagnetic waves. According to this equation, the frequency increases as the wavelength decreases. Therefore, radio waves with their long wavelengths have low frequencies, and γ rays with their short wavelengths have extremely high frequencies. The radio waves of an AM station broadcasting at a frequency of 1000 kilohertz (1000 kHz), or 10 6 s –1 , emits electromagnetic waves with a wavelength of 300 m. Quantum Theory. In 1900 Max Planck solved a long-standing riddle when he provided an explanation for the energy radiated by an object at a certain temperature. Heated bodies emit radiation usually in the infrared. However, a body will emit visible radiation if it is hot enough. According to classical physics, an object can acquire or lose any amount of energy no matter how small. But all attempts by scientists to explain the wavelengths of electromagnetic radiation emitted by heated bodies using classical theory have failed. In Planck's quantum theory, however, there is a lower limit to the smallest increment of energy (a quantum) that an atom Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website

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SGCh07 - Chapter Seven QUANTUM THEORY AND THE ELECTRONIC...

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