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Chapter 7: Atomic Structure and Periodicity: 7.1 Electromagnetic Radiation Book Title: Chemistry Printed By: Abdulrahman Abonayan ([email protected]) © 2018 Cengage Learning, Cengage Learning
7.1 Electromagnetic Radiation One of the ways that energy travels through space is by electromagnetic radiation (radiant energy that exhibits wavelike behavior and travels through space at the speed of light in a vacuum.) . The light from the sun, the energy used to cook food in a microwave oven, the X rays used by dentists, and the radiant heat from a fireplace are all examples of electromagnetic radiation. Although these forms of radiant energy seem quite different, they all exhibit the same type of wavelike behavior and travel at the speed of light in a vacuum.
Waves have three primary characteristics: wavelength, frequency, and speed. Wavelength (the distance between two consecutive peaks or troughs in a wave.) (symbolized by the lowercase Greek letter lambda, ) is the distance between two consecutive peaks or troughs in a wave (Fig. 7.1). The frequency (the number of waves (cycles) per second that pass a given point in space.) (symbolized by the lowercase Greek letter nu, ) is defined as the number of waves (cycles) per second that pass a given point in space. Since all types of electromagnetic radiation travel at the speed of light, short-wavelength radiation must have a high frequency. You can see this in Fig. 7.1, where three waves are shown traveling between two points at constant speed. Note that the wave with the shortest wavelength has the highest frequency and the wave with the longest wavelength has the lowest frequency. This implies an inverse relationship between wavelength and frequency, that is,
, or
where is the wavelength in meters, is the frequency in cycles per second, and is the speed of light ( m/s). In the SI system, cycles are understood, and the unit per second becomes , or , which is called the hertz (abbreviated Hz).
Figure 7.1
The nature of waves. Many of the properties of ocean waves are the same as those of light waves. Note that the radiation with the shortest wavelength has the highest frequency.
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Electromagnetic radiation is classified as shown in Fig. 7.2. Radiation provides an important means of energy transfer. For example, the energy from the sun reaches the earth mainly in the form of visible and ultraviolet radiation, whereas the glowing coals of a fireplace transmit heat energy by infrared radiation. In a microwave oven, the water molecules in food absorb microwave radiation, which increases their motions. This energy is then transferred to other types of molecules via collisions, causing an increase in the food’s temperature. As we proceed in the study of chemistry, we will consider many of the classes of electromagnetic radiation and the ways in which they affect matter.
Figure 7.2
Classification of electromagnetic radiation.
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Interactive Example 7.1
Frequency of Electromagnetic Radiation
The brilliant red colors seen in fireworks are due to the emission of light with wavelengths around nm when strontium salts such as and are heated. (This can be easily demonstrated in the lab by dissolving one of these salts in methanol that contains a little water and igniting the mixture in an evaporating dish.) Calculate the frequency of red light of wavelength nm.
Solution
We can convert wavelength to frequency using the equation
or
where m/s. In this case nm. Changing the wavelength to meters, we have
and
See Exercises 7.45 and 7.46
Chapter 7: Atomic Structure and Periodicity: 7.1 Electromagnetic Radiation Book Title: Chemistry Printed By: Abdulrahman Abonayan ([email protected]) © 2018 Cengage Learning, Cengage Learning
© 2018 Cengage Learning Inc. All rights reserved. No part of this work may by reproduced or used in any form or by any means - graphic, electronic, or mechanical, or in any other manner - without the written permission of the copyright holder.
