# D2.1 Electromagnetic Radiation

The periodic table summarizes much information about chemical elements. That information can be better understood by assuming that physical and chemical properties of elements depend on the underlying structures of their atoms. An important way to learn about the structures of atoms is to study how energy, in the form of electromagnetic radiation, interacts with matter.

Activity: Preparation—Atomic Spectra and Atomic Structure

In your course notebook, make a heading for Atomic Spectra and Atomic Structure. After the heading write down what you remember about atomic spectra from courses you have already taken. Also write what you recall about the relation of spectra to atomic structure—how electrons are arranged in atoms. If there is anything you remember being puzzled about, write that down as well. We will ask you to refer back to what you have written when you complete this section.

Electromagnetic radiation consists of oscillating, perpendicular electric and magnetic fields that travel through space and can transfer energy. The oscillating fields (waves) are characterized by wavelength (λ, measured in meters, m) and frequency (ν, measured in hertz, Hz or s−1). In a vacuum, electromagnetic radiation travels at the speed of light (c):

λν = c = 2.998 × 108 m/s

Electromagnetic radiation occurs in small, indivisible quantities of energy called photons. The energy of a photon, Ephoton, can be determined from either its frequency or its wavelength:

In this equation h represents Planck’s constant; h = 6.626 × 10−34 J s.

Exercise: Photon Energy from Wavelength

Your calculation in the above Exercise showed that the energy of a single photon is quite small. Most interactions of electromagnetic radiation and matter involve lots of photons and lots of atoms. The total energy transferred is proportional to the number of photons, N. If all photons have the same frequency:

Notice that electromagnetic radiation has been described as involving wave motion and also as a number of particles (photons). Originally, scientists thought that electromagnetic radiation could be described entirely by a wave model, but that model was unable to predict all experimental observations. Consequently, both wave and particle models need to be combined for full understanding of electromagnetic radiation.

The figure below shows the enormous range of all types of electromagnetic radiation: Frequencies of 105 Hz to 1020 Hz, that is, wavelengths of 103 m (km) to 10−12 m (pm) have been observed. What we can see, visible light, is only a tiny portion (380-740 nm) of that range.

Different parts of the electromagnetic spectrum typically use different units: Low-energy photons, such as microwaves and radio waves, are specified in frequencies (MHz or GHz); mid-energy photons, such as infrared and visible light, are specified in wavelengths (μm, nm, pm, or Å); high-energy photons, such as x-rays and gamma-rays, are specified in energies (keV or MeV; 1 eV = 96.5 kJ/mol).  As the equation above shows, these units can readily be converted to one another.

Our eyes detect visible-range photons, allowing us to see the world around us. But scientific instruments allow us to “see” lots more by detecting photons over a much wider range of energies. For example, studies of atomic spectra via experiments involving interaction of gaseous matter with visible, ultraviolet, and infrared photons, led to a better understanding of the structure of atoms.

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## License

Chemistry 109 Fall 2021 Copyright © by John Moore; Jia Zhou; and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.