Atoms & Molecules Main

Electromagnetic Radiation

Historical Concepts of Atoms

Spectrum Lines and Ionization Energies

Quantum Theory, the Uncertainty Principle, and Pauli's Exclusion Principle

Electron Orbitals, Quantum Numbers, and Orbital Filling

Ionic and Covalent Bonding

Electronegativity, Oxidation Numbers, and Formal Charge

Trends in the Periodic Table

Ionic Solids: Lattice Energy

Lewis Electron Dot Symbols

Covalent Bond Orders and Energies

Molecular Shapes and Orbital Hybridization

Sigma and Pi Bonds

Paramagnetism

Molecular-Orbital Theory

Practice Problems


Electromagnetic Radiation

Electromagnetic energy surrounds us: radio waves, microwaves, infrared radiation, visible light, ultraviolet light, x-rays, and gamma rays are all types of electromagnetic (or "em" for short) energy. This type of radiation allows us to see the world around us, communicate with others, and diagnose injuries; it tans our skin (and sometimes burn us), cooks our food, and lets us see in the dark. It is also of central importance to the concept of the atom, which we will begin discussing in the next page.

As you probably know, light rays travel at the speed of light, 3 x 108 meters per second (symbol: a small, italic "c"). The same goes for all electromagnetic radiation. EM energy also travels in waves, like swells on the ocean. The radiation's wavelength (symbol: the Greek letter lambda, λ) is defined as the distance from the crest of one wave to the crest of the next, and its frequency (symbol: the Greek letter nu, ν) is the number of crests that pass a particular point in one second. The unit of wavelength is distance per cycle (one cycle being one whole unit of the wave: crest to crest, trough to trough, etc.); the unit of frequency is cycles per unit time. Multiplied together, they give distance per unit time, or speed. Therefore a direct relationship exists between a wave's frequency, wavelength, and speed:

λν = c

So, the frequency, usually in cycles per second, or hertz (Hz), multiplied by the wavelength, usually given in meters, gives the speed of the wave, in meters per second. Since the speed of light (and all EM radiation) is fixed at 3.00 x 108 meters per second, knowing the wavelength will give us the frequency, and vice versa.

The energy of a electromagnetic wave is dependent on its frequency and wavelength: the higher the frequency and the shorter the wavelength, the more energy the wave has. Scientists have divided the range of electromagnetic waves into groups, which together make up the electromagnetic spectrum. The groups of waves are detailed below:

  • Radio waves: These waves have the lowest energy of all EM radiation. Wavelengths range from kilometers (about 300,000 Hz) to about .01 meters, or about a centimeter (about 3 x 1010 Hz). Uses for these waves include AM and FM radio, TV transmissions, and radar.
  • Microwaves: These waves have wavelengths from about a centimeter to about 1 millimeter (about 3 x 1011 Hz). Microwaves are used for telecommunications, in cellular phones, and for cooking food (at very specific wavelengths).
  • Infrared waves: Infrared radiation occupies wavelengths from about 1 millimeter to about .7 micrometers (about 4 x 1014 Hz). All hot objects, including people, emit infrared radiation, so infrared cameras can "see in the dark" by tracking heat sources.
  • Visible light: The light we can see ranges from 700 to 350 nanometers (about 1015 hertz), a tiny sliver of the spectrum. Red light has the lowest wavelength, followed by orange, yellow, green, blue, indigo, and violet (composing the familiar acronym ROYGBIV, or "Roy G. Biv").
  • Ultraviolet light: Ultraviolet rays are more energetic than visible light, and ultraviolet radiation from the sun is what tans and burns our skin. The ozone layer protects earth from the more harmful UV rays. Ultraviolet light has wavelengths from 350 nanometers to about 10 nanometers (about 1016 hertz).
  • X-rays: More powerful still than ultraviolet rays are x-rays. They pass directly through soft tissue, being stopped only by the dense bones in our bodies; hence, X-rays are used to examine our skeletons. The wavelengths of X-rays vary from 10 nanometers to 1 nanometer (about 1017 hertz).
  • Gamma rays: The most energetic radiation in the EM spectrum, gamma rays have wavelengths from 1 nanometer to 50 femtometers or even smaller (almost 1022 Hz!). Gamma rays are severely damaging to living matter; luckily, our atmosphere shields us from gamma rays generated by the sun and the even more powerful gamma rays (sometimes called cosmic rays) from galactic and intergalactic sources.

Max Planck, a prominent physicist of the early twentieth century, discovered the relationship between the energy of an electromagnetic wave and its wavelength and frequency:

E = hν = h * c/λ

Where E is energy in joules, h is Planck's constant (6.62 x 10-34 joule-seconds), and ν is the frequency (because of the relation between the speed of light, frequency, and wavelength, you can use the term c/λ instead of ν).

We will return to electromagnetic radiation a little later in this chapter; the next page will discuss the changing view of the atom over the past few hundred years.

| |