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Section 6: Light from Elements
Fireworks, which date back more than 1,000 years, are enjoyed by people worldwide to celebrate a new year, an anniversary, or a victory. The Chinese, who invented fireworks in the 7th century CE, knew that the many colors produced by fireworks come from different chemical compounds. It would not be until the second half of the 19th century that scientists would have the tools to understand the elements present in these compounds and have the skills to observe the light that they produced. This combination led scientists to learn more about the structure of the atom for each element in terms of the behavior of its electrons, which are actually responsible for the light the elements are creating.
Figure 3-11. Fireworks Display
Fireworks manufacturers use a range of compounds to take advantage of the striking colors created by the different elements. For example, strontium compounds are used to create the bright red colors in fireworks. Other colors can be created by more common elements so that sodium compounds create the yellow fireworks, and copper compounds make the blue-green ones. These colors are visible evidence of the behavior of electrons in atoms as energy is added into an atom, but this connection took almost two centuries of investigation to uncover.
© Wikimedia Commons, Creative Commons License 3.0. Author: Kurum-Shimin, 5 August 2011.
Figure 3-12. Three Different Types of Spectra
In a continuous spectrum, instead of one or more bright lines, there is a broad range of colors. This is the kind of spectrum produced by a hot, glowing object, such as an incandescent light bulb or a coil on a stove. In a line spectrum or emission spectrum, signature bright lines at certain colors are produced by the different elements. This is the kind of spectrum produced by a flame test or gas discharge tube. In an absorption line spectrum, the light from a continuous source travels through a gas or other transparent material, and the atoms and molecules of that material absorb light at specific wavelengths, producing a dark line, but the remainder of the continuous spectrum passes through. Note that for the same element, the emitted wavelengths of light should match perfectly with the dark spaces, or absorbed wavelengths.
© Science Media Group.
In the 1800s, scientists used increasingly sophisticated spectroscopes to closely examine the pattern of light given off by many different processes. The way this light spectrum looked could be categorized in one of three ways shown in Figure 3-12. In the middle of the 19th century, two German scientists, Gustav Kirchhoff (1824–1887) and Robert Bunsen (1811–1899), collaborated at the University of Heidelberg to study spectroscopy. Kirchhoff and Bunsen realized that when they placed very pure samples of specific elements in a flame, they always saw the same colors and emission spectra through their spectroscope. They categorized the spectra of several elements (sodium, lithium, and potassium) and, expanding their tests further, saw unique, deep blue lines in a sample of mineral water from Dürkheim. After concentrating the dissolved salts from 44 metric tons of this water, they had enough to identify a previously unknown element: cesium. Later, they went on to discover the element rubidium using the same method. In effect, each element has its own "spectral signature," one that is constant, no matter how the light is generated. As discussed in the next section, this is an important clue to the electronic structure of the atom.
Figure 3-13. Robert Bunsen and the Bunsen Burner
Left panel: A Bunsen burner is used to create a bright yellow flame by heating a small quantity of table salt. Right panel: Robert Bunsen (1811–1899) was a German chemist whose name is familiar to legions of chemists and chemistry students through his most famous invention, the Bunsen burner, which allowed a hot, clean-burning flame to be created safely in the laboratory. A pioneer in the field of spectroscopy, his tool enabled the identification of several new elements.
© Left panel: Wikimedia Commons, Creative Commons 3.0. Author: Søren Wedel Nielsen, 13 June 2005; right panel: Wikimedia Commons, Public Domain. Author: C. H. Jeens.
In 1868, there was a full solar eclipse that offered a special opportunity to use spectroscopy to analyze the elements present in solar prominences, the large, red flames erupting from the Sun's surface. Observers noted that the prominences were mostly made of hydrogen, but there was a yellow band of light that did not match up with the spectrum of hydrogen or any other element scientists had seen on Earth. Assuming that there was a yet undiscovered element present on the Sun, they named this element "helium" after Helios, the Sun god of the ancient Greeks. It would be another 27 years before helium was identified on Earth.