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Ask Dr. Knowledge

What is strontium and its connection to TV?

May 24, 2010

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Channel 38 refers to strontium (element 38) in its commercials and says it used to be used in TV tubes. What’s this all about?

The chemical elements are numbered according to how many protons their atoms have in their nuclei. This is also the number of electrons they have going around those nuclei when the element is electrically neutral: one electron for each proton.

Element 1 is hydrogen, with one proton in the nucleus and one electron. Element 2 is helium, with two protons in the nucleus. If you keep adding protons, eventually you get to element 38, strontium. Its name comes from a Scottish village called Strontian, because the element was discovered in lead ores from mines there, around 1800.

As a pure element, it oxidizes (combines with oxygen) rapidly, taking on a yellowish color. Finely powdered strontium ignites spontaneously if you toss it into the air. The heat released is enough to set fire to the little grains of metal.

Strontium is not actually volatile (as the TV commercial claims). Volatile means that something evaporates easily, which strontium does not. Its salts do, however, impart spectacular red color to flames, making it useful for fireworks.

Now for television: Strontium is an important constituent of the glass used in older TVs with picture tubes. As electrons sweep across the screen to make an image, they produce X-rays. Glass with about 8.5 percent strontium oxide (and various other things) is quite effective at protecting viewers from getting a dose of radiation. Mixing lead into glass would work, too, but glass that includes lead tends to turn brown when it absorbs X-rays, so the cheaper lead-glass option is useful only for parts of the tube other than the face.

Before flat screens, 70 percent of all strontium in the United States went into television picture tubes.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.