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Physical Properties of Ionic Compounds

Characteristics shared by ionic compounds

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Sapphire Cell Phone Screens?

Sapphire Cell Phone Screens?

Credit: gemteck1
Source: http://www.flickr.com/photos/jacbt/374123816/
License: CC BY-NC 3.0

For most of us, the word “sapphire” conjures up an image of the deep blue gemstone shown above. Seems like a poor choice for a cell phone screen, doesn’t it? First of all, it’s blue. Second, naturally occurring sapphires are extremely expensive. On the other hand, sapphires are among the hardest naturally occurring minerals, second only to diamond. Such a material would be very resistant to scratching, so if a colorless, inexpensive type of sapphire were available, it might make a very good screen for your phone.

News You Can Use

  • Sapphires are composed almost entirely of the colorless ionic compound aluminum oxide (Al2O3), which is sometimes referred to by the older name corundum. The blue color occurs only when some of the aluminum cations are replaced by Fe2+ and Ti4+ ions. Only about 0.01% of these cationic impurities need to be present before the blue color becomes visible.
  • Different transition metal cations in different ratios produce sapphires of other colors. One of these has a special name; if a sufficient amount of the aluminum cations are replaced with Cr3+, the gem has a deep red color and is referred to as a ruby instead of a sapphire.
  • Although it is more expensive than glass, synthetic aluminum oxide is not nearly as pricey as naturally occurring gemstones.
  • Broken smartphone screens are very expensive to replace, which is why demand for shatter proof sapphire screens is rising. 
  • Credit: Incase
    Source: http://www.flickr.com/photos/goincase/4625597566/
    License: CC BY-NC 3.0

    Many people choose not to replace the screens due to the high cost [Figure2]


  • Learn more about the possibility of sapphire screens by watching the following video: 


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With the links below, learn more about gemstones, minerals, and transition metal ions. Then answer the following questions.

  1. Would you be able to scratch a sapphire screen with a metal coin? What about a piece of broken glass or a kitchen knife?
  2. If they are so hard, how can large sapphire crystals be cut into desired shapes?
  3. Sapphires are blue because they absorb light in the orange-yellow range, leaving behind light that is perceived as the complementary color. This absorption is accompanied by an electron transfer between Fe2+ and Ti4+ ions that are close to one another. Which ion do you think donates an electron, and which gains an electron? Why? (Hint: Think about electron configurations, not just overall charge.)
  4. Pure metals tend to be much softer (easier to dent) than ionic compounds like aluminum oxide. Explain this phenomenon based on the types of bonds holding these substances together.

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Image Attributions

  1. [1]^ Credit: gemteck1; Source: http://www.flickr.com/photos/jacbt/374123816/; License: CC BY-NC 3.0
  2. [2]^ Credit: Incase; Source: http://www.flickr.com/photos/goincase/4625597566/; License: CC BY-NC 3.0

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