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This lesson is on crystals. Here, we’ll review what the four types of crystals are and the different physical structures possible. We will also go over the properties of the different types of crystals and give examples.

What are Crystals?

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On the table, crystal glasses shine as they are filled with champagne. When you think of crystals, all of these beautiful decorations most likely come to mind. But, there is so much more to crystals! Crystals are used for solar-powered devices, transformers that transmit electricity, solving important biological puzzles in medicine, creating monitors for computers and television, and serving as tasty snacks!To figure out how crystals play a role in all these functions, we need to cover some basics. A crystal is any solid material with its atoms, or smallest units of matter, organized in a repeating pattern. Look at this diagram to see an example of how atoms are arranged in a crystal.

Atoms are shown in purple and green, arranged in a specific, repeating pattern
Pyrite (Cube) Crystal
Crystal Cubes

Hexagonal crystals have six sides of even length, and two hexagon shaped ends parallel to each other. Tetragonal crystals look like rectangles. There are also orthorhombic crystals, which are not rectangular, but rather look like two pyramids put together with a cylinder in between.

Trigonal crystals look similar, but the shape is two pyramids attached end to end. Monoclinic crystals form prisms and triclinic crystals do not have a regular geometric shape but make abstract forms.

Lesson Summary

In summary, a crystal is any solid material with its atoms organized in a repeating pattern. There are four types of crystals. Covalent crystals occur when the atoms form strong, covalent bonds. Such crystals, like diamonds, are very tough. Ionic crystals, like table salt, are hard and stable but are made of atoms connected by electrical charges.

Metallic crystals are formed from metal elements and conduct electricity well. Molecular crystals, like rock candy, are formed through hydrogen bonds and are generally weak and have a low melting point. There are seven main structures of crystals, depending on the shape of the crystal. These are cubic, hexagonal, tetragonal, orthorhombic, trigonal, monoclinic, and triclinic.

Crystal Vocabulary & Definitions

Structure of Crystals
  • Crystal: any solid material whose atoms are organized in a repeated pattern of one of four types: covalent, ionic, metallic and molecular.

    • Covalent crystal: the atoms share electrons; diamond
    • Ionic crystal: the atoms are drawn together by opposite charges; table salt
    • Metallic crystals: atoms are metal elements; gold nuggets
    • Molecular crystals: atoms form weak hydrogen bonds; rock candy
  • Crystal Systems: Atoms in a crystal bond into specific shapes and patterns know as systems. The four types of crystals may appear as one of seven structural/system types: cubic, hexagonal, tetragonal, orthorhombic, trigonal, monoclinic and triclinic.

Learning Outcomes

After students have completed this lesson, they should be ready to:

  • Define crystals and crystal systems
  • Describe the four types of crystals, including bond types
  • List examples of the four types of crystals
  • Outline the seven crystal systems

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