Crystal: Definition, Types, Structure & Properties

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  • 0:00 What Are Crystals?
  • 1:13 Types Of Crystals
  • 3:25 Structure Of Crystals
  • 4:37 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

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?

Stunning earrings and diamond rings accent guests at a wedding. 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
crystal structure

As you can see, the atoms are shown in purple and green, arranged in a specific repeating pattern. There are four types of crystals: covalent, ionic, metallic, and molecular. Each type has a different type of connection, or bond, between its atoms. The type of atoms and the arrangement of bonds dictate what type of crystal is formed.

Types of Crystals

Covalent crystals are crystals whose atoms are connected with covalent bonds. Covalent bonds exist where the atoms share electrons. These bonds are extremely strong and very hard to break. Because of this, the crystals themselves are also very strong and have high melting points. Imagine gluing together beads with super glue. The super glue is the covalent bond. Now think of trying to glue beads together with a glue stick. They probably wouldn't hold together very well. The glue stick is analogous to another type of crystal we will talk about later. An example of a covalent crystal is a diamond, which is one of the hardest substances known to man.

Ionic crystals are crystals whose atoms are held together with ionic bonds, or charged bonds. With these ionic bonds, one atom is negatively charged and is attracted to other atoms in the crystal that are positively charged. They are arranged in a pattern based on the charges. These crystals are typically solid with a high melting point. An example of an ionic crystal is table salt.

Metallic crystals are crystals made of metal elements. These crystals sparkle with the lustrous sheen we think of metals having. They are extremely good conductors of heat and electricity. Copper can be extracted from copper crystals to form copper wire used to transmit electricity in our homes. The melting point of these crystals depends on the metal used in the crystal. Gold nuggets are an example of metallic crystals.

Molecular crystals are crystals formed from weak bonds called hydrogen bonds. These have to do with how tiny charged particles on atoms, called electrons, are arranged between different hydrogen atoms. These bonds are very weak and are analogous to the glue stick adhesive we talked about before. Because of this, molecular crystals typically have lower melting points than other crystals. A familiar example is rock candy, the crystalline sugar candy on a stick. Other examples are ice crystals and dry ice, or frozen carbon dioxide. Scientists use molecular crystals to identify the shape of various microscopic proteins inside cells.

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