Crystals

What are Crystals? When you think of crystals, you may think of pretty gems that are used in jewelry, like diamonds (April birthstone), amethysts (February birthstone), and peridot (August birthstone). Some different types of crystals are shown below. Crystals like the ones shown here are defined as solid substances whose molecules are arranged in a repeating pattern.

Other examples of crystals include table salt (NaCl), sugar (C₁₂H₂₂O₁₁), and ice (H₂O in its solid form).  With the exception of crystal water (ice), all the examples of crystals here (salt, sugar, diamonds, amethysts, peridot, and all the crystals shown above) are solids at room temperature.

Table salt is one type of crystal. Crystals are defined as solid substances whose molecules are arranged in a repeating pattern. It is this repeated molecule arrangement that gives crystals qualities like shininess and colors that makes them look pretty to us.

• For example, table salt is one type of crystal because it is made of a repeating pattern of NaCl molecules. One NaCl molecule is shown below:

• The molecules are arranged like in the picture below on the right. As you can see, the repeated pattern of salt crystals is very simple.

• If you put a string in saturated salt water, NaCl molecules would start building up on the string to form a pattern of repeating NaCl molecules. The NaCl molecules are attracted to the molecules that make up the string due to electric forces. Negatively charged areas of the string attract positively charged Na+ atoms (or the Na+ side of NaCl molecules). Positively charged areas of the string attract negatively charged Cl- atoms (or the Cl- side of NaCl molecules).

Ice (the solid form of water, or H₂O) is another example of a crystal. This means that (Select one) ice has mystical properties. ice can be used as jewelry. water molecules form a repeating pattern in ice. all of the above are true.

A water molecule is shown below.

In ice, H₂O molecules are arranged in repeating patterns. There are many types of patterns that H₂O molecules in ice can form. Two possible patterns are shown below. The pictures below are "ball and stick" representations of ice crystals: regular patterns of H₂O molecules. Oxygen atoms are shown as darker blue balls, hydrogen atoms are shown as lighter blue balls. The bonds between atoms are black lines (solid to the left and dashed to the right). (Remember that bonds are not physical "things", like atoms, and just represent attractive forces between certain atoms.)

In both patterns, notice that the hydrogen atoms (smaller) in general are closer to oxygen atoms (larger) than other hydrogen atoms. And the oxygen atoms are closer to hydrogen than other oxygen atoms. (Some oxygen atoms look like they are closest to other oxygen atoms, but that's because it's hard to show 3 dimensions in a 2 dimensional picture, and the hydrogen atoms may be hidden by oxygen atoms that are in front of them.)

• Some kids mistakenly think of "crystals" as ice. But ice (frozen water) is just one type of crystal. Table salt (NaCl) and sugars are also types of crystals. But salt and sugar (and many other crystals) are not frozen or ice! These crystals exist at room temperature. If you touch salt or sugar, it will not feel cold.
  
  Crystals are any type of substance that has repeating molecule patterns.

• Also, some kids wrongly think that crystals are alive (because they are said to "grow"). They're not alive or living! Crystals are minerals, like rocks. And (of course), rocks are not alive or living. They are not made of cells or have DNA, which living things have. When people say that crystals "grow," what they really mean is that molecules build on other molecules to make larger crystals (like Legos).