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Reading Selection, Lesson 21

The Periodic Table

All matter is composed of one or more elements. In lesson 20, you obtained two elements, hydrogen and oxygen, by splitting water, a compound. You have come across other compounds, such as copper sulfate, in this module.

You may know what some other elements look like, such as gold, silver, and aluminum. But could you identify silicon, the second most common element in Earth’s crust? (Oxygen is the most common.) Or calcium, which is found with other elements as a compound in bones and teeth? Did you know that each time you breathe you inhale the elements argon and neon?

If you don’t recognize many elements in your daily life, you’re not alone. It took scientists hundreds of years to identify the more than 100 elements we know today. Why? Most elements are reactive. That is, they tend to combine in chemical reactions with other elements to form compounds. Therefore, they usually exist in chemical compounds rather than by themselves. Water, for example, is produced when hydrogen combines with oxygen. You observed that reaction in Lesson 20.

It can be useful to group large collections of different items. In your kitchen at home, for example, you may have a drawer that contains flatware; the drawer is likely separated into compartments for knives, forks, and spoons. Similarly, scientists classify the elements according to their characteristics. Then they can predict properties of the elements, such as how they will react with other elements and how they can be used. The classification they use is called the Periodic Table. Scientists even use this classification system to predict the existence of elements that have not yet been discovered.

In this lesson, you will try your hand at classifying elements.

Click on each colored element tile of the Periodic Table for more information about that element.

Key for element colors: Green (such as Hydrogen, H)= nonmetals; Blue (such as Calcium, Ca)= metals;
Light blue (such as Carbon, C)= metaloids

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