# Compounds

## Introduces matter made of different elements.

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Compounds

What is this strange-looking object? Can you guess what it is? It’s a model of a certain type of matter. Some types of matter are elements, or pure substances that cannot be broken down into simpler substances. Many other types of matter are compounds. The model above represents a compound. The compound it represents is carbon dioxide, a gas you exhale each time you breathe.

### What Is a Compound?

A compound is a unique substance that forms when two or more elements combine chemically. For example, the compound carbon dioxide forms when one atom of carbon (grey in the model above) combines with two atoms of oxygen (red in the model). Another example of a compound is water. It forms when two hydrogen atoms combine with one oxygen atom. You can learn more about compounds and how they form by watching the video at this URL:

Q: How could a water molecule be represented?

A: It could be represented by a model like the one for carbon dioxide above. You can see a sample Figure below.

### Summary

• A compound is a unique substance that forms when two or more elements combine chemically. A compound always has the same elements in the same proportions.
• The properties of compounds may be very different from the properties of the elements that form them.
• Some compounds form rigid frameworks called crystals. Other compounds form individual molecules. A molecule is the smallest particle of a compound that still has the compound’s properties.

### Vocabulary

• compound: Unique substance that forms when two or more elements combine chemically.
• crystal: Rigid, lattice-like framework of many ions bonded together that is formed by some compounds such as table salt (NaCl).
• molecule: Smallest particle of a compound that still has the compound’s properties.

### Practice

Investigate compounds at the following URL. Be sure to view the rotating 3-D models of molecules and crystals. Then answer the questions below.

1. What are ionic compounds?
2. What types of elements form ionic compounds? Give an example of an ionic compound.
3. What are covalent compounds?
4. What types of elements form covalent compounds? Give an example of a covalent compound.

### Review

1. What are compounds? List three examples.
2. How do the properties of compounds compare with the properties of the elements that form them?
3. Compare and contrast crystals and molecules.

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