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Look at all the colors you can make by mixing together just a few colors of paint. In the photo above, the rainbow of colors on the brush formed from just four paint colors: green, yellow, red, and blue. The same thing is true of matter in general. By combining just a few different elements, you can form many different chemical compounds.

What Are Compounds?

A compound is a unique substance that forms when two or more elements combine chemically. Compounds form as a result of chemical reactions. The elements in compounds are held together by chemical bonds. A chemical bond is a force of attraction between atoms or ions that share or transfer valence electrons. For a video introduction to compounds, go to this URL: http://www.youtube.com/watch?v=-HjMoTthEZ0 .

Water is an example of a common chemical compound. As you can see in the Figure below , each water molecule consists of two atoms of hydrogen and one atom of oxygen. Water always has this 2:1 ratio of hydrogen to oxygen. Like water, all compounds consist of a fixed ratio of elements. It doesn’t matter how much or how little of a compound there is. It always has the same composition.

All water molecules have two hydrogen atoms (gray) and one oxygen atom (blue).

Q: Sometimes the same elements combine in different ratios. How can this happen if a compound always consists of the same elements in the same ratio?

A: If the same elements combine in different ratios, they form different compounds.

Same Elements, Different Compounds

Look at the Figure below of water (H 2 O) and hydrogen peroxide (H 2 O 2 ), and read about these two compounds. Both compounds consist of hydrogen and oxygen, but they have different ratios of the two elements. As a result, water and hydrogen peroxide are different compounds with different properties. If you’ve ever used hydrogen peroxide to disinfect a cut, then you know that it is very different from water!

Q: Read the Figure below about carbon dioxide (CO 2 ) and carbon monoxide (CO). Both compounds consist of carbon and oxygen, but in different ratios. How can you tell that carbon dioxide and carbon monoxide are different compounds?

A: You can tell that they are different compounds from their very different properties. Carbon dioxide is a harmless gas that living things add to the atmosphere during respiration. Carbon monoxide is a deadly gas that can quickly kill people if it becomes too concentrated in the air.

Types of Compounds

There are two basic types of compounds that differ in the nature of the bonds that hold their atoms or ions together. They are covalent and ionic compounds. Both types are described below. You can learn more about the properties of covalent and ionic compounds at this URL:

http://www.youtube.com/watch?v=bQ6_8iMd8YM

  • Covalent compounds consist of atoms that are held together by covalent bonds. These bonds form between nonmetals that share valence electrons. Covalent compounds exist as individual molecules. Water is an example of a covalent compound.
  • Ionic compounds consist of ions that are held together by ionic bonds. These bonds form when metals transfer electrons to nonmetals. Ionic compounds exist as a matrix of many ions, called a crystal. Sodium chloride (table salt) is an example of an ionic compound.

Summary

  • A compound is a unique substance that forms when two or more elements combine chemically.
  • A compound always consists of the same elements in the same ratio. If the same elements combine in different ratios, they form different compounds.
  • Types of compounds include covalent and ionic compounds. They differ in the nature of the bonds that hold their atoms or ions together.

Vocabulary

  • compound : Unique substance that forms when two or more elements combine chemically.

Practice

Watch the video about compounds at the following URL, and then answer the questions below. http://www.youtube.com/watch?v=LRVW0tgSLRI

  1. What force holds together atoms in compounds?
  2. Name a gas that consists of two atoms of the same element bonded together. Do the molecules of this gas represent a compound? Why or why not?
  3. Identify at least one property of water that differs from the properties of the elements that form it.
  4. Which two elements make up the compound named butane? What is the ratio of these two elements in butane? How would you use chemical symbols to represent butane?

Review

  1. What is a compound?
  2. A mixture is a combination of two or more substances in any proportions. An example of a mixture is lemonade, which contains water, lemon juice, and sugar. How do compounds differ from mixtures such as lemonade?
  3. Compare and contrast ionic and covalent.

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Difficulty Level:

At Grade

Grades:

7 , 8

Date Created:

Oct 31, 2012

Last Modified:

Oct 27, 2014
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