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Classification of Elements and Compounds 

Now let's return our focus to compounds and elements and review where they fit into the scheme of things by reconsidering the classification of materials. At least two different classification schemes can be used. (Different formats for these tables are shown in Example 1 in your workbook.) The section of this lesson titled "Classification of Materials" elaborates on the content presented here.

The first one shown here is based on the kinds of observations and separation techniques you worked with earlier in this course. By looking at a material we can decide whether it is heterogeneous or homogeneous. If it is heterogenous, we call the material a "heterogeneous mixture." (Sometimes simply called a "mixture.") If it is homogeneous, we next check to see whether the material can be separated into components using phase change techniques like distillation. If it can be separated in this way, we call the material a "solution." ("Homogeneous mixture" would also be appropriate.) If not, the material is called a "pure substance," and we check further to see whether the material can be decomposed by electrolysis or some other chemical reaction. If so, the material is a "compound," and if not, it is an "element."
Materials
Heterogeneous Homogeneous
Separable by phase changes Inseparable by phase changes
Heterogeneous Mixtures Pure Substances
Solutions (homogeneous mixtures)
Separable by chemical reactions Inseparable by chemical reactions
Compounds Elements

The second classification scheme shown here is based on knowing something about the composition of the material. It is simpler and more common, but it requires a different kind of analysis of the material.  Nevertheless, we end up with the same four categories.
Materials
Mixtures
(variable composition)
Pure Substances
(fixed composition)
Heterogeneous Homogeneous Can be decomposed into two or more elements Cannot be decomposed into two or more elements
Heterogeneous Mixtures Solutions (homogeneous mixtures) Compounds Elements

Using either approach we have the four categories - heterogenous mixture, solution, compound, and element - that we need to explain. Let's focus on the elements and compounds. There are a number of important questions that can be asked about them.

Why is it that elements cannot be decomposed?
Why is it that compounds can be decomposed?
Why do they have a composition that is consistent from one sample to another?

Related to this idea of "breaking things down" is the opposite idea of "building things up." Elements can somehow be put together very securely in ways that make compounds with fixed proportions.

How does that happen?

Elements and compounds both can be put together more loosely in mixtures which have indiscriminate proportions. This lesson will focus on the elements themselves, along with how compounds are put together from them. We will leave the mixtures for later.

The concept or theory that there are elements and that those elements mix and combine with one another to make the materials we observe has been around for a long time. It probably began from a desire to see things more simply or to simplify things that we were able to observe. The elements are something fundamental. At one time people talked about the elements of air, earth, fire, and water. Chemically, the idea focused on elements being those building blocks of all material things, which themselves could not be decomposed. That concept had to change in the 20th century when we started to understand radioactivity and nuclear reactions and realized that those materials (like hydrogen and oxygen) that had been listed as elements were themselves composed of still more fundamental things. Thus, we no longer look at elements as being the ultimate fundamental building blocks. But they are still the materials from which our compounds and mixtures are made. The analogy is still good. We use different kinds of building blocks to make different kinds of things, even though we know that the blocks themselves are made of smaller things.

 

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E-mail instructor: Eden Francis

Clackamas Community College
1998, 2002 Clackamas Community College, Hal Bender