Density Curriculum

Section 3—Lesson 8: How Does Atomic Mass Contribute to Density?

Background Information

Density as Caused Versus Defined

One way to think about the cause of density is that density is a characteristic of stuff—a property of matter. Aristotle talked about "material cause"—in the sense that a "cause" of the existence of a building is its material stuff—that which constitutes it. Density also has "material causes." However, students tend to say that density isn't "caused," it just "is." They see it as a defining property. By taking a look at the causes of differences in density at the micro- and macro-levels, this section offers students information that enables them to resolve some lingering questions about density—for instance, whether there is air or space in between particles. Or how a very small piece of metal can have much more mass than a large piece of another metal.

Students also tend to see density as static. However, there are some respects in which density is dynamic. An understanding of density as dynamic is crucial to understanding all sorts of scientific phenomena such as weather patterns, how a thermometer works, convection currents, and so on. This section attempts to shift students towards a more flexible understanding of density—one that enables them to see instances where density can change. This involves having a greater understanding of why there are differences in density and which of the variables contributing to differences in density can change.

Causes of Differences in Density: Atomic Mass

In order to help students understand density more deeply and flexibly and to offer them the knowledge that they need in order to analyze real-world instances of density, this section addresses three variables that cause differences in density: 1) atomic mass; 2) the strength and structure of atomic and molecular bonds; and 3) mixed density.

This lesson focuses on the first cause: atomic mass. To consider it, we have to zoom in to the micro-level. Some types of atoms have more protons and neutrons than others. This contributes to the mass or weight of the material because protons and neutrons have a lot of mass or weight. (Electrons are very light relative to protons and neutrons, and therefore have little effect on atomic mass.) It also results in more stuff in a given amount of space. Atoms will weigh more or less, depending upon the number of protons and electrons they have. This information can be found on the periodic table.

This lesson asks students to use information from the periodic table to predict differences in density. They will find that atomic mass accounts for many of the differences. However, they will also find some discrepancies. This invites students to begin to discover that they can't directly compare elements based upon the number of protons and neutrons to come up with the density, because density can have multiple contributing causes. (For example, the strength of the atomic or molecular bonds and subsequent crowdedness of the atoms may compensate for the heaviness of individual atoms.) These other causes, and what it means to have multiple contributing causes, are considered in Lesson 11.

Other Causes to Be Explored in Lesson 9 and 10

A second cause of differences in density is the strength and structure of the atomic and molecular bonds. This is most important when thinking about solids and liquids. A third cause of differences in density is mixed density—for instance, when gas molecules are spread out with other molecules in between, or when something is hollow inside and therefore is a combination of air and the material around the outside. These causes are explored in the next few lessons.