Density Curriculum

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

Lesson Plan

Materials

Prep Step

Analyze Thinking

Step 1: Reflecting on What We Think Causes Differences in Density at the Microscopic Level

Explain to your students that in this lesson we will zoom in to consider the causes of density at a more microscopic level. Ask, "What do you think causes differences in density at the microscopic level?" Encourage students to put forth their theories about how come different materials are denser than others. Collect as many possibilities as students can think of.

Students may mention atoms and molecules. If they don't talk about particles of any kind, introduce the topic by asking, "Do you think the particles that make up matter are part of the explanation for why one substance is denser than another?" Guide them to the idea of particles (or molecules or atoms) and spaces.

Say, "Scientists refer to at least three causes of density. We'll talk a lot about one of them and then we'll come back to the other two in the next two lessons."

Explore Causality

Step 2: Introducing Atomic Mass

Explain that in order to think about the first cause, we will need to "zoom in." Tell the students that they need to zoom in just like they are looking through a very powerful microscope.

Put up the Cause #1 poster. Ask the students to consider the pictures showing models of two different atoms. Remind the students what atoms are. Then ask them to look at the differences between the atoms. What do they notice? They should notice that one has more protons and neutrons than the other, and that there is more stuff in a given amount of space.

One of the causes of density has to do with how many protons and neutrons the atoms that make up the material have. Protons and neutrons are big and heavy. The atoms might also have different numbers of electrons, but these don't really make a difference because they are so light. There could be huge differences in the numbers of electrons between one type of atom and another, but it wouldn't really matter to the density.

Step 3: Can an "Air Molecule" fit inside an Atom?

Draw the students' attention to the spaces inside the atoms. An atom is mostly empty space. Most of the mass is packed into the center part which is called the nucleus. Say, "If another student said that there was lots of air in these spaces, what would you say? Would you agree or disagree?" Gather the students' ideas and supporting reasoning for why they would agree or not.

Show the Illustration of Aluminum Atom and Some Molecules that Make up Air. Explain to the students how the molecules that make up air could not possibly fit inside an atom. (Even if the gas that makes up the air was made up only of single atoms, the single atom still wouldn't fit inside another atom.) Besides the issue of fit in terms of size, there are other chemical and electrical reasons why an atom doesn't "go inside" another atom.

Remind the students that we sometimes say "Air Molecules," but what we mean are the molecules that make up the air (such as nitrogen, carbon dioxide, oxygen, etc.) There really is no such thing as an actual air molecule.

Help the students come to the realization that air is like any other substance: it is matter and can be part of an object or substance, but it is no different from any other matter when it comes to determining density. Make sure that by the end of the discussion the students understand that the empty space in the atom IS NOT air. It is empty space. In fact, there is even space within the atoms that make up the air that cannot be filled with air.

Step 4: The Role of Atomic Mass in Density

Pass out and read the Thinking about Atomic Mass and Density sheet together. Discuss it. It introduces the concept of atomic mass and helps students to see why some atoms have more mass than others.

Step 5: Exploring the Implications of Atomic Mass

Pass out the sheet, The Role of Atomic Mass in Density. If mass of atoms were the only cause of density, which of each pair would you expect to be denser? Have the students go through the list and check the atomic mass of each type of atom. Have them infer from this which materials are likely to be denser.

Next, hand out Densities of Common Substances Under Standard Conditions Table and check each element on the worksheet against the density chart. Ask your students, "Were you able to make accurate predictions using atomic mass? Why or why not?"

Review, Extend, Apply

Step 6: Introducing a Puzzler

Pose the following puzzler about platinum and gold. "How can it be that Platinum has a density of 21.4 g/cm3 and Gold has a density of 19.3 g/cm3 if Platinum has a Mass Number (or Atomic Mass) of 195 and Gold has a Mass Number (or Atomic Mass) of 197?"

Ask students to draw some models and give a written response.

Discuss the puzzler together. Ask students if they think that atomic mass is still a cause of density, as discussed in the lesson, or if they think that the Cause #1 poster is wrong. Is there any way that it can be true and platinum still be denser than gold? Ask the students what they think about it now. Collect some of their ideas. Have the students think about it during the course of the next lesson, and explain that you will come back to this question again later.