Unit E: Kinds of Matter

What is the most common element in the universe?

1. Get Set to Explore

Vocabulary

  • compound: A substance made up of two or more elements.
  • element: A substance that cannot be broken apart into other substances; there are just over 100 known elements.

Building Background

  • Review the vocabulary words and their definitions with the class. Run a brainstorming session for students to name all the elements they can think of, with you listing the elements they name on the board. Have volunteers name one element at a time. Try to involve all students in adding to the list. If students happen to name a compound rather than an element, use the opportunity to differentiate between elements and compounds, reinforcing the definitions. Keep the list of elements up on the board.
  • Ask students to talk about the universe and the types of objects in it. If necessary, have them look back at Student Edition Chapter 11. Guide students to realize that stars are the most common objects in the universe. There are more than 200 billion stars in an average galaxy like the Milky Way. Scientists estimate that there are about 125 billion galaxies.
  • Challenge students to recall the main elements that form stars. Write down their answers on the board. Suggest students answer without looking at their textbook.
  • Ask the Discover! question. Referring to the list of elements on the board, let volunteers name the one they think is the most common in the universe and explain why. Let the class vote, tallying their votes on the board. Keep the tally to refer to later.

2. Guide the Exploration

  • Let students launch the Discover! Simulation. They should click each of the five objects shown to learn the main elements found in each. The percentages are figured by using the total number of atoms, not by using mass or volume.
  • You may wish to explain that the set of lines shown for each star is what scientists use to identify the elements in a star. Stars are made of very hot gases. Any gaseous element can be identified by heating the gas and passing the light through a spectroscope. The spectroscope resolves the light into a unique set of lines. Scientists compare the lines that result when starlight passes through a spectroscope with those given off by different elements to deduce the elements present in stars.
  • Students should spend some time comparing the different objects and hypothesizing which element is most common. Before students go on to Step 3, have them share their hypotheses and reasons for them. Let the class vote again on the most common element in the universe. Then, tell students to complete the simulation.

3. Review/Assess

  • Discuss Step 3's Wrap-Up text with the class. Point out that since stars and galaxies are the most common and most massive items in the universe, hydrogen—which composes 90% or more of most stars and galaxies—is the most common element.
  • Have students return to the simulation to work on the Extension questions. Assign them to work in small groups and help them understand the following answers:
    • Helium is the second most common element in the universe, because it makes up 10% of galaxies and most stars.
    • The composition of Earth differs as Earth contains little helium.

If time permits, present students with the following questions:

  • Critical Thinking Synthesize The simulation explains that since the densest part of Earth includes the crust, mantle, and core, and these are composed mainly of iron, oxygen, and silicon, these three elements are the most common on Earth. Why is density important to determine the most common elements on Earth? Answer: Density is a ratio of mass to volume. The higher the density, the greater the amount of mass per unit volume. A greater mass, in this case, implies a larger number of atoms. The larger the number of atoms, the more common the element.
  • Inquiry Skill Compare Review Student Edition pages E16–E17—the periodic table. How does the atomic number of the most common elements in the universe compare with those of less common elements? Give a hypothesis about why this might be so? Answer: Hydrogen and helium are the most common elements and they are the lightest elements. One hypothesis might be that the other elements form only from lighter atoms fusing together; the conditions under which this occurs may not be common.

4. Reaching All Learners

On Level: Logical/Visual Learners

Encourage students to make circle graphs to compare the composition of the five different space objects shown in the simulation. Let students use their circle graphs to help others better understand the composition of the different space objects.