Elements are pure substances with unique properties. The smallest particles of elements are atoms. Compounds are unique substances that form when two or more elements combine chemically. The smallest particles of compounds are molecules, but some compounds form crystals instead.
- SCI.CA.8.PS.3.b, c
- MCR.6-8.SCI.8.1, 2, 4, 7, 8
- NSES.5-8.B.1.1, 2, 3
- AAAS.6-8.1.A.4; AAAS.6-8.1.C.1; AAAS.6-8.4.D.1, 2, 3, 8, 17, 19; AAAS.6-8.10.F.1, 2
- Describe elements and atoms.
- Describe compounds, molecules, and crystals.
- Define mixture, and identify types of mixtures.
atom: smallest particle of an element that still has the element’s properties
colloid: homogeneous mixture in which the particles are large enough to reflect light and be seen but too small to settle or filter out of the mixture
compound: unique substance that forms when two or more elements combine chemically
crystal: rigid, lattice-like framework of many atoms bonded together that is formed by some compounds such as table salt
element: pure substance that cannot be separated into any other substances
mixture: combination of two or more substances in any proportions
molecule: smallest particle of a compound that still has the compound’s properties
solution: homogeneous mixture in which particles are too small to reflect light and be seen and also too small to settle or be filtered out of the mixture
suspension: heterogeneous mixture in which particles are large enough to reflect light and be seen and also large enough to settle or be filtered out of the mixture
Introducing the Lesson
Students are likely to have learned about elements and atoms in previous science classes. Call on volunteers to share what they can recall about them. Tell the class they will learn about elements, atoms, and other types of matter in this lesson.
Reinforce the important idea that a compound is a unique substance that always has the same composition. Show students samples of two compounds that each contains only hydrogen and oxygen: water (H2O) and hydrogen peroxide (H2O2). Explain how the proportions of hydrogen and water differ in the two compounds. Also discuss how their properties differ (e.g., hydrogen peroxide is so reactive that it kills germs and bleaches color from hair).
Work with any students who need extra help to make a concept map of types of matter. A sample concept map is shown below:
Encourage a few creative students to make three-dimensional models of a crystal of sodium chloride. For example, they might use Styrofoam balls of different colors to represent the sodium and chloride ions and “bond” them together with glue or toothpicks. Invite students to explain their models to the rest of the class. Afterward, put the models on display in the classroom.
Have groups of students devise procedures for separating the following mixtures into their components based on their different physical properties:
- pepper and iron filings
- sugar and sand
Give groups a chance to share their procedures and offer each other feedback. You can have groups carry out the procedures if time allows.
A common student misconception is that atoms are “mini-versions” of the elements they represent. For example, students might think that atoms of copper are copper-colored and the atoms of gold are gold-colored. Correct this misconception by explaining that atoms consist of smaller particles—called protons, neutrons, and electrons—that are identical in all atoms, regardless of the element. Atoms of different elements have different numbers of these particles and this is what gives the atoms different properties.
Reinforce and Review
Copy and distribute the lesson worksheets in the CK-12 Physical Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.
Lesson Review Questions
Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.
Check students’ mastery of the lesson with Lesson 3.2 Quiz in CK-12 Physical Science for Middle School Quizzes and Tests.
Points to Consider
The properties of matter are not fixed. In fact, matter is always changing.
- What are some ways you have seen matter change?
- What do you think caused the changes?