Unit Plan 2 (Grade 8 Science): Atoms, Molecules & Structures

Focus: Develop and use models to describe the atomic composition of simple molecules (like H₂O, CO₂) and extended structures (like salt crystals or metals). Build on prior particle models to distinguish between atoms, molecules, and different kinds of structures of matter.

Grade Level: 8

Subject Area: Science (Physical Science • Modeling)

Total Unit Duration: 5 sessions (one week), 50–60 minutes per session

I. Introduction

Students move from general particle models to more detailed atomic and molecular models. They learn that all substances are made of atoms, which can be the same or different, and can be arranged as individual atoms, molecules, or extended structures such as crystals or networks. Using ball-and-stick, 2-D diagrams, and digital tools (if available), students build models of common substances and compare different types of structures. They also discuss what models show well and what they leave out, strengthening their understanding of how scientists use models for invisible structures.

Essential Questions

• What is the difference between an atom, a molecule, and an extended structure?
• How can we use models to represent the composition and arrangement of atoms in different substances?
• Why do substances with different atomic structures have different properties (e.g., crystals vs. individual molecules)?
• What are the strengths and limitations of various models (particle diagrams, ball-and-stick, formulas) for representing atomic composition?
• How does understanding atomic composition prepare us to explain chemical reactions and properties of matter later?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. Distinguish between atoms, molecules, and extended structures using words, diagrams, and examples.
2. Interpret simple chemical formulas (e.g., H₂O, CO₂, O₂, NaCl, SiO₂) as representing the types and numbers of atoms in a substance.
3. Develop and revise models (2-D drawings, physical models, or digital) to show the atomic composition of simple molecules and extended structures.
4. Compare different model types (particle diagram, ball-and-stick, structural formula) for the same substance and describe what each model shows well and what it leaves out.
5. Use models to explain why some substances exist as individual molecules and others as extended structures (like repeating crystal lattices).
6. Communicate in writing or orally how their models represent atomic composition and identify at least one limitation of each model.

Standards Alignment — 8th Grade (NGSS-based custom)

• MS-PS1-1 — Develop models to describe the atomic composition of simple molecules and extended structures.
  • Students create and revise models of substances like water, carbon dioxide, oxygen, and ionic/covalent crystal structures to show how atoms are arranged.

Success Criteria — Student Language

• I can explain the difference between atoms, molecules, and extended structures and give examples of each.
• I can read a chemical formula and say how many of each type of atom it represents.
• I can build or draw a model of a simple molecule or extended structure and explain what each part of the model stands for.
• I can compare two different models of the same substance and describe what they show well and what they don’t show.
• I can identify at least one limitation of my own model and suggest how it could be improved.