Science · 6th Grade · Molecules in Motion · Weeks 1-9

Atoms and Molecules in Reactions

Students model how atoms rearrange to form new substances during chemical reactions.

• Common Core State Standards
• MS-PS1-2

About This Topic

This topic asks students to look inside a chemical reaction and understand what is happening at the atomic scale. Aligned with MS-PS1-2, students learn that during a chemical reaction, the atoms of the reactants are not destroyed: they are rearranged into new combinations to form the products. This is the molecular basis of the Law of Conservation of Mass and a foundational idea in all future chemistry.

A chemical equation is essentially a bookkeeping system for atoms. On the left are the reactants (starting materials); on the right are the products (new substances formed). The atoms on both sides must match because atoms cannot be created or destroyed during a chemical reaction. In 6th grade, students work with simple word and formula equations, focusing on identifying reactants and products and tracking atoms rather than formally balancing equations.

Modeling is central to this topic. Physical manipulatives, colored beads, blocks, or cut-outs representing different atoms, allow students to literally take molecules apart and reassemble them into new arrangements. This hands-on process makes the abstract reality of atomic conservation tangible and memorable. Active, collaborative model-building is one of the most effective instructional approaches available for this concept.

Key Questions

1. Explain how atoms are conserved during a chemical reaction.
2. Construct a model to represent the rearrangement of atoms in a simple chemical reaction.
3. Analyze the difference between reactants and products in a chemical equation.

Learning Objectives

• Identify the reactants and products in a given chemical reaction word equation.
• Construct a physical model using manipulatives to represent the rearrangement of atoms during a specified simple chemical reaction.
• Explain, using a model, how atoms are conserved during a chemical reaction.
• Analyze a simple chemical equation to determine the types and numbers of atoms present on both the reactant and product sides.

Before You Start

Properties of Matter

Why: Students need to understand that different substances have distinct properties to recognize that new substances are formed in a chemical reaction.

Elements and Compounds

Why: Understanding that matter is made of elements and that compounds are formed from different elements is foundational to grasping how atoms combine and rearrange.

Key Vocabulary

Atom	The basic building block of matter, consisting of protons, neutrons, and electrons. Atoms are the smallest unit of an element that retains the properties of that element.
Molecule	A group of two or more atoms held together by chemical bonds. Molecules can be made of atoms of the same element or different elements.
Chemical Reaction	A process where starting substances, called reactants, are transformed into new substances, called products, by the rearrangement of atoms and the breaking and forming of chemical bonds.
Reactant	The starting substance or substances in a chemical reaction. Reactants are found on the left side of a chemical equation.
Product	The new substance or substances formed as a result of a chemical reaction. Products are found on the right side of a chemical equation.

Watch Out for These Misconceptions

Common Misconception

Students often think that atoms are destroyed during a chemical reaction because the original substance disappears.

What to Teach Instead

Physical model activities where students cut apart molecule representations and rearrange them into new molecules are the most direct correction. Seeing that all the original pieces are still present, just in new arrangements, makes atom conservation concrete rather than declarative.

Common Misconception

Many students confuse atoms with molecules, treating them as the same thing.

What to Teach Instead

Use consistently differentiated visuals: an atom is a single particle (one bead), and a molecule is atoms bonded together (beads connected). Peer model-building activities that require students to physically separate individual atoms from molecules reinforce the distinction through repeated practice.

Active Learning Ideas

Peer Teaching: Atom Rearrangement Models

Using colored balls or beads to represent different atoms, student pairs model a simple reaction such as hydrogen and oxygen forming water. One student assembles the reactant molecules and breaks them apart; the other reassembles the atoms into products and checks that no atoms were gained or lost.

30 min·Pairs

Inquiry Circle: Counting Atoms

Groups receive printed models of reactant molecules, cut them into individual atoms, and then reassemble those atoms into the correct product molecules. They record how many atoms of each type they started with and ended with to verify that the count is identical.

40 min·Small Groups

Think-Pair-Share: Where Did the Carbon Go?

The teacher presents simple combustion in accessible terms: methane (one carbon, four hydrogens) burns in oxygen and produces carbon dioxide and water. Students discuss with a partner where the carbon atom went after the reaction and trace its path from reactant molecule to product molecule.

15 min·Pairs

Real-World Connections

• Bakers use chemical reactions when they combine ingredients like flour, sugar, and yeast to create bread. The yeast causes a reaction that produces gases, making the bread rise, and transforms the ingredients into a new substance with different properties.
• Chemists at pharmaceutical companies design new medicines by understanding how atoms rearrange in chemical reactions. They must ensure that all atoms from the starting materials are accounted for in the final drug molecule to guarantee its safety and effectiveness.

Assessment Ideas

Quick Check

Provide students with a simple word equation, such as 'hydrogen + oxygen -> water'. Ask them to draw a diagram showing the atoms involved before and after the reaction, demonstrating conservation of atoms. They should label reactants and products.

Exit Ticket

Give students a card with a chemical reaction represented by colored beads or blocks (e.g., 2 red + 1 blue -> 1 red-red-blue). Ask them to write the word equation for this reaction and identify the reactants and products. They should also state whether atoms were conserved.

Discussion Prompt

Pose the question: 'Imagine you are burning a log in a fireplace. What are the reactants and what are the products? Where did the atoms in the ash and smoke come from, and where did the atoms in the original log go?' Facilitate a class discussion focusing on atomic rearrangement.

Frequently Asked Questions

What is the difference between a reactant and a product in a chemical reaction?

Reactants are the starting materials that enter a chemical reaction. Products are the new substances that form as a result. In the reaction of hydrogen and oxygen forming water, hydrogen and oxygen are the reactants, and water is the product. The arrow in a chemical equation points from reactants to products.

Do atoms change identity during a chemical reaction?

No. Atoms keep their identity throughout a chemical reaction. A carbon atom stays a carbon atom no matter what reaction it goes through. What changes is which atoms are bonded together and how. The rearrangement into new bonding patterns creates molecules with completely different properties.

How can active learning help students understand atoms and molecules in reactions?

Physical models make the abstract tangible. When students pick up 'atom' pieces, form them into reactant molecules, break those apart, and rebuild product molecules, they experience atom conservation directly as a hands-on process. This is far more effective than watching an animation because students must decide where every single atom goes.

Why does a chemical reaction produce substances with different properties?

New substances form because the atoms are bonded in new arrangements. The specific way atoms are bonded determines a substance's properties. When atoms are bonded differently, the resulting substance has different properties even though the same types of atoms are present.