Learning Set 2 • What Is in Air?

2.4 Investigate

The Molecules in Air Using Models to Observe Phenomena That Are too Small to See You already know that scientists use models and simulations to explore processes and objects that are too small or too large, too fast or too slow, or too dangerous to experience easily. You will be using an atomicmodel kit to explore how atoms interact with each other to form molecules. Actual atoms are so small that scientists can observe them only with very specialized tools. Your atomic-model kit will allow you to experience the arrangements of these very small particles. You will use small spheres with holes in them to represent atoms, and you will use gray rods to represent the connections between atoms to form molecules. Of course, real atoms do not have holes and are not connected by gray rods. However, using spheres and rods, you will be able to explore how atoms interact with each other. Understanding how atoms interact will help you understand how pollutants are formed and how, once formed, they cause problems. In this investigation, you will use four different kinds of objects to build molecules: blue spheres with 3 holes, red spheres with 2 holes, silver spheres with no holes, and gray rods. The blue spheres represent nitrogen atoms, the red spheres represent oxygen atoms, and the silver spheres are argon atoms. The flexible gray rods will be used to connect the atoms together.

Project-Based Inquiry Science

AQ 78

2.4 Investigate

Materials • atomic-model kit • Model Molecules page

Investigation 1: Build Stable Molecules A stable molecule is one with atoms that are fully connected. Stable molecules will tend to stay together. They do not easily break apart after they form. Using the atoms in your kit, you will be able to recognize stable molecules by following these two rules: • A stable molecule will have all of its holes filled. • Both ends of each gray rod will be attached to an atom.

Procedure 1. Using the blue, red, and silver spheres in your kit, build as many stable molecules as you can. Be sure that you follow the rules for making stable molecules. Remember that a molecule has at least 2 atoms. 2. As you complete each molecule, sketch a diagram of it on your Model Molecules page. Label the atoms.

AQ 79

AIR QUALITY

Learning Set 2 • What Is in Air?

Analyze Your Data 1. How many different stable molecules were you able to build? Why could you not build more molecules? 2. How many different molecules did you build with argon? What does this tell you about argon? 3. How many different molecules did you build that contain both nitrogen and oxygen? Why could you not build more?

Molecular Formula

molecular formula: a shorthand method of representing the types of atoms and the numbers of atoms in a molecule. subscript: a number written below the line. In molecular formulas, it shows how many atoms of that type of element are in a molecule.

Project-Based Inquiry Science

You sketched and labeled the atoms in each molecule. This is a time-consuming method of sharing your ideas with others. Scientists also sketch and label, but they do not spend time sketching and labeling common molecules like those you built. Instead, because scientists are already familiar with the form of those molecules, scientists use a shorthand method, called a molecular formula, to refer to those molecules. A molecular formula requires two important pieces of information. First, it includes the chemical symbol for each element in a molecule. Second, the molecular formula contains a number to show how many atoms of each element are in the molecule. This number is written below the line as a subscript. The subscript numbers in formulas show how many atoms of each type of element are in the molecule. An atom with no subscript represents one atom of that element. As you saw in your models, an oxygen atom can be attached to another oxygen atom to make a stable molecule. The formula for an oxygen molecule is O2. O is the chemical symbol for oxygen, and 2 (written as a subscript, below the line) shows how many atoms are connected to each other in an oxygen molecule.

AQ 80

O

O

O2

symbol for element subscript shows the number of atoms of the element that are in a molecule

2.4 Investigate

Reflect Record the molecular formula for each molecule you built next to each of your sketches.

Investigation 2: Build Oxygen and Nitrogen Molecules You will now use the modeling materials to build oxygen and nitrogen molecules. Make sure you follow the rules for building stable molecules when you build these molecules.

Red spheres represent oxygen atoms.

Blue spheres represent nitrogen atoms.

Procedure 1. Build a model of an oxygen molecule (O2) and a model of a nitrogen molecule (N2). Make sure you fill all of the holes to make the molecules stable. Have your teacher check your models after you have built them. Remember that the oxygen spheres are red, and the nitrogen spheres are blue. 2. Sketch the molecules on a Model Molecules page. Record the molecular formula for each molecule next to each sketch.

Analyze Your Data 1. How many oxygen atoms are in each of your oxygen molecules? How many gray rods did you use to make your oxygen molecules? 2. How many nitrogen atoms are in each of your nitrogen molecules? How many gray rods did you use to make your nitrogen molecules? 3. Why are there not more atoms in each molecule you made? Why is there a different number of gray rods in each molecule?

AQ 81

AIR QUALITY

Learning Set 2 • What Is in Air?

Investigation 3: Build More Molecules Return to your atomic-model kit. This time you can break the rules and make molecules that have atoms with unfilled holes and unattached gray rods.

Procedure 1. Make at least 3 new molecules using N, O, and Ar. Remember, these molecules do not have to be stable. 2. Sketch the molecules on a Model Molecules page. Record the molecular formula for each molecule next to each sketch.

Analyze Your Data 1. Why do you think it is difficult to make a stable molecule that contains both nitrogen and oxygen? 2. Why do you think nitrogen molecules have 2 atoms? Why do you think oxygen atoms have 2 atoms?

What Do the Holes in Your Models Represent? You have now used your atomic-model kit to build models of different molecules. You already know that the blue spheres represent nitrogen atoms, the red spheres represent oxygen atoms, the silver spheres represent argon atoms, and the gray rods represent connections between atoms. But what about the holes? What do the holes represent? As you built molecules, you found that the holes have something to do with how atoms can combine with other atoms. In the kit you are using, the holes represent the ways atoms can connect to other atoms. An atom with holes in it forms molecules by combining with other atoms to fill the holes of all the atoms in the molecule. If all of the atoms in a molecule have their holes filled, then the molecule is stable. You built stable oxygen molecules by using 2 rods to join one oxygen atom to another oxygen atom. You built stable nitrogen molecules by using 3 rods to join one nitrogen atom to another nitrogen atom. You know that stable molecules are molecules with all the holes filled. unstable: can easily change.

Project-Based Inquiry Science

But, if after combining, a hole remains in a molecule’s atoms, then the molecule is unstable. Because it has a hole, it is incomplete. During the last investigation, you built some unstable molecules. When an unstable molecule is in a mixture, the molecule can combine with other atoms to fill the hole.

AQ 82

2.4 Investigate

Reflect With your group, answer the following questions. Be prepared to discuss your answers with the class. 1. Pollutants are very unstable molecules. Do you think N2 and O2 are pollutants? Use what you know about stable and unstable molecules to support your answer. 2. Do you think argon might be a pollutant? Why or why not? 3. What else do you think you need to know about atoms to know how air becomes polluted? 4. What else do you think you need to know about how atoms form molecules to know how air becomes polluted?

Update the Project Board You know many things now about atoms and molecules. You also have generated questions about atoms and about molecule formation. Begin updating the Project Board by focusing on the What are we learning? column. Record in that column what you now know about atoms and molecules. Use the conclusions from building atomic models as evidence, and record your evidence in the What is our evidence? column. Then think about the questions you generated about atoms and molecules. Add those questions to the What do we need to investigate? column. As the class works on the class Project Board, remember to record the same ideas and questions on your own Project Board.

AQ 83

AIR QUALITY

Learning Set 2 • What Is in Air?

What’s the Point? Atoms form molecules by combining with other atoms. When a molecule contains atoms that are fully connected to each other, it is a stable molecule. In the atomic-model kit you are using, stable molecules are those for which all of the holes in the atoms are connected to other atoms by gray rods. Stable molecules tend to stay bound together and do not easily break apart. However, when a molecule contains atoms that are not fully connected to other atoms, the molecule is unstable. In the atomic-model kit you are using, unstable molecules are those that have atoms with empty holes or gray rods that do not connect at both ends to an atom. In a mixture, unstable molecules combine to fill any holes. Because atoms are too small to see, you are doing what scientists do when they need to explore something too small to see. You are using an atomicmodel kit with pieces large enough for you to see to build models of atoms and molecules. Atoms do not look exactly like the spheres in your kit, and the connections between them do not look like the gray rods in your kit. However, with the kit, you can experience the way atoms combine with each other and what affects the combinations they make. Scientists use molecular formulas to communicate the names of molecules to each other. A molecular formula uses letters representing the names of elements to show what kinds of atoms are in the molecule. A molecular formula contains numbers to show how many of each type of atom are in a molecule. The formula N2 represents a molecule that includes two nitrogen atoms. This is the formula for naturally occurring nitrogen. The formula O2 represents a molecule that includes two oxygen atoms. This is the formula for naturally occurring oxygen. Argon does not form molecules. It exists as individual atoms of argon in the air mixture.

Project-Based Inquiry Science

AQ 84