Description

This is an introduction to the fundamentals of nuclear science and the forces that hold the atom together. This lesson will teach students the basics of the subatomic particles that make the atom, the way the electrostatic force holds the electrons to the nucleus, and how the nucleus is held together by the strong nuclear force. This lesson will provide a basis for students to understand the energy generation reactions in nuclear science and the basics of radioactivity.

Background Information

Atoms are made of three types of subatomic particles: electrons, protons, and neutrons. Protons and neutrons have the same mass, and they make up the nucleus, which is the center of the atom and the location of most of the mass. Electrons are lighter and orbit the nucleus. Each particle has its own value of electric charge, with electrons being negative, protons being positive, and neutrons having no charge. There are the same numbers of protons and electrons to balance out the charge of the atom, and the number of neutrons depends on the isotope.

Three forces define the interactions between these particles: the electromagnetic force, the strong nuclear force, and the weak nuclear force.

The electromagnetic force is the interactions of electric charge and magnetism. Electric charges can be positive or negative, and they produce fields of force that allow for interaction at a distance, with interactions increasing with decreasing distance. Like charges repel each other, and opposite charges attract. Magnetism has two poles, which interact at a distance, which increase with decreasing distance. However, unlike electric charges, magnets are always dipoles with both north and south parts.

Electric charges and magnetism are deeply related. Moving charges are what make up electricity. They also produce magnetic fields. Magnetic fields can also move electric charges. Light fundamentally is just magnetic and electric fields that continuously produce each other and move in space.

The strong nuclear force is what holds together protons and neutrons in the nucleus. One thing to note is that it can hold together protons and neutrons, but it cannot hold together protons by themselves or neutrons by themselves. It is the strongest of all the fundamental forces and can overcome the electrostatic repulsion of protons. However, it only acts over extremely short distances. To make fusion happen, scientists work to overcome the electrostatic repulsion between atoms to bring them close enough that the strong nuclear force takes over. To make fission happen, scientists introduce a neutron into a heavy atom where electrostatic repulsion and the strong force are just barely in balance. The introduction of the neutron unbalances the atom enough to cause it to split.

The weak nuclear force is responsible for producing radioactive decay, but it does not play a role in holding particles together.

Why This Matters

The defining characteristic of nuclear science is that it deals with behavior and interactions with the nuclei of atoms. Some of these interactions are driven by forces that are present in everyday life, while others only emerge at the subatomic scale. This lesson shows what makes nuclear science special.

Student Objectives

• The student will describe the forces that hold electrons to the nucleus and the forces that hold the nucleus together.

Learning Objectives

1. Atoms have charged particles. 
2. A charge can be negative or positive.
3. Opposite charges (positive and negative) attract one another.
4. Like charges (either two positive or two negative) repel one another.
5. The strong nuclear force holds positive particles together, but only if they are very close together.
6. Nuclear fusion is trying to smash two small atoms together until the strong nuclear force kicks in and merges them.
7. Nuclear fission is trying to make one large atom so big that the strong nuclear force can't hold it together and it breaks into two smaller atoms.

Materials (click on items for example supplies)

• Two pairs of bar magnets per student
• Velcro dots
• electroscope, one per 5-6 students
• Balloon, one per student
• Slideshow of useful images

Material Preparation

Bar Magnets

Attach opposite Velcro dots to like faces, one set per student:

Electroscope

This lesson will use an electroscope to teach lessons about charge. This device can be easily built using common household materials and can be a good hands-on activity for a class. The link below is a guide to building an electroscope.

https://www.instructables.com/how-to-make-an-electroscope-easily/

Making an electroscope is not necessary for the lesson, and the link below provides one suitable for use.

https://www.arborsci.com/products/electroscope

Laboratory Instructions

The video below is a demonstration of the hands-on portions of the lesson.

Lesson plan

Prepare materials for presentation and have them available for students. Have the image slideshow ready to either copy onto a board or show directly to students.

Questions for students

1. Did you know an atom is made of smaller particles? What are they?
2. Do you know how an atom is held together?

Exploration Activity 1: Electroscope and Introducing Electric Charge

1. If time allows, have students build their own electroscope using the given instructions and materials. If not, use a pre-built electroscope to demonstrate. Tell them that this device will allow them to directly see electrostatic forces.
2. Begin electroscope demonstration.
   1. Have the electroscope diagram up (slide 4) to reference during the experiment.
      
   2. Have the students take a close look at their electroscope. Point out that the leaves, rod, and collector are all made of metal. What makes metals special? Point out that metals conduct electricity, which is the movement of electrons.
   3. Take the inflated balloon, and have a student rub it on their hair.
   4. Have them bring the balloon to the collector and notice that the leaves now fly apart. Ask the students- why does this happen?

Explanation: What is Charge?

1. Explain that this happened because of the behavior of electric charge.
2. Introduce the concept of charge.
   1. Charge is a property of matter, like size or mass.
   2. There are two types of charge, which we call positive and negative, as their effects cancel out.
   3. Objects with an electric charge can push and pull each other at a distance.
   4. Like charges attract, and opposite charges repel.
3. Say that charge comes from the particles inside of an atom.
4. Present the image of the helium atom (slide 2). Describe it as a diagram of the atom. Ask students to identify the nucleus, and the particles that make up an atom.
   
5. Describe the three particles that make up an atom, and their properties.
   1. Electron: lightweight, negatively charged, orbits the nucleus
   2. Proton: Heavier, positively charged, in the nucleus
   3. Neutron: Same mass as proton, no charge, in the nucleus
      
6. Use this to explain the behavior of the electroscope.
   1. Show the diagram of the electroscope and the movement of charges.
   2. Ask students to brainstorm- how can rubbing the balloon on their head remove electrons from the ballon, giving it a positive charge? 
   3. Remind how metals let electrons flow freely.
   4. Have them consider- where would the electrons move if you put a positive charge nearby?
   5. Let the students realize the leaves are now positively charged, and this is what pushes them apart. 

Exploration Activity 2: Magnets and an Intuitive Understanding of Forces

1. Now that the students have seen electric charge in action, begin the magnet demonstration to have them get an intuitive grasp of the behavior of charges, and tell them so.
   1. Begin by describing how magnets behave similarly to charges:
      1. North and south are like positive and negative.
      2. Magnets act at a distance.
      3. Likes attract, and opposites repel.
   2. Make sure students are clear that we are using magnets as a model, but magnetism is not the same thing as electric charge. Use a metaphor like roller skating and ice skating to show they are similar but different.
   3. First, have students understand the difference by using the magnet on the electroscope, and seeing nothing happens.
   4. Then, have the students interact magnets with each other. Ask them to test out the opposites attract and likes repel rule. Have them see at what distance the push/pull begins. Make sure they notice the force gets stronger the closer that the magnets get together.

Exploration Activity 3: How is the Atom Held Together?

1. Now that the students have an intuitive understanding of the behavior of charges, show the image of a hydrogen atom. Ask them why the electron stays attached to the nucleus. Describe that the positive and negative charges attract and hold the electron to the nucleus.
   
2. Show the diagram of the helium atom. Ask them to notice what doesn't make sense, based on what they've learned so far. Have them realize that it doesn't make sense for two positive charges to be so close together, and that there must be something else in the picture.
   
3. Hand out the magnets with the Velcro. Have them notice that the Velcro does nothing unless it is touching, but once it is, it can hold the two like poles of the magnets together. Say that something similar is happening in the atom.

Bringing it all together: The Strong Nuclear Force and Nuclear Reactions

1. Introduce the idea of the strong nuclear force.
   1. It is a new force that holds together protons and neutrons.
   2. It only acts at really close distances, but is stronger than the electrostatic repulsion.
   3. It acts just like the magnet example earlier.
      
2. Describe the two main nuclear reactions that are the source of nuclear energy: fission and fusion.
   1. Show the diagram of the two reactions.
      
   2. Describe that fission is when the electrostatic repulsion of a heavy element is too much for the strong force, and the atom splits. This is the reaction in today's nuclear power plants.
   3. Describe that fusion is when light atoms are pushed together hard enough that they overcome the repulsion, and the strong force takes over. This is the force in the sun.
   4. Have them think about why fission is with heavy elements, and fusion is with light elements. Relate it to the number of protons and the fact that more protons mean more electrostatic repulsion.

End of lesson. Have students ask any remaining questions. Tell them that future modules will flesh out these basic principles to explain radiation, nuclear reactors, and other nuclear phenomena.

Suggested Evaluations

• Ask students to identify how the electron remains attached to the nucleus, and why an extra force is needed to explain how a nucleus is held together.
• Ask students to explain why fission happens with heavier elements, and fusion with lighter elements.

Supplemental Materials

• A more detailed slideshow for the lesson
• DIY electroscope
• Lesson on the strong nuclear force
• Description of electroscope and use for detecting radiation