Lesson 2: Modeling the Nucleus of an Atom

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Prerequisite Knowledge

  1. Atoms are made of particles with different charges. 
  2. The strong nuclear force holds the nucleus together, but only if the nucleons are close.

Description

 

Students should now understand the forces holding atoms together (Lesson 1), so we'll learn why we care about the differences between atoms. In this lab, students will familiarize themselves with concepts surrounding the structure of different atoms and how that affects material properties. We will build nuclei of other isotopes with magnets, become familiar with the periodic table and reaction formulae, understand the concept of atomic stability, and model different nuclear reactions. 

 

Isotopes are all around us and are useful in medicine, archaeology, geology, and nuclear science! 

 

Why It Matters

This unit will provide the fundamental intuition and knowledge to explain why atoms decay at all. If all atoms were stable and held together equally by the strong nuclear force, there would be no radioactivity to discover, many fields of science would be set back, and we would not understand crucial parts of how the matter that surrounds us came to be. It is one thing to hear atoms are unstable and another to feel your model fall apart in your own hands. Additionally, being familiar with basic concepts like the periodic table and how to read chemical formulas will be helpful for later lessons in the curriculum and simplify complex processes.

 

Background

The periodic table is essential for understanding trends in material properties and organizing critical information, but it initially seems overwhelming. There are numbers and letters of different sizes and sometimes even different colors. Ultimately, the purpose of the periodic table is to list atoms of increasing size. Let's see how this is done:

Fundamentally, an atomic symbol or a box on the periodic table needs to have three key features: the atomic number, the mass number, and the elemental symbol. The atomic number is usually in the bottom left, but you can always find it by reading the smaller number. The atomic number or Z represents the number of protons in an atom, giving each element its distinct identity. The atomic number matters most for chemical problems because the number of protons equals the number of electrons in an atom, and the electrons do all the chemical bonding and interaction. The atomic mass is generally shown in the top left and is the bigger of the two numbers. Atomic mass represents the total mass of an atom. It is the sum of the protons AND neutrons in a nucleus. *Note that the number of neutrons must follow this equation: neutrons = Atomic Mass - Protons. There is also the element's symbol, usually a letter or two representing the element.

Every atom with the same atomic number is the same element. Every atom with 6 protons is a carbon atom. Atoms of the same element can have different numbers of neutrons, which leads to isotopes. Isotopes are atoms of the same element that have different masses. Isotopes and the number of neutrons in a nucleus do not matter to average chemists, but they do matter to nuclear scientists and engineers.

Isotopes have different stability. Stability is how well a nucleus holds itself together. As we learned in lab 1, the strong nuclear force only works over very short distances, so as you add nucleons (the term for protons and neutrons) to a nucleus, it gets larger and less tightly bound together. This can eventually lead to unstable atoms decaying or fissioning when they release certain particles or break apart to become more stable.  As you move down the periodic table, you can observe that elements become less stable because radioactive elements begin to be prevalent.

Here are some examples of different nuclear reactions you can explore and build with students:

Fission:

Fusion:

Radioactive Decay:

Student Objectives

  • The student will build different isotopes of the same element.
  • The student will create an unstable nucleus and see what is likely to happen to unstable nuclei.

Learning Objectives

  1. Protons (positively charged particles) and neutrons (uncharged or neutral particles) are in the nucleus of an atom and are roughly equal in mass.
  2. The number of protons in an atom tells us what element it is. We call this number an element's "Atomic number" or "Z."
  3. The number of protons and neutrons in an atom tells us its mass and which isotope it is. 
  4. Some isotopes are stable, and some are unstable. Generally, larger atoms are more unstable.
  5. Unstable atoms undergo radioactive decay. We don't know when precisely a single atom will split, but we can know the average time it takes for half of the substance to decay--this is called the half-life.
  6. There are different ways an atom can break apart. Alpha, beta, and gamma decay are all ways atoms try to become more stable. Particular atoms can also undergo fission. You'll focus more on the types of radiation in the next lab.
  7. Read Atomic symbols on the periodic table and in chemical equations. 

Materials

  • ~10 ball magnets (~5mm) per student
  • ~10 steel ball bearings of the same size (~5mm) per student (These need to be magnetic steel, which can be hard to acquire. We strongly recommend buying from the same source to ensure your bearings are magnetic and can be used.)
  • Paper plates or bowls - one per student
  • An image or physical copy of the periodic table

Material Preparation

  • Pre-separating the ball magnets and bearings for students can help enable quicker distribution.
  • You may consider doing a small introduction prior to passing out the materials because the magnets can be very distracting to students during lecture-style teaching.

Demonstration Video

https://youtu.be/UfwP9exDbKI

 

Laboratory Instructions

Find a digital Google Drive copy here: https://docs.google.com/document/d/1oqdkjkSd-CqAQiKHbpjlOG14A09MRtv4/edi...

 

  1. Now that your teacher has introduced some key concepts about building atoms, it's your turn to try for yourself. Using what you have learned, create a C12 isotope (remember the colored magnet balls are your protons, and the steel ball bearings are your neutrons).
  2. How would you change the nucleus you just built into C11 or C13?
  3. If you put 4 protons together and five neutrons, what is the atomic symbol and name of the isotope you built?
  4. Let's now look at trends in stability. Stability is how much a nucleus (or your collection of bearings and magnets) wants to stick together. Try taking only 1 or 2 magnets and getting as many neutrons as possible to stick to them. Can you carry them all at once? How well do new neutrons stick? What starts to happen?
  5. Your nucleus has an imbalance of protons and neutrons, making it less stable. Making big nuclei also makes them less stable since, remember, the nuclear force only acts over short distances. Where on the periodic table might you find unstable elements?
  6. What happens if you take your unstable and neutron-rich nucleus and drop it into your bowl? (Remember, only drop into your bowl; you don't want your particles getting lost!)
  7. What you've just demonstrated is fission! Your nucleus broke apart and released some neutrons. In the real world, very big nuclei generally split into two smaller nuclei while spitting out several neutrons. Unstable atoms are susceptible to fission when something energetic hits them. In this example, hitting the table provided the energy. In the real world, energetic particles like neutrons can hit unstable atoms, causing them to fission.
  8. Atoms fall apart naturally when they get too big, but nature also builds atoms through fusion. Try making the chemical equation below: H2 + H3 = He4 + n
  9. You just built a fusion reaction like the ones powering our sun!

Suggested Evaluations

  • Prompt students to build a particular isotope ask them how they would do it using their materials and a periodic table.
  • Ask students where on the periodic table they think the unstable elements would be. 
  • Collect Lab Worksheet

Supplemental Materials

  • For advanced students: Consider introducing the concept of the Table of Nuclides. Like the periodic table, this chart shows atomic symbols. Unlike the periodic table, the table of nuclides shows all known isotopes and is usually color-coded by their half-life which represents their stability. It can also reveal how different isotopes decay. Try exploring here: https://pripyat.mit.edu/KAERI/
  • A picture showing Am241 decay chain. Unlike fission, we know exactly what product will come from an alpha decay and can calculate how much of each of the elements down the chain will be around at a particular time using half-lives.