GO IN DEPTH

Isotopes of Pennies

Materials

For each pair of students, you will need:

• a balance
• an empty 35 mm film canister
• a sample of ten pennies to place in the film canister

By Roman Oleinik (Own work) [CC-BY-SA-3.0], via Wikimedia Commons

Purpose

To demonstrate that isotopes of an element have different masses; that isotopes are atoms of the same element that have different numbers of neutrons; and that atomic mass is the weighted average of the naturally occurring isotopes of an element.

Context

This is the first in a three-lesson series about isotopes, radioactive decay, and the nucleus. The second lesson, Radioactive Decay: a Sweet Simulation of Half-life, introduces the idea of half-life. The final lesson, Frosty the Snowman Meets His Demise: An Analogy to Carbon Dating, is based on gathering evidence in the present and extrapolating it to the past.

This lesson helps students build their understanding of the properties of matter, specifically it will help them understand that average atomic mass is not a simple average, but is weighted according to percent abundance. Before working on this lesson, students should be familiar with the periodic table and should have had some basic instruction in the following concepts: isotopes, mass number, and atomic number. Students should be able to describe an atom and its basic structure.

This lesson helps students understand the important notion that neutrons in the nucleus add to an atom's mass. Prerequisite understanding for this lesson can be found at the 6-8 level, particularly the idea that "atoms of any element are alike but different from atoms of other elements." (4D Structure of Matter (6-8) #1) The ideas in this lesson are essential for building an understanding of the concept that the nucleus of radioactive isotopes spontaneously decays.

Electrically neutral particles (neutrons) in the nucleus add to its mass but do not affect the number of electrons and so have almost no effect on the atom's links to other atoms (its chemical behavior). A block of pure carbon, for instance, is made up of two kinds, or isotopes, of carbon atoms that differ somewhat in mass but have almost identical chemical properties. Scientists continue to investigate atoms and have discovered even smaller constituents of which electrons, neutrons, and protons are made.

According to research, students may at first take isotopes to be something in addition to atoms or as only the unusual, unstable nuclides. The most important features of isotopes (with respect to general scientific literacy) are their nearly identical chemical behavior and their different nuclear stabilities. Insisting on the rigorous use of isotope and nuclide is probably not worthwhile, and the latter term can be ignored. (Benchmarks for Science Literacy, p. 79.) In this lesson, pennies of different compositions represent isotopes. Students can readily understand that pennies of different masses are still pennies.

Before the lesson, prepare the canisters in the following manner:

1. Put a piece of tape on the sides of the canisters and write a code letter on the top of each canister. Be sure to keep a record of the code letters on a separate sheet of paper.
2. Weigh the canisters with their tops. Record the mass on the tape on the side.
3. Place a penny sample in the canister. Record the number of old and new pennies next to the appropriate code letters on your separate sheet of paper.
4. Seal the canisters with a small amount of Superglue.

Note: These sealed canisters may be kept from year to year.

Motivation

Refer students to the Pictorial Periodic Table. If students are working offline, they can look at the periodic table in their textbooks or you can print out a copy of the Web Elements: Printable Periodic Table for them to use.

As students look at the periodic table, ask them if they see any atomic masses that are whole numbers. Then ask them: "Why do all the atomic masses on the periodic table include decimal points instead of just whole numbers?" Accept all answers and ask students to record their answers to this question in their science journals. Later in the lesson, students will revise their answers.

Tell students: "Atomic mass refers to average atomic mass of the naturally occurring isotopes of any element. Since it is an average of various isotopes, it is generally a decimal number. Mass number, on the other hand, is the count of protons and neutrons for any single isotope and is always an integer."

Display a "collection of atoms" such as a beaker of a solution or a glass of milk. Ask students: "In this collection of atoms, are all the atoms of a given element exactly the same?" Let them explain that isotopes of an element have the same chemical properties but different weights.

Development

Tell students: "In 1913, T. W. Richards found two atomic masses for lead. In 1919, F. W. Aston separated neon atoms into two different isotopes, after he invented the mass spectrograph. Since that time, many isotopes of the elements have been discovered. They are all listed in many places, including the websites used in this lesson."

Remind students that ALL atoms are isotopes. Naturally occurring chemical elements are usually mixtures of isotopes, and so their atomic masses are weighted averages of the masses of the isotopes in the mixture.

Distribute the Science NetLinks lab packet, Isotopes of Pennies, to each student. You may group students in any size group, but working in pairs involves and engages each student.

Tell students: "In this activity, you will find weighted averages of the masses of two kinds of pennies. Then you will find the number of each type of penny in your mystery sample, using the concepts you developed in the activity."

On the first page of the lab packet, ask students to define the following terms in their own words: isotopes, mass number, and atomic number. At the end of the lesson, students will be asked to reflect upon and revise, if needed, these definitions. You may wish to collect these sheets to check for student understanding. If students appear to need more instruction on isotopes, you should review these concepts before proceeding with the lab.

Students should be able to complete parts A, B, and C in one class period. You can assign the Isotopes of Pennies assessment sheet as homework.

Assessment

Have students complete Isotopes of Pennies assessment sheet so that you can assess student understanding of the concepts in this lesson. In the first part of the assessment, students apply what they have learned to various elements. Also, they write a short summary explaining how the lab has illustrated the concepts of isotopes, mass number, and atomic number.

Have students refer back to their answers to the question in the motivation. (Why do all the atomic masses on the periodic table include decimal points instead of just whole numbers?) Ask them to revise or expand upon their answers based on what they have learned in this lesson.

Extensions

Aston, Francis William - 1922 Nobel Biography, on the Nobel eMuseum website, provides information about Francis William Aston, the British chemist and physicist who won the 1922 Nobel Prize in Chemistry for discovering isotopes of elements by means of the mass spectrograph.

Students can browse the website Photograhic Periodic Table of the Elements to see the chart of all the elements and every known isotope.