Summary

In this demonstration, helium balloons and clothespin weights are used to demonstrate how adding an electron makes a negative ion, and removing an electron makes a positive ion, a concept that is often confusing to students.

Grade Level

Middle School, High School

NGSS Alignment

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• MS-PS1-1: Develop models to describe atomic composition of simple molecules and extended structures.
• HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
  • Developing and Using Models

Objectives

By the end of this demonstration, students should be able to:

• Predict the charge of an ion.
• Connect charges with subatomic particles.
• Differentiate between anions and cations and how they are formed.

Chemistry Topics

This demonstration supports students’ understanding of:

• Subatomic particles
• Ions and Atoms

Time

Teacher Preparation: 15 min to make the helium balloon(s) neutrally buoyant with clothespin weights
Lesson: 30 minutes

Materials

• 1 or 2 (or more) Mylar® balloons with ribbons of equal length attached
• 6-12 mini clothespins, or as many as needed, to attach to the ribbons as weight to adjust the floating/sinking of the balloon
• Tape, to add on to clothespins to act as small amounts of mass to adjust buoyancy
• Permanent Marker, to write an “e^–” symbol on each mini clothespin

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• Students should be familiar with the three subatomic particles before this lesson. Therefore, the background reading should be review, and could be assigned with the pre-activity questions as homework before doing the demo in class. Additionally, knowledge of isotopes and writing isotope symbols is expected in the extension exercise but can be skipped if this is used as a middle-school activity or as an introduction to ions and charge.
• Students often assume that adding something automatically means “positive,” and taking something away equates to “negative.” Of course, the opposite is true when adding negatives, like electrons in atoms. The goal of this demonstration with balloons and weights is to show that adding to something can make it more negative and can help students overcome this common misconception, which is a frequent cause of confusion when introducing the concept of ions and how they form.
• Since latex allergies are becoming more common, Mylar® is recommended. Select Mylar balloons of the same size and shape. A latex balloon will work, but it will deflate within a day or two.
• You only need one balloon for this demonstration, but you could get more than one balloon, filled with different amounts of helium to represent different atoms with different numbers of protons in their nuclei, thus requiring different numbers of electrons for a neutral atom.
• In this demonstration, weights are used as a negative force pulling down and helium is used as a positive force pulling upward. The addition of weights represents the addition of a negative charge (electron). The use of a helium balloon is like the positive charge on a nucleus.
• Practice explaining the concept before class. Each clothespin represents one electron for the atom, therefore a three-pin balloon represents an atom with 3 electrons, or lithium.
• Demonstration Procedure:
  • Before starting the class, attach enough mini clothespins to the balloon ribbon(s) to weigh the balloon(s) down enough that it does not rise or sink. If you find that you cannot add an exact number of clothespins to get the balloon to be neutrally buoyant (doesn’t rise or sink), add pieces of tape to the clothespins to make minor mass adjustments.
    
    • This does not need to be perfect, but it should be close to neutrally buoyant so the balloons are not on the floor or on the ceiling all period. You may notice that air currents in the room affect the balloons.
      
    • Be sure that the ribbons and any attached clothespins are not resting on a desk/table or the ground.
      
  • Draw a vertical number line on the white board or chalk board with zero at the middle of the vertical line. (There is no need to write positive and negative numbers on the vertical number line, only indicate that positive is above zero and negative is below zero.)
    
  • Place the pre-weighted balloon(s) so that it hovers around zero on the number line.
    
  • Explain to students that the helium that causes the balloon to float represents the positive charge of the protons in the nucleus, and the clothespins that weigh the balloon down represent the negative charge of the electrons in the electron cloud.
    
  • Ask students if they can identify the atom the balloon represents. They should realize that the number of electrons (clothespins) in a neutral atom (the balloon when it is neutrally buoyant, at zero) equals the number of protons, which can be used to identify the atom.
    
  • Ask students what will happen if a clothespin is added or removed, and what will happen when the original number of clothespins is restored.
    
  • Demonstrate that as an electron (clothespin) is removed from the balloon, the balloon rises into the positive region on the vertical number line drawn on the board. This indicates a positively charged ion (cation).
  • Return the clothespin to the balloon. It should return to its neutral buoyancy. Line it up with the zero point on the number line again if it has shifted.
  • Demonstrate that as an electron (clothespin) is added to the balloon, the balloon sinks into the negative region on the vertical number line drawn on the board. This indicates a negatively charged ion (anion).
  • You can repeat the procedure with additional balloons with varying amounts of helium that would require different numbers of clothespins (representing different atoms).
• Although this exercise focuses on electrons and charge, the calculations section reviews isotope symbols, numbers of subatomic particles, and mass number in addition to the concept of charge.