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Modeling Radioactive & Stable Atoms


Use this as a demonstration or as a hands-on activity for students.


Radioactive atoms emitting beta particles will change into the element which is one atomic number higher, but they will have the same atomic mass as before.

In this exercise, a zip-close plastic bag represents the nucleus of an atom and holds representation for protons and neutrons. If the atom is stable, zip the bag closed. If atom is radioactive, bag is left open to emit ionizing radiation (alpha particles, beta particles and/or gamma rays).


Activity Materials:

Preparing the Marshmallows

Note: If you keep the marshmallows in a cool, dry place or place a packet of silica gel into the zipper-type plastic bag with your marshmallows, your “atoms” will last for several years.



  1. Remove the marshmallows from the packaging and let them “air” for a day.
  2. Mix the water and food coloring in a cup.
  3. Insert a toothpick into a marshmallow.
  4. Dip the marshmallow “fondue style” into the colored water for about 2-3 seconds.
  5. Remove from colored water and stick in a Styrofoam tray to dry.
  6. Let dry for several days or until not sticky to the touch.
  7. Use marker to indicate protons with a “+” (plus sign).

Suggestion – make the protons PINK – because Pink Protons are Positive.

Put in plastic bags, i.e., 7 neutrons (yellow) and 7 protons (pink) = nitrogen.




  1. Mark 7 large marshmallows with a positive (+) sign. They represent protons.
  2. Select 7 unmarked large marshmallows to represent neutrons.
  3. From the group above, select 2 “protons” and 2 “neutrons”; use toothpicks and glue to join these into a group of four. This represents an alpha particle.
  4. Mark the sides of a mini-marshmallow with a negative (-) sign; it represents an electron. Stick, but do not glue, a toothpick into this mini-marshmallow. Glue the other end of the toothpick into the side of a “proton” (so the positive sign is partially covered). This now represents a neutron.
  5. Put the alpha particle from step #3 into an empty zip-close bag. Add 4 “protons” and 4 unmarked marshmallows (neutrons). Zip bag closed. The closed bag represents the nucleus of a stable atom. The binding energy can contain all the protons and neutrons within the nucleus; atom is stable.
  6. Open the bag. Add two “neutrons.” Leave the bag unzipped; excess neutrons have now made it unstable.
  7. To become stable, the nucleus will emit a beta particle. Find the “neutron” you made in step #4. Pull off the mini-marshmallow (now it is a beta particle) and toss it about 1-2 feet from you. Leave the remaining proton in the bag and zip it closed. The atom has changed and is stable again.



1)    How many positively charged marshmallows (protons) are in the bag? (Do not count the one whose positive sign is partially covered by the mini-marshmallow!) This is the atomic number of the atom.

2)    What element is represented by this model?

3)    How many neutral particles are in the bag? (You do count the particle where positive and negative charges cancel each other out!)

4)    What is the atomic mass of this atom? (Each large marshmallow equals 1 atomic mass unit, regardless of charge.)

5)    After Step 7: What is the atomic number of the atom now?

6)    After Step 7: What element does the atomic model represent now?

7)    After Step 7: What is the atomic mass of the atom now?

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