The Atomic Marshmallow Kit
Now that you have all of your supplies ready, it’s time to guide your group through the construction of a model atom.
Start by handing out the marshmallows and ice-cream topping pieces. With younger participants, it can maintain focus if you mention that there are extra supplies for snacking on afterwards.
Start with the marshmallow. Most of an atom is empty space. And most of a marshmallow is nothing but air frothed into sugar. So this marshmallow represents the âemptyâ space of an atom. For older participants, you can encourage them to think of the sugar of the marshmallow as representing not only the energy that permeates what we call âemptyâ space but also the forces that hold the atom together.
For a very long time, the atom was believed to be more-or-less of uniform density, an amorphous mixture of tiny negative particles called electrons swirling around in a positively-charged “pudding.” In 1911, Ernst Rutherford and his team completed a series of experiments that shocked the physics community by revealing that most of the mass of an atom is concentrated in a tiny, central nucleus containing all of the positive charge. For our model, in honor of Rutherford, we’ll build a helium (He) atom, which has a nucleus containing two protons and two neutrons. (Much of Rutherford’s research focused on the alpha particle–which happens to be exactly the same as a helium nucleus.)
Let your dark-colored candies be protons and your light-colored candies be neutrons. (It doesn’t really matter, but textbooks often draw protons as dark dots and neutrons as white dots.)
Candy Nucleons 
2 Neutron, 2 Protons
Using the wooden skewer or toothpick, drill a small hole in the side of the marshmallow. Now use the same toothpick or skewer to push those nucleons (a word which here means “candy pieces representing protons and neutrons”) into the center of the marshmallow.
Pathway for the Nucleus 
1 Down, 3 To Go
This is a good time in the activity to stop lecturing and instead gather suggestions from the participants and sketch their ideas on a board if you have one, or to gather around some sketching paper for discussion purposes. You can expect to see pictures that look much like a planetary system, because thatâs the way the atom often (still!) is drawn in textbooks. You might have a knowledgeable participant who’ll shout out something like, “Shells! The electrons are in shells!” or “They’re in the Cloud!” Regardless, during the discussion, build on these volunteered suggestions to reach a description of the electrons as whirling around the nucleus in a cloud, going so fast that you canât really tell exactly where they are, only that you know roughly how far they are from the nucleus.
At this point, we have a positively charged ion, because we havenât added any electrons yet. A helium atom needs two electrons, negatively-charged particles, to balance out the two positively-charged protons. Once it was established that the positive charge is concentrated in the nucleus, where did researchers decide that the electrons belong?
Our helium atom’s two electrons do indeed share an electron âshellâ, a layer of electrons a known distance from the nucleus. So let’s put a very thin, energetic, sparkly shell around our atom.
Before setting up the shell supplies, pause to demonstrate the procedure. If you’re working with younger students, you may need to stress that everyone will get their turn. If the “mess” part of the activity is an issue, set up a protected area where the messy activity is OK and let the participants queue up to build their atoms in assembly-line fashion.
To create the âelectron shellâ skewer the marshmallow firmly on the wooden stick, then very briefly dunk it into the water, then tap off any excess water into the water container. Tapping off excess water is important, because otherwise the marshmallow can get soggy, which makes for a less-attractive candy atom.
Each group needs a container with about a cup of water in it and another container with a packet of dry gelatin mix emptied into it. (For fun, choose a gelatin color in keeping with whatever events are ongoing, or a local sports team’s colors…anything to drive interest.)
Finally, gently swirl the damp marshmallow in the gelatin mix.
Adding the Electron Shell
Set the decorated marshmallows aside on a sheet of waxed paper or a plate.
Two Finished Helium Atoms
As time permits, participants can make other atomsâŚstuffing different numbers of protons or neutrons into marshmallows and adding a shell of electrons.
