Cooking With Kuiper: The Instruction Set

January 11, 2013| cometary| 0 Comment | 1:42 am

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Hands-On Science

(update: 2/18/2015)

Time to build a comet!

If you have adult or older-student assistants, ask them to take charge of crowd control; that is, keeping the audience from crowding around the demonstration. Everyone will get to see the comet! Spare a minute for a brief lecture on the hazards of dry ice. You may have participants who know that dry ice can “burn”, but not all will understand that idea at first. However, no one wants to get hurt. Mention that you will be protecting your hands with gloves and your eyes with safety goggles (or safety-rated eyeglasses).

Supplies for Comet Making (Just Add a Cooler-Full of Dry Ice)

Supplies for Comet Making (Keep your cooler-full of dry ice in a safe spot.)

Participation opportunities include: helping move the materials and equipment to a mess-tolerant location, measuring ingredients, and smashing dry ice. The trauma of allotting slots to help out is one important reason to try the exercise at least twice. (Crowd-control tip: sometimes it helps to announce “I’ll only choose helpers from those who do not raise hands and call out to volunteer.”) As a first step, take one of your plastic bags and cut it open along one side, then use it to line your mixing bowl. Take 2 other bags and put one inside the other to make a double-thickness bag.

In the first stage, your chosen helpers will take turns measuring all the “safe” ingredients into the bag-lined mixing bowl. Working with the dry ice needs closer control, so keep your supply of CO2 off to one side for now. As you introduce each ingredient, explain why it’s being included. You can use the short explanations provided here as a starting point, adding your own facts or curriculum tie-ins, but remember to keep it brief or you’ll lose your audience’s attention.

Let’s start with water: most comets are composed primarily of water ice. During the early formation of the solar system, the planets were bombarded by comets—so some of the water you will use in this experiment may have actually originated in the Kuiper Belt! (For a popular-science overview, check out this article from Time Magazine.) Your helper will add 2 cups of water.

Next, add sand or gravel: most comets incorporate at least some rocky material. Have your helpers measure out about 2 Tablespoon (TB) of grit.

Next, you’ll add ammonia: real comets typically contain NH3, the active ingredient in this cleaning solution. (Regrettably, few, if any, comets show up to help when it’s time to clean house.) If you’re using a squirt bottle to store the solution, your helper just needs to add one “squirt” of ammonia solution. Otherwise, your helper should measure in 1 Tablespoon.

A Dirty Soup of Rocks, Water, and Organics

A Dirty Soup of Rocks, Water, Ammonia, and Organics

And, for our last step before major excitement sets in, stir in a touch of ice-cream topping: these contain organic molecules, which are a normal component of comets. The organic molecules in real comets are not this delicious–they include hydrogen cyanide and formaldehyde–but comets often contain complex and interesting compounds such as amino acids. Researchers at NASA’s Ames Research Center have shown that amino acids from comets striking Earth long ago during the Solar System’s early eons would not only survive impact but would form even more important compounds for life under the heat of impact. So it may be that we are here to enjoy ice cream (and sugary toppings) thanks to ancient comets. Let your helper squirt in one squeeze-worth (it will be about a tablespoon).

Now, finally, it is time to add the dry ice. Comets contain significant quantities of frozen gases, especially carbon dioxide, which just happens to be the gas that we call “dry ice” when frozen. This stage of your demonstration is a two-step process. First, you will put on safety goggles and work gloves and use the hammer to tap off about 2 pounds of dry ice (1/4 to 1/3 of your supply). Place the chunks into the doubled plastic bag and twist the opening closed. Then, and only then, one lucky volunteer will be asked to don a set of goggles and, once protected, may proceed to smash the contained dry ice with the hammer.

Crushing Dry Ice with Flat Side of Hammer

Have your crusher use a two-handed grip (this helps deflect the temptation to also handle the bag of dry ice and also limits the range of motion, protecting bystanders from the crusher’s swing) and turn the hammer sideways, to smash with a broader surface area.

Once that stage is completed, ask the crusher to rejoin the group. Make sure that the wooden stirring spoon is at hand and that you are still wearing your work gloves and goggles. Then open the bag and quickly scoop out roughly two cups of crumbled dry ice.

2 Cups of Ice-Cold CO2

Give the mixture a stir and then swiftly add the dry ice, stirring vigorously. There will be some dramatic vaporization of CO2 and in moments the dry ice will freeze the water solution to a slushy slurry. Quickly wrap the plastic bag around your slushy mass and—keeping those gloves on—form the contents into a snowball, using firm pressure to shape the contents.

Comet’s In the Bag

You will feel the mass harden as you form your iceball. At that point, it is time to unwrap the comet and reveal it to your onlookers. You will have something that looks surprisingly like the common description of a comet—“a dirty snowball”. You may even want to use your snowball-making skills to firm up the comet a bit once you remove it from the bag–remember to keep your gloves on!

Firming Up the Comet

Your finished comet

Set the comet aside on a cold-safe surface, in a location where the eventual water-ice-melt will not damage anything. The comet will continue to outgas CO2 vapor. If you are working outdoors, any breeze will push this plume into a fair imitation of a comet’s tail.

Gases (CO2) immediately begin to sublime from the comet’s surface

Your experiment team will undoubtedly want to repeat this process. A typical group of students will demand about four comets. After 2 or 3 builds, it will be time to set up fresh plastic bags for mixing and crushing. If the group is larger, find ways for students to share participation tasks. For instance, two students can take turns as dry-ice crusher, two can each measure one cup of water into the mix, and so on. As you proceed, instead of repeating the descriptive information yourself, invite the students to call out more of what they remember about the components represent.

Here’s one small starter comet, let’s call this one “Dirty Little Snowball”

These model comets will last a long time, up to a few hours depending on their size and the conditions. You can explain that the comets which get our attention are much larger–Comet Halley is estimated to be about the size of Manhattan Island–and between visits to the inner Solar System, they orbit back to where it is too cold for water, ammonia, or CO2 to be anything other than solids. By no means do you need to make any effort to create spherical, smooth comets. In fact, as you create successive comets, allow them to be different, irregular, and, well, messy. Here are a few samples from a few of my comet-making sessions:

That's one frosty, rocky, comet: "Before"

That’s one rocky comet, frosted with ice crystals of H2O and CO2

That's one slimy, partly-dissociated comet

Here’s a comet with conspicuous dark patches

That’s one tall, frosty, cone-shaped comet

If your schedule permits, allow some time to pass and return to look at the comets after they have lost more material, as if you are checking in on a comet as it approaches the sun and some of its ice has been drawn off under the combined forces of the sun’s radiation and the solar wind…forming the comet’s tail.

Holey Comet, Batman!

You might also like to read:

Chasing Comets: Notes for Project Leaders #1Chasing Comets: Notes for Project Leaders #1

In this activity, the most important idea is to explore and experiment with models and games to understand how a comet’s tail behaves as the comet hurtles around the sun. The key concept is that the comet’s tail is being pushed away from the sun by the ionizing radiation, solar wind and even the light itself blasting out of the sun. This means that when the comet is inbound, approaching the sun, its tail streams behind it, like a horse’s tail. But on the outbound journey, as the comet leaves the sun behind, its tail flies out in front of it. What we hope the participants will take away from these activities is a picture of what a comet looks like as it moves and the knowledge of why it looks that way.

Comet-tail behavior simply makes sense when “experienced” from the comet’s point of view. If by any chance some of these facts are a discovery for you, too, don’t feel like you have to keep it a secret that you are learning–have fun with it. A key ingredient in the formula for growing a scientist is that finding out how the universe works is fun. Or, in the words of one physicist profiled in the film Particle Fever: The real answer to “why do we do this is . . . because it’s cool.”)

Keep in mind the constraints of your particular situation when assembling your materials and pre-planning the project. For instance, if there aren’t enough classroom scissors or if session time is tightly constrained, you can pre-cut the ribbon for the individual comet models into 3-foot lengths. Be aware of opportunities for participants with special needs—for instance, the comet-running activity does require at least one person to be standing still. In return, that one who just can’t stand still could be a pinch-runner. If the group as a whole isn’t particularly fast-moving, the “running” game can be done at whatever pace suits the team. (One can be a “student” at any age—most of us middle-aged folks are not exactly speed-demons.) If you’re planning this as a home-schooling project, this is one you’ll want to save for a get-together with other home-schoolers–you need at least three players and it is ever so much more fun with a group.

Stage 1: The Small-Scale Experiment

This description may look long, but that’s just to let you walk through it easily and to share some photos to help. This whole Stage 1 should take about fifteen minutes, tops. I’ll spare your weary eyes and park the “Stage 2” and “Stage 3” activities in the next posting–but don’t worry, the entire activity fits into a single science session if you can claim an hour’s time to play with.

Before distributing materials, bring out one individual model comet, the sample to be used for the models everyone will take home. It’s simply an ordinary badminton birdie with long streamers of ribbon tied to it. For now, keep the ribbons bunched up inside the net of the birdie. Explain that the ball at the end of the birdie is the comet’s nucleus, the frilly part can be its atmosphere, or coma, which begins to form as the gas and dust which jets away from the outer layers comet as it warms up.

One Small Comet

Notes: I’d suggest that you relax and let your sample comet be imperfect—comets are messy creatures by nature and you don’t need that one super-meticulous individual slowing down the whole event by striving to exactly matching a perfect sample. If you have an older, more experienced group of comet enthusiasts to work with, you can interject the extra information about the distinction between the ion and dust tails—perhaps even represent them by different ribbon colors. On the other hand, if you’re working with anyone between the ages of 5 and 15, and you don’t want to deal with distracting snickers and giggles erupting through the group, simply refrain from using the technical term for a birdie. Oh, come on, you know why.

OK, back to it. The ribbon represents those gases and dust particles that make up the comet’s tail(s). Now, if we toss our model across the room, what happens to the streamers tied to it? Right . . . they float out behind. They don’t stretch out in front or clump in a bunch around the head of the “birdie”. You can demonstrate by trying to throw your comet backwards: hold the tail in front and toss, but the tail will just fall back to the head and—if your throw is a mighty one—end up in back again..

Now, invite answers to a key question: why does the ribbon float behind? What pushes the tail behind the cone as it flies through the room? With a little nudging, you should get general agreement that it is the air pushing on the lightweight streamers, shoving them behind the “head” of our comet.

But now we must turn to a more difficult line of questioning. Pull out playground or soccer ball (a handy model for the sun), and ask one student to stand and hold up your Sun so everyone can see the next portion. Bunch up the comet’s tail in the back of the shuttlecock again, and carry the comet in a “flight” around the “Sun”. As you move, ask the students to think hard about what happens to the comet’s tail as it whips around the sun.

Start easy. Shake out the streamers, and stretch them out with your free hand. Move the comet towards the sun. Which way should I point the streamers? Everyone will be quick to tell you to pull them backwards, away from the sun. Now, place the comet at its closest approach to the sun, just before it curves back to head into deep space again. “I’m at the Sun now,” you can say, “zooming around the back of it. And moving as fast as I’ll go in this journey. Which way should the streamers point?”

Usually this question generates some disagreement. A reasonable argument would be that you should hold the streamers behind the comet, as it moves, which would mean the comet’s tail would point along a tangent to its orbit around the Sun. (Even if the students are covering tangents in math, please don’t interrupt yourself to pause and discuss tangents right now! Use this lesson later to enliven the math session.)

Tail Behind?

Tail In Front?

Tail Sideways?

Some students may suggest—quite logically–that when you are that close, the Sun’s gravity should pull the tail towards it. If the group is large enough, you should also get someone who can argue that the tail should point away from the sun—for now, it doesn’t matter if this is a knowledge-based claim or just a contrarian viewpoint from snarkiest person in the room. Whatever hypotheses are offered, just accept them as proposed solutions and demonstrate what each would look like.

Finally, move to the “outbound” portion of your comet’s orbit. “Our comet now flies on away from the sun, perhaps to return in another century or two. Now, which way should the comet’s tail point?” Again, if you have managed to keep a poker face so far, the most popular answer is likely have the tail streaming behind the comet. As before, accept and demonstrate each of the guesses. If students have reasons for their theories, let everyone hear them. Discussing and justifying hypotheses is an integral part of the real scientific process.

If you have access to a blackboard (oh, well, it’s modern times, so, okayokayokay, you can use your smelly whiteboard or that fancy tablet-linked projector), now is the moment to leave off demonstrating with the model and sketch the competing hypotheses for everyone to see. Your picture will look kind of like this. Please remember to Keep It Messy.

Discussing Possible Tail Directions

Have you ever read one of those annoying mystery stories in which the author leaves you in the dark about a critical fact that solves the entire case? Well, here too, we have denied our puzzle-solvers an important clue. So, tell the group it’s time for a change of topic. But actually what we’re doing is rolling out the narrative twist that makes the whole thing so cool.

Here on Earth, it is air that pushes the streamers on our comet model. But how much air is there out in space? (So little that you might as well say “zero”!) But without air, why should any comet have a tail at all?

What comes out of the sun? You should hear the following answers: heat, light, maybe even radiation. But has anyone heard of the solar wind? The sun blasts out particles, too? The sun is shooting out plasma, protons and electrons flying through the solar system at thousands of miles per hour. This is the solar wind, which blows through the solar system all the time, at thousands of miles per hour. The particles are tiny, not even as big as atoms, so it is an invisible wind. And like wind, it’s not perfectly even, it gusts and changes from moment to moment as the Sun itself changes.

All of those things we named help to make our comets look the way they do. Consider your audience…

Explanation #1: You are all correct. All of that stuff blasting out of the sun–light, radiation, heat, and the solar wind–shove all that stuff leaking out of the comet into a tail. And since all that stuff is coming from the sun, the only way the tail can point is away from the sun.

Explanation #2: All of those answers are correct . . . and they all combine to make a comet’s tail. The heat of the sun warms the comet to free the gases and dust. The solar wind blasts the gases—and the particles in the solar wind also interact with those gases, stripping some of their electrons to make that part of the tail a glowing stream of ionized gas. The radiation from the sun actually can push things, and that pressure is just strong enough to shove those tiny dust particles enough to counteract their tendency to fall towards the sun. And the visible sunlight reflects from the spread-out cloud of dust, making the comet shine in our night sky.

Again, with older/experienced participants, now is the time to clue them in that radiation pressure—the totally cool idea that sunlight itself exerts pressure—exists because light is electromagnetic radiation and electromagnetic radiation is a wave and a wave [http://physics.info/em-waves/] pushes on the objects it encounters. You may not feel battered and bruised by the TV and radio waves powering through you day and night or be physically bowled over by the sunlight forming a gorgeous rainbow. But: it’s enough to push fine grains of dust. The only sad thing about radiation pressure is it’s not common knowledge yet—it’s been proven since 1873.

To represent these solar forces, we need to make a breeze. For that job, a fan does the trick. When we turn it on, it blasts a healthy “solar” wind. (Be sure to experiment in advance with your fan and sample comet–there’s a lot of variation in fan settings.)

Inbound Comet

Hold the comet in the “inbound” position, with the front of the birdie pointed at the Fan Sun. Yes! We were all correct: the tail points behind the comet as it moves towards the sun.

If the fan is strong enough, you can also use the model to hint at how the length of the comet’s tail changes. Far from the sun, the comet has no tail; far from the fan, our streamers dangle to the floor. A little closer in, a real comet’s tail appears as a pale streak behind it; as you approach your fan, the model’s streamers lift up and begin to flutter weakly behind it. Near the sun, the tail stretches out millions of miles behind a real comet’s head; near the fan, the your streamers stretch their full length.

Now, what about when the comet is heading away from the sun? Which way will the tail be pointing, now that we know about the solar “wind”? Nearly everyone will see, now, that it must point away from the sun.

Outbound Comet

Demonstrate that this works: you point the birdie’s nose away from the fan, turn on the blast, and the streamers flow out over the front of the birdie. The shape of the birdie helps emphasize the incongruity of our expectation—that the tail goes behind—with the reality: the solar forces push the tail.

If the class has patience for one more test, add the third question: what happens when the comet is rounding the far side of the sun, and is pointed “sideways”? Hold the comet model perpendicular to the flow of the fan.

Comet At Perihelion

Let everyone see how the tail sweeps out to the side of the comet. It always points away from the sun, no matter what direction the comet is pointing.

Here’s 13 seconds of one model comet in action:

Coming Real Soon: Stage 2

Walking to Pluto: Step 3Walking to Pluto: Step 3

Step 3: Making the Journey

If you skipped Part 1, then you need to know know that in this activity, you will build a scale model of the Solar System as far as Pluto. You will use familiar objects and easy, approximate measurements—mostly simply pacing off distances. This is not a project about being extremely precise; the goal is to develop a strong perception of just how big the solar system is and how small the planets are within that system.

For preparation, you need only to assemble the collection of properly-sized objects listed in the requirements table (See Step 2) and print out the “cheat sheet” you’ll carry on the Walk. A glance at a map of your local area will help you decide which way to take your expedition and to identify some landmarks to stand in for more-distant things like the far edge of the Oort Cloud. To build your own interest and enjoy some discoveries of your own, check out some of the links I’ll include in the references section (Step 4).

You can feel free to substitute alternate model planets, using the scaled sizes as a guide; however, most of the items called for can be found in an average family home, borrowed from classroom parents, or purchased at a very modest outlay. While modern kids may not find the contents of kitchen spice jars terribly fascinating, using an allspice or peppercorn seed as your “Earth” model will give them a lifelong reference point–they’ll be smelling pumpkin pie or watching a chef grind pepper and that spark of memory will remind them of this project.

Because the scaled planets range from the size of a pin point to the size of a jacks ball, it also makes sense to attach each object to something larger, such as an inverted cup or a 4 by 6 index card. If you have access to sports equipment, the bright-colored cones often used for laying out a temporary playing field are helpful. You can position the planet-holder and also tape a “Please Leave Our Experiment Here” sign to the top of the cone. And the bright colors and signs help the explorers to look back and spot the distant planets. Again, be creative! There is no need to run out and buy sports equipment—any handy rock or a brick will do to keep your objects and notes in place.

Here’s my Walk kit, ready to go.

When reviewing the Cheat Sheet, you’ll see that this model describes our solar system as far as the outer edge of the Oort Cloud. However, to go all the way to the Oort Cloud in this model is a journey of 75 miles (100 km), so don’t expect to travel that far. Instead, as part of your preparation, identify a few local landmarks 1 or 2 miles from your start point and also pick some regional and further-off destinations to match the scaled distances for such key locales as the Oort Cloud, the heliopause, the estimated positions of the Pioneer and Voyager spacecraft, the far edge of the Kuiper belt, and our further neighbors in the Universe. If you’re too short on time, the Cheat Sheet includes some general destinations, but your own localized ones will be much more meaningful to the group. If your group won’t have time to walk all the way to Pluto, find out where Pluto would be in that locale and point ahead to that location before you do turn back.

Once in the classroom, before launching your exploratory mission, start with a quick review of the concept of scale. Regardless of your target age group, toys which are also scale models of cars or airplanes or trains are helpful examples. Quickly walk through a sample of numerical proportions to give a sense of how it goes when you are creating your own scale model: for instance, sketch on the board or a sheet of poster paper a rough scale drawing of the classroom room at 1 inch per foot (5 cm per m). Rather than slowing down the project with extra work, prepare for this session by making your own rough measurements of the classroom dimensions in advance—simply pace off the length and width and note any additional features to the room. Remember, the idea is to illustrate your point, not to create an architectural drawing.

Moving on to the Solar System, start with the Sun…an 8-inch-diameter playground ball or an ordinary soccer ball fits our scale. Ask if anyone can guess what size the Earth should be to go with this “Sun”. The guesses are very likely to be way off, because most “models” used in classrooms and the pictures in the textbooks are not at all to scale. In those, Earth is shown as a recognizable ball appearing as much as a tenth the size of the Sun.

Once you have a few guesses on record, share the key data. Write on the board or a flip chart as you go, to keep the presentation lively. (Nothing kills attention like a PowerPoint!) The Sun’s diameter is about 800,000 miles (1400 thousand km), and we’re using an 8-inch (18 cm) ball, so each inch stands for 100,000 miles (or, a cm stands for 75,000 km). The Earth’s diameter is only 8,000 miles (12,700 km). So how big will the model Earth be? It turns out we need something less than 1/10^th of an inch across, only 0.08 inches (0.17 cm). So now you can pass around your “Earth”…a peppercorn will work, so will an allspice seed. (And, yes, you can get away with crumbling up a bit of paper and claiming it’s a spitwad you found.) If you have a spice-jar worth of seeds, everyone can have their own Earth to keep. Let the students take a moment to actually compare the sizes of Earth and Sun. It’s a dramatic difference, nothing like what their textbooks show.

Now it’s time to figure out where the Earth and Sun should be to fit in with this scale. Start by inviting students to guess…they will likely assume you can fit the Earth-Sun model easily inside the room. So now, add the distance data they need and we can “step” through the necessary calculation:

The Earth is roughly 93 million miles (150 million km) from the sun.
In our scale model, that’s 930 inches (2000 cm)
or 78 feet (20 m),
or 39 steps of about 2 feet (40 steps of 0.5 m)

Notes:

In our model we’re using a pace distance reasonably close to the average woman’s step length and not too far off the step length of a child who is supposed to be walking but can’t resist running. If your group is adult men or tall women, you can use the worksheet to adjust the number of steps accordingly.
Our scale in SI (Système international, or metric) is slightly different than in English units, so that those using the SI version can also use simple round figures.

At this point, try to keep a straight face while pretending to start building the model inside the classroom. Dramatically place the “Sun” at one end of the room and try to pace off 39 or 40 steps. Unless you’re doing this activity in a large lecture hall or a cafeteria, you will quickly run out of space (pun intended). By now, it should be clear to the students that this is to be an outdoor activity.

If the group is not too insanely anxious to get outdoors, you can take one more minute to assemble a part of the model which will fit in the room—the Earth-Moon system. Our Moon is nearly ¼ the diameter of Earth, so it’s actually an important body in its own right. And it’s close by. In our scale model, the Moon—which can be represented by a single nonpareil or cake “décor” candy—is 2 3/8” or 5 cm from Earth—so Earth & Moon can be stuck to a card or piece of paper. Keep in mind that if your group is too anxious to get outside, you can choose to save this step for your arrival at the Earth’s position in the model outside.

Earth and Moon are stuck together

Set the very few ground rules for the mission plan. The model is built by counting steps—the students will be the ones to do the counting and you (the project leader) will expect them to try hard and in return will not be too fussy about precision or how the measurement accuracy may be affected when leadership shifts from short to tall students. The group will remain cohesive, so no-one misses out on any important discoveries—and no one will charge ahead lest they get “lost in space”. And everyone should understand the time constraints.

When the group is large, I’ve had success assigning small subgroups to accompany one adult leader as the “vanguard” to each planet, leaving the rest behind until they have “landed,” then allowing the followers to run full-speed to catch up. If you do this, it’s important to ensure everyone has a turn to be in the vanguard at least once. If the students have been studying the planets, the vanguard students can also be asked to provide just a few key bits of information to the other explorers as features they have “discovered” about the planet they just reached. However, resist the urge to turn each stop into a seminar—the goal is to travel as far as possible across the system quickly enough to return before class time ends.

Remind the group that it’s a long walk across the solar system and then get started for real. Carry your Sun to a central location outside. If you can park Sol near a tall landmark (such as a flagpole), you’ll find it easier to point back to the “center of the Solar System” as you move further away. Take your Cheat Sheet in hand (the page from the resource kit listing your step-off distances) and read out the number of steps from the sun to Mercury. Send the Mercury explorer team ahead to place Mercury in its position, and quickly join them with the rest of the group. If the vanguard has some cool facts to share about Mercury, give them time to speak. And move on to Venus and the rest of the inner planets.

The asteroid belt portion is the first region containing many objects. If you pause at Ceres, the biggest dwarf planet in the inner Solar System, it helps reduce the stigma of Pluto being “only” a dwarf planet. The fun part in these “belt” regions is to pretend to dodge the small asteroids or other objects—while you may mention that there really isn’t any significant risk of running into an asteroid, that is no reason to turn down the chance to pretend you’re in a crowded mess of obstacles just like in the movies. Even Neil deGrasse Tyson, in his reboot of Cosmos, includes a sequence in which his Ship of the Imagination zigs and zags through, first, a crowded Asteroid Belt and later a densely-packed Oort Cloud.

If time is short or you are working with younger children, it is reasonable to make it to Jupiter (don’t forget to dodge the asteroids on the way out) point out roughly where the outer planets, Pluto, and the further objects would be found and then head back to Earth.

In any case, carry some ordinary first-aid supplies and be sure to have extra adults on hand to slow down those who want to jump to lightspeed. Don’t worry if you don’t have a straight route to use…twisting and turning your way around the streets of a neighborhood is equally impressive. If time will permit, participants can bring lunches and picnic in the Kuiper Belt before returning. And remember, as you return to collect the planet models, it is just as fun to rediscover the distances on the way back.

View looking down of supply kit: pink powder (gelatin) in a white up, water in a clear plastic cup, a round red plate, two wooden skewers, two marshmallows, four small candies in a small container. — 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 “empty” 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 “empty” 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.)

Assorted small candies in various colors, blue, green, purple, white. — Candy Nucleons

Two dark and two white little candies (close-up) — Candy Nucleons

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.

Close-up of marshmallow with hole poked in center, showing marshmallow-covered tip of wooden skewer. — Pathway for the Nucleus

Close-up of marshmallow, with one dark candy visible inside hole in center, and two white and one dark candy waiting on top. — Pathway for the Nucleus

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 that’s 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 can’t 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 haven’t 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 “shell”, 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 “electron shell” 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.

Marshmallow on skewer dunked into clear plastic cup half-full of water. — Dunk

Wet marshmallow held by skewer on edge of plastic cup of water, drops of water dripping off. — and un-dunk.

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.

Marshmallow on a skewer, being rolled in pink gelatin in a round white bowl. — Adding the Electron Shell

Set the decorated marshmallows aside on a sheet of waxed paper or a plate.

Two gelatin-coated marshmallows, on skewers, set to dry on a round red plastic plate. — Two Finished Helium Atoms

As time permits, participants can make other atoms…stuffing different numbers of protons or neutrons into marshmallows and adding a shell of electrons.

Cooking With Kuiper: The Instruction Set

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