Bond Breaker Lessons

Curious to know more about Bond Breaker?

This game is chock-full of science -- from protons to lasers, to Scanning Tunneling Microscopes. Feeling lost? Want to learn more? You've come to the right place.

You are a Proton


Protons are unimaginably tiny particles. Smaller than your goldfish, smaller than the cells in your body, smaller even than atoms. Their size is about 1.6 femtometers, which is 0.0000000000000016 meters across. That’s fourteen zeros to the right of the decimal place, for those of you keeping track at home. Put another way, if you lined up 50 billion protons side-by-side, they would just stretch across a human hair.

Protons are extremely common particles. You’ll find protons in the nucleus of every atom in the universe, their positive electric charge holding the atom together. You’ll find them zooming around the plasma in stars. You’ll even find them sailing wild and free on their own, with no atoms in sight. You start the game as just such a carefree proton.

Read more about protons on the Of Particular Significance blog.

You seem to live Forever


One of the big open questions in physics is whether protons can decay. If we wait long enough, will the proton break apart into smaller pieces? Many particles *do* decay. Radium, Plutonium, and Uranium are common examples of radioactive elements that break apart into other elements if you wait long enough.

So far, though, nobody has seen a proton decay. And there have been experiments out there watching, trying to measure the lifetime of a proton. The latest results seem to show that protons’ average lifetimes must be above 6x10^33 years ( That’s roughly a trillion trillion times the current age of the universe. And yes, I did mean to repeat the word ‘trillion,’ this number is simply that big.

This just means that a proton, if sitting still, practically lives forever. But that’s not to say we can’t destroy a proton if we try.

You may have heard of the Large Hadron Collider (LHC) in Geneva. Recently, it was used to search for and discover the Higgs Boson. The way it works is by smashing two protons together at extremely high speeds, colliding them together with enough energy that they break apart into a bunch of new particles.

Which is to say, in this game, you should strive not only to avoid the spikes -- but it is probably best to keep your proton away from Geneva, too.

Read more about proton decay on Wikipedia, or on Matt Strassler’s Blog.


Protons have a Positive Charge


If you rub a balloon in your hair, you’ll notice two things. a)You hair starts to attract up to the balloon... and b)Your friends give you strange looks. The first of these can be explained by the Electric Force.

This force is created and felt by Electric Charges, of which there are two types: positive and negative. Like charges get pushed apart, and opposite charges attract together. All protons in the universe have a positive charge, which means if you get two of them together, the Electric Force is going to push them right apart.

The Electric Force between two objects gets stronger the closer they are. It’s what’s known as an inverse-square law. If you double the distance between two protons, the force repelling them will drop to one quarter of what it was. You’ll notice this in the game, when you try to push other protons around. They repel you harder the closer you are.

Read more about the electric force on Wikipedia.

Proton + Electron = Hydrogen


So we’ve got protons in the game. A lot of lonely, lonely protons. But it just takes one little particle (an electron) to turn a proton into an Atom.

Atoms, in general, are built of three different types of particles. In the nucleus, or central core, of an atom, you’ll find (positive) protons and (neutral) neutrons. Around the outside of the atom, you’ll find the (negative) electrons. The electric force keeps the electrons from flying away.

In this game, the atoms we’re dealing with most are Hydrogen, the first and most simple element on the Periodic Table. It consists of just a single proton and a single electron.

We often think of electrons as orbiting around the nucleus, like the planets orbit the sun -- in fact, in the game they’re drawn this way -- but this isn’t accurate. In the tiny world of atoms and particles, Quantum Mechanics takes hold. The electron isn’t in one definite spot at any given time, but is instead spread all around the atom. Think of it like a cloud of probability.

Read more about the Hydrogen atom on Wikipedia, or see a cloud of probability.

Hydrogen attracts other Protons


With one proton and one electron, the Hydrogen atom is electrically neutral. (+1 and -1 add up to zero.) And if it doesn’t have an electric charge, it shouldn’t be affected by the electric force anymore, right?


Sure, if you’re a proton very far away, a Hydrogen atom looks completely neutral. But get closer, and the individual parts of the atom become important. The proton in the atom tends to repel you, and the electron will tend to attract you in. Who wins?

For the case of a proton sitting near a Hydrogen, the attractive force wins. Think of the electron as kind of living between the two protons, helping to pull them both in. The full description of what goes on is more complex (and cool), and involves Quantum Mechanics.

Read about the 2 proton, 1 electron system on Wikipedia, or read a clear explanation for why Hydrogens bond together, from the University of Illinois Urbana-Champaign Department of Physics, or read more about the Quantum Mechanics that is going on, as taught by Richard Feynman..

Scanning Tunneling Microscopes


Electrons, like protons, are all around us -- along with the neutron, those three particles are the building blocks of all atoms. However, if you want to add a single electron to a single atom or molecule, you’ll need a very specialized tool. At CaSTL, that tool is the Scanning Tunneling Microscope.

Microscopes that you are most familiar with use lenses and light to help you see things closer up. This works great for looking at butterfly wings or amoebas. But you can only zoom in so far with light. Not because of any technical limitations, but due to the laws of physics itself. Compared to an atom, a light wave is huge. Trying to take a picture of an atom using light would be light trying to pick up a grain of sand while wearing oven mitts. You just wouldn’t have the precision.

If we can’t use light to look at an atom, we’ll need to get more creative, and a Scanning Tunneling Microscope (STM) does just the trick. Instead of light, it uses the electric force and quantum mechanics. A STM has a very sharp, very fine metal tip that we bring very close to the object of interest. Then we apply a voltage between the two objects. If the STM were touching the object, electrons would flow between them. Instead, we increase the distance to the point where no electrons should be able to make the jump. But due to quantum tunneling, some *can* make it across.

By measuring how many electrons jump across the gap, we can figure out exactly what the distance is.

This makes an STM great for making what is basically a topological map of an object -- drawing the nano-scale peaks and valleys. It also makes the STM a useful tool for adding electrons to a very precise spot, like a single molecule.

Learn more about STMs from Wikipedia, or read about the limits of optical microscopes.



Molecules are a group of atoms, held together by bonds. Even though you often see bonds depicted as little sticks, the reality is far more complex, involving both electric forces and quantum mechanics.

Take the very first molecule you can make in Bond Breaker. It consists of two protons and a single electron. With it, you can make H2+, the positive ion of the Hydrogen molecule.

At a very simple level, you can think of the electron as acting like glue, attracting the two protons together. The electron doesn’t really exist in a single spot, but is spread out in a cloud of probabilities -- places where you might find it. In this molecule, the electron tends to spend most of its time between the two protons, and since negative charges attract positive charges, the electric force can keep the molecule together. The protons do repel from one another, but if they are far enough apart, that repulsive electric force is weaker.

If they get closer than this ‘bond length,’ though, then the repulsive force of the protons grows big enough to push the atoms apart once again. This is what makes the molecule stable. The protons cannot easily get closer or further apart, so they are locked together in their molecular bond.

This description, while being useful, is oversimplified. To really understand what is going on, we need to understand more complexities of Quantum Mechanics. For instance, according to QM, we can think of the electron as bouncing back and forth between the two protons - even when they are very far apart. For a neat description of this, check out the Feynman Lecture linked below. Or, if you care more about the details of how strong the atoms attract or repel, read more about the Morse Potential, which we used in this game to calculate the atomic forces.

Read more about the Quantum Mechanics that is going on, as taught by Richard Feynman, or learn what the Morse Potential is on Wikipedia.

Molecular Ions


An ion is an atom or molecule that has a non-zero electric charge. If we take a neutral Hydrogen atom, and strip it of its electron -- we get a positive Hydrogen ion, which is just a proton.

A neutral Hydrogen molecule consists of two protons and two electrons (H2). Remove one of the electrons, you’ll have a positive Hydrogen molecular ion (H2+). The protons aren’t held together quite as strongly, making H2+ is easier to split than H2. The protons also tend to stay a bit further apart from one another.

On the other hand, if you add an extra electron, you’ll get the negative ion (H2-). How does this behave? Well, with too many negative charges, it turns out the molecule is no longer stable, and it’ll break right apart.

Read about the 2 proton, 1 electron system on Wikipedia.

Two’s Company, Three’s an Explosion


As you’ll notice, your Hydrogen molecule can’t have any arbitrary number of molecules. With a single electron, you’ve got a stable ion (H2+). With two electron, you have an even more stable molecule (H2). But with three electrons, the molecule will split apart (H2-).

Three electrons are simply too many for the Hydrogen molecule to hold. The exact reason for the instability gets pretty complex, involving electric forces, quantum mechanics, and Coulomb repulsion. Basically, though, in the constant tug-of-war between attraction and repulsion: repulsion wins.

Are Atoms Mostly empty Space?


You may have heard people say that an atom is mostly empty space.

Comparing the size of the Hydrogen atom and the size of its nucleus (the proton), we find that the width of the atom is about 60,000 times larger than the width of the nucleus. This means if the nucleus were the size of a quarter, the atom would be a mile across. If the nucleus were the size of the sun, you’d find the electron ten times further away than Neptune.

Which is to say: the game is not drawn to scale. The proton is drawn HUGE, so that you’re able to see it. But in reality, it should really take up far less than a single pixel.

A great description the Jefferson Lab on how absolutely empty atoms are.

Chemistry at the single molecule scale


The CaSTL research group’s name stands for: Chemistry at the Space Time Limit. **At the Space Time LIMIT**!


Researcher here are trying to do chemistry on both the smallest scales (of single molecules) and the shortest time spans (“quick, hit it with the laser before it moves!”). To read more about their research, go here.

Separating atoms and molecules?


In the game, there are green and pink force fields that separate out the atoms from the molecules, but in the real world, keeping these things straight is harder.

One of the best tools to turn to would be a Mass Spectrometer. This device, as you might well guess given its name, helps you figure out the mass of the molecules you’re dealing with. Ions (molecules or atoms with an electric charge) are sent hurtling through a magnetic field, which would tend to deflect them. The less massive the ion, the more its path gets bend. It’s as if you had two trucks, one filled with lead, and the other empty. One way to tell them apart would be to see how easily you could speed up, or turn the truck.

Learn more about Mass Spectrometry on Wikipedia.

Laser Light and Energy Levels


So far in the game, we’ve been dealing with atoms in their ground state -- the electron has as little energy as possible. Using lasers, we can give the electron an extra kick of energy. More energy means the electron can live further out from the nucleus -- it’s able to ‘fight’ the pull of the proton better.

According to quantum mechanics, electrons can’t exist at just any radius, though -- it can only be in discrete energy levels. It can be in the first level, the second level, and so on. You’ll never find it at level one-and-a-half.

This means that when we hit the electron with some laser light, we need to make sure we’re giving it the right energy. If it gets exactly the energy it needs, it makes the transition, otherwise it doesn’t. For instance, in the case of a Hydrogen atom, to go from the first energy level to the second, the electron needs 10.2eV. (eV, or electron Volts, are a measure of energy.) If we send in 5 eV, the electron won’t go anywhere, and the laser light will pass right by. If we send in 12 eV, the electron still won’t budge. It needs 10.2 eV to make the jump.

Each color of light is associated with a certain energy - so to get the 10.2 eV we need, we’d need to use an ultraviolet laser.

Read more about energy levels by the Sloan Digital Sky Survey here, or get a more math-y description from Hyperphysics.



The energy levels an electron can be in go higher and higher. But the energy of each successive level gets closer together, and there is an upper limit to the possible bound energies the electron can have. Give an electron *too* much energy, it can fly right off. This turns the atom into an ion, hence the term: ionization.

The binding energy for an electron is the energy it would take to tear an electron in the ground state completely away from the atom. In the case of Hydrogen, the binding energy is 13.6 eV. If we give the electron more energy than this, it can escape.

So take our ground state electron from before. If we shoot it with a laser once, we add in 10.2 eV, which brings it to its first excited state. If we shoot it again, we’ve added a total of 20.4 eV, more than enough to ionize the atom.

Read more about ionization energy from Wikipedia.

Colors are Important


Different colors of light are associated with different energies. Basically, how much energy each photon (particle of light) carries. The ultraviolet lasers in this game send out photons with 10.2 eV of energy each. The orange lasers consist of photons with 1.89 eV each, precisely enough to get the electron to jump from the second level to the third.

You need to choose your lasers carefully. If you have a ground state Hydrogen atom, an orange laser won’t do anything to it. (1.89 eV isn’t enough energy to make the first jump, so nothing happens.) If you have a Hydrogen atom in the first excited state, a violet laser will ionize it.

Read about some of the colors of like that Hydrogen can absorb on Wikipedia.

Laser beams are Huge


Picture a cat chasing the red dot of a laser pointer. The laser seems incredibly precise, a tiny little speck of light. But if you get close to it, you see it is in fact a little circle a few millimeters across -- which means it hits many millions of atoms.

More advanced lasers can focus their light into even tighter beams than this, but even there, there is a limit. Due to diffraction, the best you can hope to focus a laser is roughly to the size of its wavelength. Our ultraviolet laser light has a wavelength of about 120 nanometers, which means even with the best laser imaginable, it’d hit a circle about that big. That’s 1000 times the size of a hydrogen atom - which is why you can’t hit just a single atom with a laser.

Read more about diffraction on Wikipedia.

Real colors of Laser Light


In the game, you use two different colors of lasers: ultraviolet light with a wavelength of ~120nm, and orange light at ~650nm. How does this compare to what the scientists actually have in their labs?

The super-fast lasers that CaSTL researchers need for their work tend to be Titanium Sapphire lasers. These operate only at certain wavelengths, generally longer than about 500nm, which means you can make visible and near-infrared light.

The verdict? Those ultraviolet lasers only exist in the game, at least for now. The orange lasers, however, do exist.

Some of their lasers with longer wavelengths aren’t used to electronically excite electrons (by jumping them to a higher energy level), but instead to thermally excite molecules. These lasers add in heat energy, much in the same way that your finger does in the game; getting molecules to wiggle and move.

Learn more about the Titanium Sapphire lasers we use on Wikipedia.

The Heat is On


One of the ways we can manipulate molecules is by adding thermal energy, which really just means cranking up the temperature. When we’re talking about the tiny world of particles, higher temperature means more random, bouncing kinetic energy.

What does this thermal energy look like in atoms and molecules? Well, it can mean that the particles simply move around quicker. And for atoms, that’s really the only place the energy can go.

For a molecule, there are more possibilities. In addition to moving faster, the molecule can also vibrate and rotate. Vibration means that the two atoms oscillate further apart and closer together. And rotation means that the molecule spins around its center of mass. In the case of H2, that’s all we have. For larger and more complex molecules, you can have even more different types of jiggling and wiggling going on.

Just as you can stretch a rubber band far enough apart that it breaks, if you give molecules enough of this thermal energy, you can even tear the atoms right apart.

Check out these beautiful drawings of molecules vibrating, or read a more modern take on the topic from UC Davis.

Van der Waals Forces


We’ve seen how neutral atoms can stick together into molecules, but it turns out that neutral molecules can attract together as well.

Intermolecular forces between two neutral molecules are known as Van der Waals forces, and they tend to be very weak. In the game, you’ll notice that H2 molecules tend to drift together. This is caused by a type of VdW force known as the London Dispersion Force.

H2 molecules are electrically neutral, so at first glance, it might seem as if the electric force would have no reason to pull the molecules together. But the electrons in the molecule are constantly moving around, and sometimes you’d find them on one side, other times on the other side. At any given moment, it’s likely that the net electric charge in a molecule won’t be completely balanced. One side will be more negative and the other side will be more positive.

This means that the positive end of one H2 can start attracting to the negative end of another H2. It’s extremely weak, not only because the asymmetry in any given molecule is minor, but also because it is constantly changing. But in certain cases, even this weak force can be enough to bring molecules together.

A great video from Khan Academy that describes the different types of Van der Waals forces.

Intermolecular forces, like liquids and geckos


Van der Waals forces attract molecules together, but they are so weak that it’s often hard to notice them. If you place your hand against a wall, it doesn’t stick there. But not all intermolecular forces are created equal.

Take water, for instance. Water molecules do an amazing job sticking together, existing as a liquid at room temperature. This is because they attract with a particularly strong VdW force known as a Hydrogen Bond. (Note: this isn’t like the other bonds we’ve talked about, it’s just an attractive force.) Water molecules are extremely polar, with a negative side and a positive, which makes their intermolecular force extremely strong.

Other times, it isn’t so much the strength of each force, but the sheer number of molecules you get near one another. Geckos are able to climb walls by simply placing their hands against them. Why can they do it, and we can’t? Their fingers aren’t as smooth as ours, on close inspection, they intricately feather out into ever smaller hair-like ‘branches,’ which gives their feet an extremely large surface area. By having more molecules close to the molecules in the wall, they can climb by using the VdW force.

Read more about Van der Waals forces on Wikipedia.



It turns out that even though electrons, protons, and neutrons are extremely common, they are not the only types of particles out there. Electrons, in fact, have two similar particles -- alike in every way, but much more massive: the muon and the tau. A muon has 200 times the mass of an electron, while the tau has a whopping 4000 times the electron’s mass.

The muon is also relatively common. At this very moment, there are quite a few muons raining down on you from space, for instance, created in high energy particle collisions in the upper atmosphere. They don’t tend to last very long, though, with a lifetime of around a millionth of a second. (The tau fares even worse, living only about a billionth of a second.)

Since muons behave exactly like electrons (except for their mass and short lifetimes), you could make a new type Muonic Hydrogen atom out of a proton and a muon. This would be like a standard atom in some ways. It would be electrically neutral. Just like with an electron, you could use lasers to boost up the muon’s energy level. You could even ionize atom, as before.

But since the muon is so massive, there are some significant changes. For one thing, the muon lives much closer to the nucleus than an electron. The size of an atom is determined in part by the Heisenberg Uncertainty Principle, which connects what we can know about position and momentum. Since the muon is more massive, it has a larger momentum than an equally fast electron. This means that we can know more about the position of a speedy muon than a speedy electron, and thus it can live closer in towards the nucleus.

The distance is in precise proportion to the mass -- which means the muon lives 200 times closer to the proton than the electron would.

Learn about Muons on Wikipedia, or about all sorts of interesting muonic-matter from ChemistryWorld, or how Muonic Hydrogen was recently used for some cutting edge science about the size of protons.

The Muon-Helium Trick


If Muonic Hydrogen atoms act so differently, it’s probably no surprise that substituting a muon would affect Hydrogen molecules, too. Since the Muons live 200 times closer to the protons, the Muonic H2 molecules have a much closer bond length, too. So close, in fact, that there’s a high chance that the two protons would get near enough to fuse.

Fusion occurs when the strong force is able to combine groups of Hadrons (protons or neutrons) together. This happens routinely in stars, which fuse Hydrogen into Helium under the extreme pressure created by gravity. Turns out, just adding a muon into H2 can do the very same thing, without the need for a star!

Sadly, this isn’t a step on our quest towards cheap and safe energy through fusion. Creating muons takes a fair amount of energy, and they last a very short time. Nonetheless, a surprising way to create fusion!

Read more about Muons causing fusion on Wikipedia.

Plasmonics and Nanospheres


Nanospheres? Plasmonics? Weird words, real science.

Nanospheres just refer to tiny nano-scale spheres of metal. They tend to be about 30nm and 100nm across, which means they are only a few hundred times bigger than a Hydrogen atom. They are small, they are good conductors, and importantly, you need two close together.

When a laser strikes two nearby nanospheres, it sets up a positive feedback loop. The electromagnetic waves of laser light cause the electrons in the nanospheres to slosh back and forth (the term Plasmonics refers to this sloshing electron motion). These accelerating electrons, in turn, create more light waves. The end effect is that you get a much stronger pulse of light between the nanospheres.

This can be helpful when you are trying to use laser light precisely. Recall from above that laser beams are huge compared to an atom. Even with a perfect laser you cannot focus the light smaller than the diffraction limit (roughly half the wavelength of the light, many thousands of Hydrogen atoms across).

Nanospheres let you sidestep this limitation. It no longer matters quite how spread out the laser light is coming in, it will be focused and amplified in the gap between the two spheres. This lets you aim your laser light much more precisely than you otherwise could.

Read more about Plasmonics on Stanford’s website.