How to play
Pick a tool and tap the box. Proton, Neutron and Electron add one particle; drag before you let go to throw it, and the longer the drag the faster it goes. How many per tap adds up to 200 at once (protons, neutrons, electrons, atoms or molecules), spread out around where you tap. Atom adds a whole neutral atom of any of the 118 elements, picked from a periodic table, and Molecule drops in a ready-made molecule like water or acetylene. Light shines a beam of the color you pick in the direction you drag (tap for a flash every way), and Heat warms or chills just the spot you hold. Wall draws walls, freehand or as straight lines, that everything except light bounces off: make rooms, funnels or a maze, and use Erase to take them away. Move drags things around and shows what you tapped, Zap knocks an electron off an atom, and Erase removes things. The In the box table lists everything in the box with its formula, count and molar mass (g/mol); tap a column heading to sort by it. Element data, under the panels, shows the real numbers the lab uses for each element, and sorts the same way. The temperature slider runs from 10 K, colder than anywhere on Earth, to 100 million K, hotter than the middle of the Sun.
The experiments on the side set up a box for you: build hydrogen from scratch, make water and table salt, burn methane, run a star core or watch radioactive decay. Your box is saved on this device, and so is the list of elements, isotopes and molecules you have made.
Saved setups keeps a box by name so you can load it again later, or download it as a file to keep or share. Record keeps everything that happens in the box (up to two minutes); Watch replay plays it back, and you can pause, drag along it, slow it to a quarter speed, or carry on from any moment. Download recording saves it as a file, and Open a file plays it again.
Things to try
Atoms. Put an electron near a proton. Opposite charges attract, and the electron settles into the proton's shell: a hydrogen atom. A nucleus keeps taking electrons until it is neutral. Shells fill 2, 8, 8, then the 4th, so sodium has one lonely outer electron and chlorine is one short of a full shell. From scandium to zinc the new electrons tuck into the third shell instead, which grows to 18, so those transition metals all keep two outer electrons; then gallium to krypton fill the 4th shell the same way aluminum to argon filled the 3rd, and bromine behaves like chlorine. Further down the table the same pattern repeats with bigger shells: every atom has its real arrangement of electrons, so gold is 2, 8, 18, 32, 18, 1 and cesium gives up its one outer electron even more easily than sodium.
Heavy elements. Past bismuth no element has a stable isotope. Uranium-238 throws out an alpha particle (a helium-4 nucleus) and becomes thorium-234, which beta-decays in turn, the start of the chain that ends at lead. The very heaviest, like californium-254 and most of the elements past 100, can also split in two (spontaneous fission) and send out a few neutrons. Every isotope decays the way it really does, at its real half-life on the lab's sped-up scale.
Bonds. Atoms bond to fill their outer shell: hydrogen makes one bond, oxygen two, nitrogen three and carbon four, so you get H₂O, NH₃ and CH₄. Oxygen and nitrogen pair up with double and triple bonds (O₂, N₂). Helium, neon and argon already have full shells and never bond. Water comes out bent at about 104.5° because oxygen's two unshared electron pairs push the hydrogens together, while carbon dioxide is straight.
Salt. Sodium holds its outer electron loosely and chlorine wants one badly, so when they get close the electron jumps across. Now one is a + ion and one a − ion, and they stick together: a tiny crystal of sodium chloride. The same happens with lithium, magnesium, potassium or calcium next to fluorine, oxygen or chlorine.
Heat. Hotter means faster. Strong bonds survive a kitchen oven but come apart in a flame, so methane and oxygen sit side by side until you heat them, then rearrange into water and carbon dioxide. Above about 10,000 K electrons are torn off atoms and you have a plasma.
Nuclei. Neutrons have no charge, so they slip into any nucleus and make a heavier isotope. Two positive nuclei repel each other, so they only fuse when they hit hard, which takes star temperatures and bare nuclei. Some combinations don't last: too many neutrons and a neutron turns into a proton (beta decay, out comes an electron and an antineutrino); too few and a proton turns into a neutron (out comes a positron, which vanishes with the next electron it meets). A free neutron decays too.
Light. When an atom catches an electron it gives off the energy as a photon, drawn in the color of its wavelength: hydrogen's 13.6 eV is ultraviolet at 91 nm. A positron meeting an electron makes two 0.511 MeV gamma rays flying apart.
Water sticks to water. Oxygen pulls the shared electrons toward itself, so it ends up slightly negative and the hydrogens slightly positive. Those ends attract the opposite ends of other molecules (hydrogen bonds), so cold water clumps while methane drifts apart. A bare proton grabs a spare electron pair on water or ammonia and makes hydronium (H₃O⁺) or ammonium (NH₄⁺), which is what acids do in water.
Energy has to go somewhere. Two atoms can't bond on their own: the bond energy would just throw them apart again, so a third atom nearby has to carry it off. Turn on Insulated box to switch off the heat bath: now making bonds heats the box (hydrogen atoms pairing up can warm it by thousands of degrees) and breaking them cools it. The Measured readout shows the temperature the particles really have, and Pressure compares the push on the walls with the ideal gas law.
Chain reaction. A neutron that reaches a uranium-235 nucleus usually splits it into two middle-sized nuclei and 2 or 3 new neutrons, giving off about 200 MeV. Each of those neutrons can split another nucleus, so the count grows fast: that is a chain reaction, the heat source of a nuclear power plant. About one hit in seven doesn't split the nucleus and just makes uranium-236. Boron-10 swallows neutrons (it becomes lithium-7 and helium-4), which is why reactors have boron control rods. The fragments have too many neutrons and beta-decay, which is where spent fuel's radioactivity comes from. Uranium-238 doesn't split this way; it keeps the neutron and decays twice into plutonium-239, which does.
Light, one photon at a time. Whether light frees an electron depends on the energy of each photon, set by its color, not on how bright it is. Red light never frees cesium's outer electron however much you shine, while ultraviolet does at once: the photoelectric effect, which Einstein explained. Violet light has just enough to split chlorine's weak bond, and ultraviolet splits oxygen, which is how the ozone layer forms. Infrared is soaked up only by molecules whose bonds join two different elements, like carbon dioxide, water and methane; nitrogen and oxygen let it pass. That is why those few gases warm the planet. Gamma rays above 1.022 MeV passing a heavy nucleus can turn into an electron and a positron, and very energetic ones knock neutrons out of nuclei.
Pressure. Pressure is the push of particles bouncing off the walls. Slide the piston down to give the gas less room and the bounces come more often: half the room, about twice the pressure (Boyle's law). A piston coming down also speeds up whatever it hits, so squeezing an insulated box heats it and letting it expand cools it, which is how a bike pump warms up and a spray can gets cold. Turn on Hold pressure and the piston moves by itself to keep the pressure you set, like a balloon: heat the gas and it pushes the piston up. The gauge compares the pressure with the same particles at room temperature with the piston all the way up.
pH. Add acid puts in a bare proton (H⁺), which a water molecule grabs to become hydronium, H₃O⁺. Add base puts in a hydroxide ion, OH⁻. The pH comes from how many of each there are per water molecule: below 7 is acidic, above 7 basic. When H₃O⁺ meets OH⁻ the proton jumps across and they become two waters (neutralization), so equal amounts of acid and base give pH 7. Protons also hop from water to water, which is why acid spreads through water so fast. The box only holds a few dozen water molecules, so one extra ion makes a very strong solution (about pH 0 or 14), much stronger than lemon juice or soap.
How accurate is it?
The rules come from real physics and chemistry, and the numbers are real where it matters: the list of stable isotopes for all 118 elements, atomic weights, each isotope's decay mode, half-lives, electronegativities, ionization energies, electron affinities, atom sizes and bond energies. Bonds break with heat at the rate their real bond energy predicts, and the temperature at which atoms lose electrons comes from their real ionization energy. Fusion between light nuclei makes the real products with the real energy (deuterium + tritium gives helium-4, a neutron and 17.6 MeV, and the neutron takes four fifths of it), atoms come in their natural isotope mix (about one chlorine in four is chlorine-37), momentum is kept in every capture and decay, and partial charges come from electronegativity, giving water about the charges used in common water models.
Some things are simplified so they fit in a browser and on a screen. Real atoms follow quantum mechanics in three dimensions; here they are classical and flat, and shapes like methane's are drawn flattened. Bonds are held rigid at their length and angle instead of vibrating, which is close to how real bonds behave at room temperature. A real nucleus is about 100,000 times smaller than its atom; here it is drawn about a third of the size so you can see it. An electron is 1/1836 of a proton's mass, here 1/5. Half-lives are sped up on a log scale (a 10-minute neutron decays in about 11 seconds, carbon-14's 5,730 years take about 45 seconds), and the hottest temperatures move things more slowly than they really would. Fusion through the Coulomb barrier is a chance worked out when two nuclei touch, and proton-proton fusion is far rarer in a real star. Fission happens to any neutron that reaches a fissile nucleus, fast or slow (real reactors slow their neutrons down first), and heavier nuclei keep a neutron that hits them about a third of the time instead of by their real cross-sections. Light only interacts when you shine it with the Light tool; the light atoms give off themselves just leaves the box. Nobody has made enough of the superheavy elements to measure them, so where there is no measurement the lab uses predicted values.
Is my box saved? Yes, on this device, along with your discoveries. Nothing is sent anywhere.