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. Atom adds a whole neutral atom of the element you pick, from hydrogen to calcium. Move drags things around and shows what you tapped, Zap knocks an electron off an atom, and Erase removes things. 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.
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.
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.
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 from hydrogen to calcium, 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.
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. 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. Only elements 1 to 20 are included.
Is my box saved? Yes, on this device, along with your discoveries. Nothing is sent anywhere.