TL;DR very nice little game. Really needs better onboarding via tutorial or on-screen tips. You could even just add some more text to the web page, in a pinch.
I love the use of the chemical symbols in the recipe diagrams. Felt nerdy and appropriate for the content. Nice art style; felt very unified and clear. Love the chunky machines.
It took me a while to realize that the point was to split/add the atoms. I was running around looking for an iron deposit for like 8 minutes.
I really want there to be keyboard shortcuts for each build type. Also, it would be great if it kept the item on my cursor after I place it, so building a chain of conveyor belts is less of a chore.
Finally automating construction kits, once I figured out how things work, was really satisfying. :-)
Needing to burn things you're not using was a nice touch. Made me think more carefully about what was going on.
500 is an awful lot to expect the player to create, since creating the first single piece of fuel/O2 means the player has solved the puzzle. I'm about to walk away for a while to let it run until I win.