It’s in the weird little genre of engineering games, where you design a machine that performs some task, like “fantastic contraption”, “the incredible machine”, or “armadillo run”. They’re never really that popular, but they’re like catnip to me - puzzle games where you invent a solution instead of discovering it.
Here's an interview with the developer that I really liked.
Although it’s called spacechem, it’s only fictionally about chemistry (you’re physically manipulating atoms to perform chemical reactions... in space!). It’s really a programming game – you design a machine on a 2D grid of component machine parts, and the parts have different interaction rules. It’s kind of like visual scripting, kind of like designing a circuit. You watch the machine run, then tweak your design until your machine can perform some task.
The game does a great job of building up complexity gradually – at the beginning of the game you’re given simple components and tasks, and by the end of the game you’re doing things that would have been completely impossible for you in the beginning. It’s the good kind of mind-bending – you feel yourself getting better at the game, and you see your designs getting more efficient and elegant.
The game has a demo if it seems like your kind of thing!