Click for energy, spend twenty energy on a proton, fifty on an electron, then unlock neutrinos and neutrons and start assembling actual atoms. The goal is every element up to Oganesson, number 118, and the periodic table is the progress bar.
The author states plainly that it deliberately avoids the absurd number scaling the genre is known for: the endgame is 1e18 energy rather than 1e1000. A prestige layer that runs on pressure and neutrons rather than energy is the next thing to unlock. Expect a long, patient game with real chemistry underneath the counters.