I (as a physics guy) love the idea for your game! One thing I would change is to allow the player to make mistakes. Right now, you cannot really lose, just win less. It would be nice to be able to combine two elements even if their electron counts do not match up and have some randomized destructive reaction happen. I understand that graphics were not you key concern here, but for your next game, I would recommend you to at least create a higher contrast between the letters and the background colour of the ion for accessibility reasons. People with vision restrictions will have a hard time playing your otherwise very educational game. Overall, you managed to explain a rather complex topic very simply, great educational value!