Cold Fusion

Not bad, but give the game a bit more polish. Try adding (maybe futuristic) background music, make the tutorial a bit clearer on what everything is and make indicators if something is about to spawn or despawn. It would also be fun if you make a difference in gameplay as you progress, being a certain atom makes you quicker (like a gas atom with high energy), some atoms make you actract photons better, etc.

Definitely track progress of highest atom achieved, most protons, etc.; create goals to encourage replay. Maybe once you achieve certain atom skip first levels.

Good start! Frequent death and complicated restart makes for low replay, simply click to restart would make game faster. Esc to menu to quit or Q or something will let return to menu. Music would be nice. Tutorial could be in game on first play so you don't have to read it separate, requires you to save progress. Make replay easier will make game more attractive. Controls are really smooth, but faster motion and more collisions will make progress faster.