Really challenging, which is what makes it a fun title!
I guess this one is more of a "start out joined" and "goal is to separate them"
Most of the challenging aspects of it seem fair to me. Although, I feel like a slow atom slowly moving to you and taking away an entire health point can be frustrating. A kind of frustrating that might get me to ask for that health point back. The difficulty is dodging with the high turning speed of the object.
But despite those opinions I wrote, I think that's the charm and even the point of this game. The difficulty of splitting atoms is supposed to be hard, and getting used to it is where it's at.
For that kind of difficulty, I think a gameplay loop of this should come with something rewarding and satisfying. Like when you split an atom, if it created a sound effect to make atom splitting more addicting (In a good game design kind of sense).
But in the end, GOOD GAME DESIGN HERE, THE GAME WORKS, and that is a big thumbs up from me!