Excellent puzzler! Level 19 is driving me mad and level 25... how can that be possible?! So I'm stuck really. I'll try harder with Level 20 at some point.
It's been 2 years, yet someone may be reading this for hints rather than have the puzzle ruined with solutions.
So with puzzle 19, from careful study you can recognise that it's not possible to clear all the atoms with a single chain reaction.
(Reasoning: Because the 2-electron Helium (up/down) being parallel would require one of the up/down to interact with the middle atoms under your control - but in doing so, the middle absorbs a moving up/down electron that would be necessary.)
Everything is symmetrical, except for that solitary 1-electron Hydrogen atom. Therefore finding how to place that atom is probably the most important first concern. See what chain-reaction patterns would be possible with placing that 1-electron Hydrogen in different areas and activating it with other atoms (starting with the closest neighbouring atom).
One of those patterns is the exact opposite of the chain-reaction patterns you get with the 4-electron Beryllium being the intermediary.
Then boom. You've solved the puzzle.
Embarrassingly, I am stuck on puzzle 25. I dare not look up the solution. I could get it if I could change a horizontally pointed 1-electron Hydrogen into a diagonally pointed upwards 1-electron Hydrogen atom.