move mode: tap a letter, tap a square to drop it. tap a placed atom then another square to shift it (its lines clear). tap the tray to take one back.
bond mode: tap two in the same row/column with nothing between them.
each tap adds a line. 1, then 2, then 3, then it clears.
lines can't cross. numbers on top say how many are left.
done = everything placed, all zeros, everything joined up.
click the middle of a line to take one back. u = undo, r = clear.
after 11 it just keeps making them up, harder each time.