I'll frequently have a board why my lowest element is Carbon or Nitrogen (6 or 7), but the game still spawns Hydrogen and Helium (1 and 2). These low elements frequently don't come in pairs, so I end up with a straggling element that can't be combined with anything and I just have to wait for a - to get rid of it. This also happens at higher levels, where the highest element might be Cobalt (27) but Aluminum is still being generated (13).
At what point should I stop seeing lower numbered elements? Is this tied to the lowest element on the board, or the number of turns taken, or some other thing?