Why could those scientists predict that there were forms of matter that were unknown at the time?’
You answered that question in the preceding sentence – Because there were gaps in the order.
If you have discovered atoms containing 65,66,67,68,69,70,72,73,74,75 and 76 protons....it is reasonable to assume that there exists an element containing 71 protons that fits in the gap.
I think I have missed your point Mr G.
