Chemists love a mystery as much as anyone. And gaps in patterns can be mysterious. Mendeleev’s period table had famous gaps which led to new discovery. And so from the 1890s onwards, chemists searched for the perbromate anion, BrO ₄ ^– . It represented a gap between perchlorate and periodate, both of which had long been known. As the failure to turn up perbromate persisted, the riddle deepened.