Platinum

Platinum's configuration is an anomaly worth noticing — [Xe] 4f¹⁴ 5d⁹ 6s¹, promoting a 6s electron into the d shell — and the resulting electronic structure is a large part of why its surface binds reactants just weakly enough to turn them over and release them, the Sabatier principle in metallic form. That underpins catalytic converters, the hydrogen fuel cell, nitric acid manufacture and much of petroleum reforming. Pre-Columbian metalworkers in what is now Colombia were sintering platinum grains long before Europeans could melt the stuff at 1768 °C. From 1889 to 2019 the kilogram was a cylinder of 90 % platinum and 10 % iridium in a vault at Sèvres, and cisplatin, discovered by accident in 1965, remains one of the most effective chemotherapy drugs ever found.