Illustration by Alex Eben Meyer.One theme I ran into over and over while writing about the periodic table was the future of energy and the question of which element or elements will replace carbon as king. Plutonium seemed a sure winner in the 1950s and 1960s; nowadays it conjures up more horror than hope. As the gloomy prospects of climate change became clear a few decades later, hydrogen—either in nuclear fusion or as a liquid fuel—began to excite people as a replacement for oil. Enthusiasm waned somewhat once the practical difficulties of creating and transporting hydrogen became clear.