Hydrogen

Hydrogen is one proton and one electron, and that stripped-down structure is why it is both the fuel of every star and the particle whose concentration we call pH. Henry Cavendish isolated it in 1766 by dropping metals into acid and called it inflammable air; Lavoisier renamed it hydrogen, water-former, after showing that burning it produces nothing but water. On Earth almost none of it is free — it is bound up in water and hydrocarbons — so the roughly 95 million tonnes used each year is manufactured, over 95% of it by steam reforming of methane, which is also why hydrogen is currently a large source of CO₂ rather than a solution to it. Its hydrogen bonds are what give water a boiling point 160 °C higher than its molar mass deserves, hold DNA's two strands together, and make ice float.