Hydrogen

Hydrogen is the lightest element and the most abundant in the universe, making up about 74% of ordinary matter by mass. Its atom is as simple as chemistry gets: one proton and one electron in a 1s orbital. That lone electron gives hydrogen three options. It can lose it to become a bare proton (H+, the basis of acidity), gain a second to become the hydride ion (H-), or share it in a covalent bond. This flexibility is why hydrogen sits awkwardly atop group 1 without behaving like an alkali metal.

On Earth, free hydrogen exists as H2 gas, colorless and odorless, but the molecules are so light that Earth's gravity cannot hold them, so the atmosphere contains only traces. Nearly all terrestrial hydrogen is locked in water and organic compounds. The H-H bond is strong (436 kJ/mol), so H2 is fairly unreactive at room temperature until a spark or catalyst starts things; then it burns or explodes. The world produces roughly 95 million tonnes a year, mostly from natural gas, to make ammonia, refine petroleum and synthesize methanol.

Discovery

In 1671 Robert Boyle noticed a flammable gas bubbling from iron filings in acid, but Henry Cavendish was the first to recognize it as a distinct substance in 1766, calling it "inflammable air". In 1781 he showed that it burns to form water. Antoine Lavoisier repeated the experiment and in 1783 named the element hydrogene, from the Greek hydro (water) and genes (forming): the water-former.