Argon

Argon was found in a discrepancy in the fourth decimal place. Lord Rayleigh noticed in the early 1890s that nitrogen separated from air was consistently about 0.5% denser than nitrogen made chemically from ammonia, and rather than round the difference away he and William Ramsay chased it, removing the nitrogen and oxygen from air until an unreactive residue remained. They named it argon, the lazy one, and it opened up a whole missing column of the periodic table. It makes up 0.934% of the atmosphere, almost all of it ⁴⁰Ar produced by the radioactive decay of potassium-40 in rock — which is why argon is abundant on Earth but rare in the universe, and why potassium-argon dating works. Industrially it is the cheap inert blanket: welding shield gas, double-glazing fill and the atmosphere inside incandescent bulbs.