discovery and uses of the noble gases

The noble gases were the last whole family added to the periodic table, and they were found by chasing a tiny discrepancy that most people would have ignored. In the 1890s Lord Rayleigh noticed that nitrogen extracted from air was very slightly denser than nitrogen made from chemical compounds. That stubborn fraction of a percent, pursued with William Ramsay, turned out to be a brand-new, unreactive gas — argon, from the Greek for 'lazy' — hidden in plain sight in the air all along.

Once one was found the rest followed quickly. Helium had actually been spotted first not on Earth but in the Sun, as an unexplained yellow line in sunlight in 1868 (hence its name, from helios, the Sun), and was only later found on Earth in uranium minerals. Ramsay and Morris Travers then teased neon, krypton, and xenon out of liquefied air by careful distillation in 1898, naming them from the Greek for 'new', 'hidden', and 'stranger'. Radon, radioactive, came from the decay of radium. Their abundances vary wildly: argon is plentiful at about 0.93% of the atmosphere, while xenon is a trace at under one part in ten million, which is part of why xenon chemistry stayed unexplored for so long.

Their uses flow directly from their inertness and their physics. Argon, cheap and unreactive, shields welding arcs and fills incandescent bulbs; helium, with the lowest boiling point of any substance, cools superconducting magnets in MRI scanners and lifts balloons; neon and the others glow in characteristic colours in discharge tubes; xenon gives intense white flashes and is used as an anaesthetic and in ion thrusters. The honest footnote: helium is a genuinely limited resource — it is light enough to escape Earth's gravity once released, so the helium we use comes from slow radioactive decay trapped underground, and it does not come back.