Group 18 — full outer shells, and almost nothing to prove
Helium, neon, argon, krypton, xenon, radon and oganesson have complete outer shells and consequently almost no chemistry. For sixty years they were believed to be entirely inert. They light our signs, fill our lightbulbs, cool our magnets — and one of them causes lung cancer.
Highlighted in the periodic table
Every member
What defines them
Eight outer electrons (two for helium) is the arrangement every other element is trying to reach. Having got there already, noble gases have nothing to gain from bonding — hence the enormous ionisation energies and near-zero electron affinities.
Alone in the periodic table, they exist as single atoms rather than molecules. With no bonding to speak of, only weak dispersion forces hold them together, which is why they have the lowest boiling points in their periods.
In 1962 Neil Bartlett made xenon hexafluoroplatinate and ended the "inert gas" era. Xenon and krypton form real compounds with fluorine and oxygen; even argon has been coaxed into HArF at 40 K. The heavier the atom, the looser its grip, and the more chemistry becomes possible.
Periodic trends
Down the group: boiling point, density and polarisability rise; ionisation energy falls; and reactivity — such as it is — increases. Helium is the extreme case in every direction: the lowest boiling point of any substance, and the only element that will not solidify at atmospheric pressure no matter how cold it gets.
In the world
Cooling the superconducting magnets in every MRI scanner, pressurising rocket fuel tanks, and leak detection. Also balloons, which is a strikingly frivolous use of a finite resource that escapes Earth permanently.
The orange-red glow of classic signage. Every other colour in a "neon" sign is a different gas or a phosphor coating.
The cheap inert atmosphere: welding shields, lightbulb fill, and the gap in double-glazed windows.
High-performance window fill and specialist lighting. From 1960 to 1983 the metre was defined by a krypton-86 emission wavelength.
Camera flashes, high-intensity lamps, ion thrusters on deep-space probes, and a genuine general anaesthetic.
None — it is a hazard, not a resource.
Handling
Chemically harmless, but all of them displace oxygen: an asphyxiation hazard in confined spaces, and one with no warning symptoms. Inhaling helium from a pressurised cylinder has killed people. Radon is the exception on chemistry grounds too — it is radioactive, seeps out of granite bedrock, accumulates in basements, and is the second leading cause of lung cancer after smoking.
Worth knowing
Common questions
Not entirely. Xenon and krypton form well-characterised compounds with fluorine and oxygen — xenon tetrafluoride is a stable white crystalline solid. Radon is reactive enough to form fluorides, though its radioactivity makes it awkward to study. Helium, neon and argon remain effectively inert under ordinary conditions.
Nearly all terrestrial helium comes from the alpha decay of uranium and thorium in rock, accumulating in natural gas fields over hundreds of millions of years. Once released it rises through the atmosphere and escapes into space permanently — there is no recovery. Supply is genuinely finite on any human timescale, which is why the balloon trade is contentious.
It does not change your vocal cords; it changes the medium. Sound travels about three times faster in helium than in air, so the resonant frequencies of your vocal tract shift upward. The pitch of your vocal cords is unchanged — it is the timbre that moves, which is why the effect sounds cartoonish rather than simply high.
Keep going