Why the Noble Gases Don't React (and When They Do)

Full outer shells explained simply, how argon was found hiding in air, what each noble gas is used for, the 1962 xenon surprise and the oganesson puzzle.

The six elements in the final column of the periodic table, helium, neon, argon, krypton, xenon and radon, are famous for doing almost nothing. They are colourless, odourless gases that refuse to burn, rust, dissolve or combine with anything much. That laziness earned them the name "noble". Yet a whole group of elements hid in plain sight until the 1890s, and their supposed inertness turned out to have limits.

Full outer shells, explained simply

Every chemical reaction comes down to electrons moving between or being shared by atoms. Atoms react to reach a more stable arrangement of electrons, and the most stable of all is a full outer shell.

Helium has two electrons, which completely fills the first shell. Neon has ten, arranged 2,8, and argon has eighteen, arranged 2,8,8. In each case the outermost shell holds as many electrons as it can. There is no gap to fill and no spare electron to give away, so a noble gas atom has no incentive to do anything.

This shows up in the numbers. The energy needed to pull an electron off a noble gas atom (its first ionisation energy) is the highest in its row of the table, and helium's is the highest of any element. At the same time, an extra electron would have to start a brand new shell further from the nucleus, so there is nothing to gain by accepting one. High ionisation energy plus no appetite for extra electrons equals no reactions under ordinary conditions.

The same idea explains why the noble gases exist as single atoms rather than molecules. Oxygen atoms pair up into molecules because sharing electrons fills their shells; a helium atom is already full, so it floats about alone. The only forces between noble gas atoms are very weak, which is why their boiling points are so low. Helium boils at about minus 269 degrees Celsius, a few degrees above absolute zero, and even xenon boils at around minus 108 degrees.

Hiding in the air: a late discovery

Chemists had studied air for over a century before anyone noticed that about one per cent of it was an unknown gas. The clue came from careful weighing. In the early 1890s the physicist Lord Rayleigh found that nitrogen extracted from air was very slightly denser than nitrogen made from compounds such as ammonia. The difference was tiny but consistent, and Rayleigh could not explain it away.

He teamed up with the chemist William Ramsay. Ramsay removed everything he could from a sample of air: oxygen with hot copper, nitrogen with hot magnesium, plus the carbon dioxide and water. A small bubble of gas remained that would not react with anything. In 1894 they announced it as a new element, argon, from the Greek word for "lazy" or "idle".

There was no column for argon on the periodic table, so Ramsay reasoned that a whole family must exist, and went looking. In 1895 he found helium in a uranium mineral; it had been seen in the Sun's spectrum decades earlier, hence its name, but never on Earth. In 1898, working with Morris Travers, he separated neon, krypton and xenon from liquefied air by boiling them off one at a time. Radon followed around 1900 as a gas given off by radium. Within about six years an entire new group had been added to the table, and Ramsay and Rayleigh each received a Nobel Prize in 1904.

What each noble gas is used for

Being unreactive is exactly what makes the noble gases useful: you reach for one whenever you need a gas that will not interfere.

• Helium is lighter than air and does not burn, so it lifts party balloons and airships safely. Its more important job is as a coolant: liquid helium keeps MRI scanner magnets at the very low temperatures they need. Divers also breathe it mixed with oxygen on deep dives.
• Neon glows red-orange when electricity passes through it at low pressure, hence "neon signs". Other colours come from other gases or coatings, but neon gave the technology its name.
• Argon is cheap because there is so much of it in air. It fills light bulbs so the hot filament does not react, blankets molten metal during welding, and sits between the panes in double glazing because it conducts heat less well than air.
• Krypton is used in some high-performance bulbs and in certain lasers. For a time in the twentieth century the metre itself was defined by a line in krypton's spectrum.
• Xenon gives an intense white light in camera flashes and some car headlamps. It is also a surprisingly good anaesthetic, though too expensive for routine use. Its heavy atoms also fuel the ion thrusters that push many satellites and space probes.
• Radon has almost no everyday uses because it is radioactive. It is now mainly of interest as a hazard, which we come to below.

Try it on Periodic Bubbles. Pop the xenon bubble and compare its boiling point and ionisation energy with helium at the top of the group. The whole family shares one colour on the bubbles view, so it is easy to spot.

1962: the year xenon reacted

For more than sixty years after their discovery, the noble gases were taught as completely inert, and the name "inert gases" was widely used. A few chemists, including Linus Pauling in the 1930s, suggested the heavier ones might form compounds with fluorine, but early attempts failed and the idea was dropped.

The breakthrough came from a young chemist, Neil Bartlett, working in Vancouver. He had found that platinum hexafluoride, an extraordinarily aggressive compound, could rip an electron from an oxygen molecule, and he noticed that xenon holds its outer electron about as loosely as oxygen does. If platinum hexafluoride could oxidise oxygen, it should oxidise xenon.

In 1962 he mixed the two gases and watched a yellow-orange solid form at room temperature. It was the first true compound of a noble gas. Within months other groups had made several xenon fluorides simply by heating xenon with fluorine. The surprise was how easy it was once someone tried.

The explanation fits the picture of full shells. Xenon's outer electrons are far from the nucleus and shielded by four inner shells, so its ionisation energy is much lower than helium's. It is still high, which is why only the most electron-hungry partners such as fluorine and oxygen can pull an electron off. Krypton forms a difluoride under harsher conditions, and a single argon compound has been made near absolute zero. Helium and neon have never formed a stable compound. "Noble" turned out to mean "very reluctant", not "impossible".

Radon: the noble gas in your basement

Radon forms continuously in the ground as uranium and thorium in rocks decay. Being a gas, it seeps upwards through soil and cracks, and being a noble gas, nothing on the way traps it by reacting with it. Outdoors it disperses harmlessly. Indoors, especially in buildings on granite, it can build up to many times the outdoor level.

The danger is mostly what radon turns into: within days it decays into solid radioactive elements such as polonium, which stick to dust and can be inhaled deep into the lungs. Public health bodies recognise long-term exposure as the second largest cause of lung cancer after smoking.

In the UK, parts of Cornwall, Devon, Derbyshire and other granite or limestone regions have elevated radon, and published maps show the affected areas. Testing is cheap: a small detector sits in a room for a few months and is then posted off for analysis. If levels are high, a fan-driven sump under the floor or better ventilation usually fixes it.

Oganesson: a noble gas that may not be one

Element 118, oganesson, sits at the bottom of the noble gas column, so on paper it should be the heaviest member of the family. Chemists are far from sure it will behave like one.

Only a handful of oganesson atoms have ever been made, in a particle accelerator, and each survives for less than a thousandth of a second. That is far too brief to measure a boiling point or run a reaction, so everything we say about its chemistry comes from calculation.

Those calculations point to something odd. In an atom with 118 protons, the inner electrons move so fast that relativistic effects become large. The shells are pulled and reshaped, and the neat "full outer shell" that protects lighter noble gases becomes smeared out. Predictions suggest oganesson would be a solid at room temperature, far more reactive than xenon, and perhaps even a semiconductor. If so, the last element in the noble gas column would be neither noble nor a gas: a reminder that the periodic table describes patterns, and patterns have edges.

Key takeaways

• Noble gases are unreactive because their outer electron shells are full, giving very high ionisation energies and no reason to gain electrons.
• Argon was found in 1894 when Rayleigh and Ramsay chased a tiny density discrepancy in nitrogen; the rest of the group followed within about six years.
• Their inertness is precisely why they are useful, from helium cooling MRI magnets to argon shielding welds and xenon powering ion thrusters.
• In 1962 Neil Bartlett made the first xenon compound, proving the heavier noble gases can react with very strong oxidisers such as fluorine.
• Radon accumulating in buildings is a genuine health hazard, especially in granite regions, and is easy to test for.
• Oganesson is predicted to be a reactive solid, so the bottom of the group may break the pattern entirely.