Can Noble Gases Form Compounds? Yes, but Only With a Lot of Persuasion
Yes, some noble gases form compounds, mainly xenon, with fluorine or oxygen. Here is what has been made, why helium and neon refuse, and why it matters.
Yes, some noble gases can form compounds, but only with a lot of persuasion. Xenon does it most readily, krypton manages a little, argon does it only in a freezing laboratory trick, and radon probably does it too but is far too radioactive to study comfortably. Helium and neon have never been made to join a stable compound at all. So the old name "inert gases" was wrong, and the lesson is one of chemistry's best: always read the small print on "never".
Picture the noble gases as the aloof guests at a party, standing by the window with their drinks, politely ignoring everyone. For decades chemists assumed they would never talk to anybody. Then one very persistent guest, fluorine, walked over and would not stop chatting. This article is the story of how that conversation went.
Why were noble gases called inert gases?
"Inert" means unreactive, and for good reason. When the group was discovered, nobody could get helium, neon, argon and their relatives to react with anything. Not with acids, not with metals, not with a roaring flame. They just sat there being gases.
The explanation seemed neat. Atoms react by gaining, losing or sharing electrons (the tiny charged particles that surround the nucleus). Their outermost electrons, called valence electrons, decide how an atom behaves. A noble gas has a full outer shell: helium has 2 electrons, which fills its only shell, and the others have 8 in their outer shell. A full shell is a very comfortable arrangement, so these atoms have little reason to change. If that is new to you, valence electrons and ion charges explains how ions get their charges, and electron configuration explained shows how to write the arrangement as a code such as 2s2 2p6.
It was a lovely rule of thumb. Unfortunately, "unlikely" is not the same as "impossible", and chemists are professionally rude about the word "impossible".
Who made the first noble gas compound?
In 1962 the chemist Neil Bartlett, working at the University of British Columbia in Canada, made what is usually called the first noble gas compound. It involved xenon and a very aggressive compound called platinum hexafluoride, and the orange-yellow solid that resulted is generally described as xenon hexafluoroplatinate.
The back story is a good one. Bartlett had been studying platinum hexafluoride, a compound so greedy for electrons that it could pull an electron off ordinary oxygen molecules. He then noticed that xenon's energy cost for losing an electron (its ionisation energy, which you will meet in the table below) was very close to that of oxygen molecules. If platinum hexafluoride could take an electron from oxygen, why not from xenon? So he tried it.
A small honesty note: later work suggested the real product was messier than the neat formula first reported, and chemists still describe it in more cautious terms than the headline. But the key point stands. A noble gas had been persuaded into a compound, and the textbooks needed a rewrite. Within a short time of that 1962 result, other groups made xenon fluorides, and the field opened up.
What noble gas compounds exist?
Xenon is the star. It forms a whole small family of compounds, mostly with its two favourite partners, fluorine and oxygen, the two most electron-hungry elements around. The ones to know:
- Xenon difluoride (XeF2): a white crystalline solid at room temperature, and a handy fluorinating agent in laboratories (a reagent that passes fluorine on to other chemicals).
- Xenon tetrafluoride (XeF4): another solid, made by reacting xenon with fluorine under the right conditions.
- Xenon hexafluoride (XeF6): the most fluorine-rich of the three, and the most reactive.
- Xenon oxides and related compounds: xenon trioxide is the best-known. These are made indirectly, not by bubbling xenon through oxygen, and they are famously touchy. This is a topic for specialist labs, not school benches.
The counting is tidy, by the way. Xenon bonds to two, four or six fluorine atoms, always an even number. That pattern is a clue to how the bonding works, which we will get to shortly.
Krypton is the next most willing. Krypton difluoride (KrF2) was made not long after the xenon compounds, but it is far less stable: it is a fragile thing that needs careful handling at low temperatures. Krypton is clearly the shy cousin of the family.
Argon has a famous compound called argon fluorohydride (HArF), reported in the year 2000 by a research team in Finland. The catch is huge: as far as is known it survives only at extremely low temperatures, around a few tens of kelvin (the kelvin scale starts at absolute zero, and a cold day on Earth is about 270 kelvin, so this is far colder than anything in nature on Earth), trapped in a frozen solid. Warm it up and it falls apart. So argon "forms a compound" in roughly the way a snowman "lives in the Sahara".
Radon is expected to be the most chemically willing of all the natural noble gases, and some radon compounds have been reported. The trouble is that every radon atom is radioactive and decays quickly by chemical standards, so there is hardly any time to catch it in the act, and the radiation is a hazard. Much of what is known comes from tiny quantities and careful indirect tests, so treat the detail with caution. For the full story on that element's radioactivity, see which elements are radioactive.
The heaviest member of the group, oganesson, has been made only a few atoms at a time and lasts a fraction of a second, so nobody has made a compound with it. Its chemistry is a matter of prediction, not measurement.
Why do the heavier noble gases react more easily?
It comes down to how tightly the outer electrons are held. Going down the group, each element has an extra shell, so the outer electrons sit further from the nucleus. They are also shielded by all the inner electrons in between. Distance plus shielding means the outer electrons are held less firmly, and a fluorine atom, with its famous appetite for electrons, has a better chance of tugging one loose.
The energy needed to remove the outermost electron from a gas atom is called the first ionisation energy, and here is what the data on this site says for the whole group, in kilojoules per mole (kJ/mol, a standard way of measuring energy per huge pile of atoms):
| Noble gas | Symbol | Atomic number | First ionisation energy (kJ/mol) | Known compounds? |
|---|---|---|---|---|
| Helium | He | 2 | 2372.3 | None known that are stable |
| Neon | Ne | 10 | 2080.7 | None known that are stable |
| Argon | Ar | 18 | 1520.6 | Very few, only at extreme cold |
| Krypton | Kr | 36 | 1350.8 | A few, mostly fragile |
| Xenon | Xe | 54 | 1170.4 | Yes, many |
| Radon | Rn | 86 | 1037 | Some reported, hard to study |
Read down the ionisation column and the numbers fall steadily, from over 2300 for helium to about 1000 for radon. Helium grips its electrons more than twice as hard as radon does. Xenon sits in the sweet spot where the grip is loose enough to be persuaded but the atom is still stable and easy enough to work with. If you want to see this trend across the whole table, periodic trends explained has the full picture, and the pair pages argon vs krypton and xenon vs radon put the neighbours side by side.
Try it on Periodic Bubbles. Open the bubbles, choose Colour: ionisation energy from the colour menu (on a phone, tap Display first) and look at the right-hand edge of the table. Helium should stand out as the highest of all, and the noble gases get steadily less extreme as you go down the group. That is the tug-of-war made visible.
Why can't helium and neon form compounds?
Because they are the toughest nuts to crack. Helium has only two electrons, both very close to a nucleus with two protons, and there are no inner electrons to shield them. Neon has its eight outer electrons in the second shell, still very close to the nucleus. Their ionisation energies are the highest in the table above, so no ordinary partner can pull enough electrons away to make a stable bond. Even fluorine, the most electron-hungry element, is not strong enough in normal conditions.
Scientists have found fleeting, strange combinations in extreme settings (for example, charged particles that exist for a moment in a lab or in space, and compounds that theory predicts under crushing pressures). But nothing that you could put in a bottle at room temperature. So "no known stable compounds" is the correct, careful phrase, and "never" is a word to avoid. Chemistry has embarrassed that word before.
The two lightest noble gases are gases you can trust to just be gases: helium fills party balloons and neon lights up signs (see noble gases explained for the family tour).
How do noble gas compounds actually bond?
This is where the old "full shell" story needs an upgrade. In xenon difluoride, xenon shares electrons with two fluorine atoms. A xenon atom has eight electrons in its outer shell, but the atom is large and its outer electrons are loosely held, so they can be shared with fluorine. Older books said that xenon "expands its octet" (squeezes in extra electrons using extra orbitals). More recent thinking suggests the bonding can be explained without extra orbitals, by electrons being shared across three atoms at once. At school level, you only need the headline: heavy, large atoms with loosely held outer electrons can bond when the partner is extremely electron-hungry.
That is also why fluorine and oxygen are the usual partners. They sit at the top right of the table, and they hold their own electrons very tightly while being eager to take more. A reluctant guest needs a very good conversationalist. If you want to see where those partners live, the halogens group page covers fluorine's family.
What are noble gas compounds used for?
Honestly, mostly research. Xenon difluoride is used by chemists as a tidy way to add fluorine to other molecules, and it has been used in some specialist manufacturing processes. The compounds have also been valuable as teaching examples, because they proved that the electron rules were guidelines and not laws.
Elemental xenon itself has some wider uses, though these are the pure gas and not a compound. It is used in certain bright lamps, including some vehicle headlights, and it has been used as an anaesthetic in some medical settings. Treat those as examples, not a full list, and note that they are about the gas and not about the compounds above. For other medical elements see elements in medicine.
Is "inert" the wrong word?
For the heavier members, yes. "Inert" claims that nothing will ever happen, and xenon proved that wrong. "Noble" is a better name, borrowed from the old idea of precious metals like gold that tend to avoid reacting. A noble person does not mix with just anyone, but under the right circumstances they will. The noble gases are exactly like that: reluctant, not incapable.
You can see the family as a whole on the noble gases group page, and the pair page helium vs neon shows the two that hold out longest.
Common questions
Is xenon dangerous because it can form compounds?
The gas itself is not the issue, but many of its compounds are strong oxidisers (chemicals that rip electrons off other substances) and some are unstable, so they are handled only by trained chemists with proper equipment. You will not meet them in a school lab.
Which noble gas is the most reactive?
Among those that can be studied properly, xenon. Radon should in principle be more willing still, but it is radioactive and short-lived, so its chemistry is hard to measure. Oganesson is expected to be unusual, but it has not been made in any useful quantity.
Did Neil Bartlett really make the first noble gas compound?
He is the person usually credited with it, in 1962, and the compound is generally described as xenon hexafluoroplatinate, though chemists have noted the exact makeup was more complicated than first thought. The fluorides of xenon followed soon after and were quickly confirmed by several research groups.
Can argon form compounds at room temperature?
Not as far as anyone knows. The one reported argon compound, argon fluorohydride, has been seen only at extremely low temperatures. At everyday temperatures argon behaves as it always did: politely uninterested.
Fancy testing how much you have taken in about the noble gases? Play the Pop Quiz, or try today's Daily Bubble and see whether you can spot the quiet one in the corner.
