Noble gases

Colourless, odourless and almost impossible to provoke, the elements that would rather be left alone.

For most of the nineteenth century, nobody knew that the air contained a whole family of elements. They were missed because they do nothing: no colour, no smell, no taste, and almost no reactions. Only when chemists weighed air with great care did they find that about one part in a hundred was a gas nobody had ever named. That gas was argon, and it led to the whole of group 18.

The noble gases are helium, neon, argon, krypton, xenon and radon, with the synthetic element oganesson filling the last place in the column. They were called noble because, like aristocrats who kept to themselves, they did not mix with the other elements.

What they have in common

Every noble gas has a completely full outer electron shell. Helium has two electrons in its single shell; the others have eight in their outer shell. A full shell is the most stable arrangement an atom can have, so noble gas atoms have no reason to gain, lose or share electrons. This is why they hardly react, and it is why they exist as single atoms rather than pairing up into molecules as oxygen and nitrogen do.

Because the atoms attract one another only very weakly, the noble gases have extremely low melting and boiling points. All of them are gases at room temperature and all are colourless and odourless. Pass a high voltage through a tube of low-pressure noble gas, though, and the atoms glow with distinctive colours: red-orange for neon, blue-lilac for argon.

How they change down the group

Going down the group the atoms grow larger, and the weak attractions between them grow stronger, so boiling points climb. Helium boils at about minus 269 degrees Celsius, only a few degrees above the lowest temperature possible, and it is the only substance that will not freeze solid at ordinary pressure no matter how cold it gets. Neon boils at about minus 246 degrees, argon at about minus 186, krypton at about minus 153, xenon at about minus 108 and radon at about minus 62.

Density rises in the same direction, from helium, far lighter than air, to radon, one of the heaviest gases known. The energy needed to remove an electron falls down the group as the outer electrons sit further from the nucleus. That is why the heavier noble gases are not completely inert. Xenon forms a range of compounds with fluorine and oxygen, krypton forms a few, and radon is expected to be more reactive still, though its radioactivity makes it hard to study.

Oganesson, at the bottom, is a special case. Only a handful of atoms have ever been made, each lasting under a millisecond, and calculations suggest its electrons move so fast that the usual pattern breaks down. Its predicted boiling point is above room temperature, so it may be the first noble gas that is not a gas.

Where you meet them

Helium fills party balloons and airships, but its most important job is as a coolant. Liquid helium keeps the superconducting magnets in hospital MRI scanners cold enough to work. Neon's famous role is in glowing signs.

Argon is the workhorse of the family. It makes up just under one per cent of the air, so it is cheap, and it is used wherever a reaction with oxygen must be prevented: inside light bulbs, around the arc in welding, and in the gap between panes of double glazing. Krypton and xenon fill high-performance lamps and camera flashes. Radon has no consumer uses. It seeps out of certain rocks and soils and can build up inside buildings, where it is a health hazard because it is radioactive.

Odd ones out

Helium is the family's odd one in almost every way. It has only two electrons and its behaviour at low temperature is unique. It never solidifies under ordinary pressure, and below about two kelvin it becomes a superfluid that flows without any friction and climbs the walls of its container. Most of the helium on Earth comes not from the atmosphere but from natural gas, where it has collected from the radioactive decay of uranium and thorium in rocks.

Radon is odd because it is dangerous. Every isotope is radioactive, and it is the largest single source of natural radiation exposure for many people. Xenon is odd because it is the noble gas that most clearly proves the group is not truly inert: its fluorides are stable crystalline solids that can be kept in a bottle. And oganesson, if the predictions are right, may be a solid that is not noble at all, which would make its position in group 18 a matter of electron count rather than behaviour.

The numbers

Melting points rise down the list, from Helium at -272 °C to Radon at -71 °C.

Density in g/cm³ (g/L for gases). Values for synthetic elements are predictions.

Did you know?

• Helium has the highest first ionisation energy of any element, meaning it holds on to its electrons more tightly than anything else.
• Helium boils at about minus 269 degrees Celsius and never freezes solid at ordinary pressure.
• Argon makes up just under one per cent of the air, making it the third most abundant gas in the atmosphere.
• Xenon forms stable compounds with fluorine, showing that the noble gases are not completely inert.
• Radon, a radioactive gas from rocks, is a significant natural source of radiation exposure in some homes.
• Oganesson has been predicted to be a solid at room temperature, unlike every other member of the group.

Frequently asked questions

Written by Anthony. Data from Periodic-Table-JSON (CC BY-SA).