What Are the Noble Gases? The Elements That Refuse to React
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The last column of the periodic table holds six naturally occurring elements that were missed entirely by chemistry until the 1890s, because they do almost nothing. Helium, neon, argon, krypton, xenon and radon do not burn, rust, dissolve or combine, and for decades after their discovery no compound of any of them was known. Their laziness is not a lack of character. It is the direct consequence of an electron arrangement that every other element is trying to reach.
Why they do not react
Chemical reactions happen because atoms gain, lose or share electrons to reach a stable arrangement, and for most elements that means filling their outermost electron shell. Sodium has one electron too many and gives it up eagerly; chlorine has one too few and grabs it; the two form salt because each ends up with a complete outer shell. The noble gases start with a complete outer shell. They have nothing to gain by taking electrons and nothing to gain by giving them away, so they do neither.
That is why they are monatomic gases: single atoms drifting alone, not even bonding to each other, when almost every other gaseous element pairs up. It is also why they have the highest ionisation energies in their rows of the table, meaning it takes more energy to pull an electron off a noble gas atom than off any of its neighbours. Chemistry is largely the story of atoms trying to look like a noble gas.
How they were found
Helium was detected in the Sun before it was found on Earth. During the 1868 eclipse, astronomers saw a yellow line in the solar spectrum that matched no known element, and named the substance after the Greek word for the Sun; it was isolated from a uranium mineral only in 1895. Argon was found the previous year by Lord Rayleigh and William Ramsay, who noticed that nitrogen extracted from air was slightly denser than nitrogen made chemically, and reasoned that the air contained a heavier gas nobody had noticed. It turned out to be nearly one percent of the atmosphere.
Ramsay then went looking for the rest. Within four years he had separated neon, krypton and xenon from liquefied air by boiling it off in fractions, naming them from the Greek for new, hidden and stranger. Radon, radioactive and produced by the decay of radium, followed in 1900. For finding an entire new group of the periodic table, Ramsay received the 1904 Nobel Prize in chemistry, and Rayleigh the physics prize the same year.
Not quite noble after all
The textbook line that noble gases form no compounds lasted until 1962, when Neil Bartlett, working in Vancouver, noticed that a platinum fluoride compound could pull an electron from oxygen and calculated that xenon should be just as vulnerable. He mixed the two and got an orange solid, the first noble gas compound. Xenon fluorides and oxides followed within months, and krypton difluoride soon after.
The heavier gases react because their outer electrons are far from the nucleus and held less tightly, so a ferociously electron-hungry partner like fluorine can pry one loose. Helium and neon, whose electrons are held tight, have no stable compounds under ordinary conditions, though a helium-sodium compound has been made under pressures a million times that of the atmosphere. Nobility, it turns out, is a matter of degree.
What they are used for
Their reluctance to react is exactly what makes them useful. Argon fills incandescent bulbs so the hot filament does not oxidise, and blankets the arc in welding for the same reason. Helium, the second lightest gas and impossible to ignite, lifts balloons and airships, and because it liquefies at a lower temperature than anything else it cools the superconducting magnets in MRI scanners. Neon glows red-orange when a current passes through it, and the other gases give the other colours of the signs still loosely called neon. Xenon fills the brightest flash lamps and car headlights and is used as an anaesthetic and as propellant in ion engines on spacecraft. Radon is the exception: it seeps from granite into basements and is the second largest cause of lung cancer after smoking.
The six, with the thing each is known for:
- •Helium: balloons, deep diving mixtures, cooling MRI magnets
- •Neon: the classic red glow of signs
- •Argon: the commonest, filling bulbs and shielding welds
- •Krypton: high-performance lamps and window insulation
- •Xenon: flash lamps, headlights, anaesthesia, ion engines
- •Radon: radioactive, a hazard in poorly ventilated buildings
Where helium actually comes from
Helium is the second most abundant element in the universe but scarce on Earth, and the reason is its lightness. Any helium released into the air rises and slowly escapes into space, so the atmosphere holds only a few parts per million. The helium we use comes from underground. Uranium and thorium in rocks decay slowly and give off alpha particles, which are helium nuclei, and over hundreds of millions of years this helium collects in the same porous rock formations that trap natural gas, sealed in beneath layers it cannot pass through.
It is extracted as a by-product of natural gas, and only some gas fields contain enough to be worth separating, which is why supply comes from a small number of countries and why prices jump when a single plant shuts down. Once used and released, helium cannot be recaptured on any useful scale, so scientists have long argued that filling party balloons wastes something irreplaceable. Hospitals and laboratories increasingly recycle the helium that boils off from MRI magnets and research equipment, and newer scanners are designed to need far less of it.
A noble gas that keeps time
Argon's unwillingness to react makes it a natural clock. Rocks contain potassium, and a small fraction of potassium is the radioactive isotope potassium-40, which decays very slowly, half of it every 1.25 billion years or so, partly into argon-40. While a rock is molten, any argon produced escapes as gas. Once it cools and crystallises, new argon is trapped inside the minerals, because it forms no compounds and has nowhere to go. Measure how much argon-40 has built up relative to the potassium that remains, and you can calculate how long ago the rock solidified.
The method, potassium-argon dating, and its more precise successor, argon-argon dating, are among the main tools for dating volcanic rock, and they matter far beyond geology. In 1961 potassium-argon dates on volcanic layers at Olduvai Gorge in Tanzania showed that early human fossils there were about 1.75 million years old, far older than many had assumed, and layers of volcanic ash above and below fossils in East Africa still supply much of the timescale of human evolution. The same decay explains a curiosity of the air: almost all the argon in the atmosphere is argon-40, released from rocks over the Earth's history.
The noble gas in some people's basements
One noble gas is a genuine health concern. Radon is produced by the radioactive decay of uranium, which occurs naturally in rocks such as granite, and because it is a gas it seeps out of the ground and can collect in poorly ventilated buildings, especially basements and ground floors. You cannot see, smell or taste it.
Radon itself is inert, but it is radioactive, and when breathed in, it and its decay products can damage the cells lining the lungs. Health agencies identify radon as one of the leading causes of lung cancer after smoking. In parts of Britain with granite bedrock, such as Cornwall and Devon, home testing kits are recommended, and high levels can usually be reduced by improving ventilation or fitting a small fan under the floor.
The takeaway
The noble gases are the six elements with full outer electron shells, which leaves them nothing to gain from reacting and makes them single, inert atoms. Found in the 1890s by weighing air carefully, they were thought incapable of forming compounds until xenon was persuaded to in 1962, and their unreactivity is what makes them useful everywhere from light bulbs to spacecraft.