Noble gases

The family nobody expected

When Mendeleev drew up his periodic table in 1869, none of the noble gases was known. They do not react, so they had never shown up in chemical analysis. Argon was found in 1894 hiding in ordinary air, which is almost 1 per cent argon. Within four years helium, neon, krypton and xenon followed, and the periodic table gained a whole new column.

Full shells, no interest in bonding

Noble gas atoms have completely full outer electron shells: two electrons for helium, eight for the rest. They have no electrons to spare and no space for more, so they rarely form bonds. They exist as single atoms rather than molecules, and they are all colourless and odourless.

The word “inert” turned out to be not quite right. In 1962 Neil Bartlett made the first noble gas compound, xenon hexafluoroplatinate, and many compounds of xenon and krypton are now known. Helium and neon, however, have never been made to form a stable neutral compound.

Lighting up the night

Pass an electric current through a tube of noble gas and it glows in a colour unique to that element. Neon gives the famous red-orange, argon a pale violet-blue, krypton a whitish lavender and xenon a bright blue-white used in car headlamps and cinema projectors. Most “neon” signs in other colours actually use argon with mercury and coloured phosphors.

Uses beyond the glow

Radon is the odd one out. It is radioactive and seeps from rocks into homes, where it is a leading cause of lung cancer after smoking. Many countries recommend testing basements and ground floors for it.

• Helium: cooling the superconducting magnets in MRI scanners, and lifting balloons and airships
• Argon: a protective atmosphere for welding and inside traditional light bulbs
• Xenon: ion thrusters on spacecraft, and a general anaesthetic
• Krypton: insulating double glazing and some high-performance lamps