Neon

Because the electric discharge excites its atoms and, on returning to their normal state, releases the excess as intense orange-red light. Each noble gas has its colour: helium pink, argon blue, neon red. The 'neon' signs of other colours actually carry argon or mercury with phosphors. A well-made tube shines for decades: atoms are not consumed, they simply go up and down energy levels over and over.

Because the primitive Earth, hot and with insufficient gravity, left light gases to escape into space: hydrogen, helium and neon were lost, leaving only 18 parts per million in the air. In the Sun and Jupiter it is still very abundant. Each litre of industrial neon is a survivor: thousands of litres of air had to be distilled to capture it. We are a planet poor in neon by gravitational accident.

The helium-neon laser, the classic red ray of laboratories and old barcode readers, uses neon as the emitting medium: excited helium passes energy to neon, which returns it as coherent light of 632.8 nm. It was one of the first lasers in history (1961) and is still used in surveying and holography for its stable and cheap beam. Without neon, the most famous red ray would not exist.

In reality most do not contain neon: the orange-red is pure neon, but blue is argon, pink is helium, and the rest are mercury vapor tubes with fluorescent powders like domestic fluorescent lamps. 'Neon' became a synonym for illuminated tubes. A glass artist chooses the gas according to the desired colour: each element signs with its light. Genuine neon only paints red.

In practice, no. Its complete octet and extremely high ionisation energy shield it against any stable bond. Only helium is more inert. Only atoms trapped in ice under pressure or molecules lasting millionths of a second excited are known. It is the champion of chemical indifference: it undergoes reactions, flames and acids without changing.

Ramsay and Travers distilled liquid air in 1898 and examined each fraction with a spectroscope: one emitted an unknown and spectacular red. As it did not react with anything, they knew it was a new noble gas and called it neos, new. The spectroscope was their magnifying glass: each element leaves a unique light fingerprint. Thus invisible ones are hunted not by their chemistry but by their luminous signature.

It is not toxic because it does not react with the body, but breathing pure neon displaces oxygen and can asphyxiate without warning, as it neither smells nor irritates. Deep-sea divers use it mixed with oxygen precisely for its innocuousness: it does not narcose like nitrogen. The danger is never chemical, but lack of oxygen.