HOW NEON LIGHTS WORK

The discovery of the most stable elements in the periodic table led to the garish lights that define big cities. JOEL F. HOOPER explains.

SINCE THE END of the 19th century humans have been infatuated with the vivid glow of neon. It has become a sign of both glitz and kitsch, leading us to the nearest open motel, lighting up strips of landmarked boulevards and symbolising those lost to the pleasures of the night. It is a gas that creates a sense of mystery, and one that also holds an extensive history.

To understand what gives neon light its colour, let’s first consider the family that neon gas belongs to: the noble gases. This group of six elements – the other five are helium, argon, krypton, xenon and radon – have properties that make them uniquely stable, seemingly participating in no chemical interactions.

This lack of interaction with other elements to form compounds is the reason these six gases are called ‘noble’ – it was considered a sign of nobility not to react when provoked, and members of the nobility were expected to be aloof.

Understanding the stability of these gases led to the discovery of the structure of the atom, and consequently to how it could be manipulated to create the red-orange glow we know as neon.

It began in 1902, when Dmitri Mendeleev added the noble gases to his periodic table. He arranged the elements in rows and columns according to their atomic weight, realising that certain elements appeared in repeating (or periodic) patterns due to their properties. The noble gases are situated on the far right-hand side of the periodic table.

Physicists struggled to find a model that would explain this curious observation. What was the significance of the number eight?

In 1912, a young Danish physicist named Niels Bohr came up with the explanation. Based on work by physicist Ernest Rutherford, who had proposed that electrons orbit the nucleus of an atom much like planets orbit a star, Bohr proposed that electrons did indeed orbit around the nucleus of the atom, but only at certain distances and nowhere in between. He titled these distances “orbitals” or “shells”.

Bohr’s model was a hit. It was a key step in the development of quantum mechanics, and in determining the stability of the noble gases.

Using Bohr’s theory, a physicist named Gilbert Lewis suggested that each electron shell was most stable when it contained eight electrons, with the exception of the very first shell, which could only accommodate two.

Lewis further explained that most atoms would go to great lengths to stay in this stable state by borrowing, donating or sharing electrons with others to make sure their outer shell was complete. This is now recognised as chemical bonding, which allows for the formation of compounds.

This is why the noble gases are so stable and very resistant to forming bonds. They have a complete outer shell of eight electrons (except for helium, which has just two), so they have no need to share, borrow or donate.

That isn’t to say they can’t undergo chemical bonding: xenon and krypton will react with fluorine gas, an extremely powerful oxidant, to form compounds such as xenon difluoride (XEF ), and helium can react with sodium under extreme pressure to form Na He.

The stability of the noble gases leads to their practical uses. Argon is used as a shielding gas in welding, while helium is used as a cryogenic coolant in MRI machines and superconductors. Because helium doesn’t burn, and is lighter than air, it is also great for filling balloons.

Most importantly, noble gases provide us with our neon lights!

One thing to remember, though, is that not all so-called neon lights use neon gas. Depending on its colour, a neon light might use another noble gas.

The process to create the light is similar to how other lamps work. A high-voltage electrical discharge is passed through a tube of low-pressure gas. The electrical discharge can excite electrons in the atoms, causing them to jump from a low-lying and stable shell to a higher shell.

This, of course, breaks Lewis’ rule of eight electrons in the outer shell, so the excited electron will eventually “relax” back to its preferred shell. As this electron relaxes back to the lower shell, it sheds some energy, in the form of a packet of light, known as a photon.

The wavelength of this light corresponds to its colour, and will depend on the difference in energy between the higher and lower shells. Each element gives off a different characteristic wavelength of light, and thus colour, when its electrons are excited, as each element has a different number of shells its electrons can jump up to.

This is why neon lights glow with an electrifying red-orange colour, while argon lamps are lavender blue and xenon lamps light blue-green. It all comes down to the movement of electrons between each of the element’s shells in order to ensure its outer shell is complete.

So we can trace both the stability of the noble gases, as well as the bright and lurid colours of the neon light back to the same quirk in quantum mechanics: Gilbert Lewis’ rule of eight.