The noble gases are the elements in the right-hand column of the periodic table. They are called "noble" for being chemically inert. None of the noble gases react with any other element except in extreme artificial conditions.
Helium is a very light gas (only hydrogen is lighter), and the gases become progressively more dense down the table. They are gaseous to low or very low temperatures as well. Xenon condenses at 165K (-163 °F). Helium condenses at 4K (-452 °F). That is 4 degrees above absolute zero, the lowest boiling point of any element.
Noble gases are used in place of vacuuum when a nonreactive atmosphere is required. For example, argon may be played over a welding site, and xenon may be used to fill an incandescent bulb. These gases fluoresce in various colors when ionized by a strong electric current. A neon tube glows orange-red, other gases glow with different colors, and the glass itself can be tinted for even greater variety.
Helium is the second-lightest element and the first to be discovered off the planet. The development of spectroscopy around 1850 led to the discovery of such elements as caesium from the light that they emit or absorb.
In 1868, the French astronomer Pierre-Jules-César Janssen observed a total solar eclipse in India. His spectroscope showed an unfamiliar line in the light spectrum. English astronomer Norman Lockyer identified it as a new element, which he named helium after the greek god Helios. This discovery was controversial until helium was detected in various minerals on earth around 1890.
Helium is a major component of stars. On earth, it occurs in minerals not because it is bound chemically but because the helium nucleus is a product of radioactive decay of much larger atoms. This helium collects underground in pockets of natural gas and it is vented into the atmosphere.
Helium is used to fill balloons and blimps. It has unique properties at temperatures approaching absolute zero. Helium tanks are used to cool anything else to near that temperature, and helium is the subject of many cryogenic experiments.
Neon was discovered after krypton (see below) by Ramsay and Travers. They separated trace components from atmospheric argon, one of which produced a crimson spectrum. Neon means "new".
Neon was the first gas used in so-called neon signs. It is relatively expensive because of its rarity.
In the late 19th Century, the composition of the atmosphere was supposedly well known. The components were oxygen, nitrogen, carbon dioxide, and varying amounts of water vapor. John William Strutt, Lord Reyleigh, was a scientist and professor at Cambridge. In firming up the densities of these gases, he found a puzzling inconsistency. Nitrogen prepared from ammonia had one density, but nitrogen from the air – after extracting the other gases – had a higher density.
This discrepancy was tackled by the Scottish chemist William Ramsay. He found that the supposed nitrogen remnant from the atmosphere contained a portion that was not reactive. Its spectrum had unfamiliar lines; this was a new gas. The new element argon, "the lazy one", introduced a new column of the periodic table.
Argon comprises about 1% of the atmosphere, which is more than carbon dioxide. Its origin is rocks containing radioactive potassium that decays into argon. Argon is by far the most common noble gas, and therefore the cheapest.
Krypton was discovered in 1898 as a trace element in atmospheric argon. William Ramsay and his assistant Morris William Travers detected unfamiliar spectral lines. The name krypton means "hidden".
Xenon is the last gas discovered by Ramsay and Travers. From large quantities of argon, they extracted the heaviest fraction, krypton, and worked to separate it further. They found xenon in 1898, whose name means "stranger".
Xenon is extracted from the air, where it occurs greatly diluted. It glows blue or violet in a gas discharge tube. As the heaviest safe inert element, it is used to fill premium lightbulbs and lamps that would otherwise be vacuum tubes, and it thereby slows the evaporation of the filament.
The heaviest of the noble gases is radioactive, a product of the decay of thorium and uranium. German physicist Friedrich Ernst Dorn in 1900 proved that radium minerals, themselves radioactive, emitted a radioactive gas, now called radon.
Uranium and thorium in the earth's crust are continually decaying into daughter products, one of which is the very heavy gas radon. Radon therefore tends to collect in certain low areas such as basements and especially mines. If it is inhaled, it presents a health risk, causing lung cancer.