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Noble Gases and Their Uses

The noble gases occupy the last group of the periodic table, i.e. group 0. They consist of Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn). They are all non-metallic elements and colorless gasses at room temperature and pressure.

They form 1% of air, and most of this is argon. Neon, argon, krypton, and xenon are obtained by fractional distillation of liquid air (Helium is also obtained in small quantities from this source) - see the table for: the boiling points and properties of noble gases. Radon is obtained from radium salt.

The very inertness of the noble gases is an important feature of their practical uses.

Helium

Helium is much less dense (lighter) than air and is used in balloons and blimps. Because of its inertness, it doesn’t burn in air unlike hydrogen which was formally used in large balloons with ‘flammable’ consequences. Helium is less soluble in water than nitrogen. When inhaled, it dissolves in the bloodstream in smaller quantities than nitrogen.

For this reason a mixture of helium and oxygen is used in place of natural air for divers and others who work under high air pressure. Breathing compressed air causes considerable nitrogen to dissolve in the blood. When a diver is brought up rapidly and begins to breathe air at normal pressure again, much of the nitrogen that dissolved under high pressure comes out of solution, forming bubbles that block the circulation of the blood.

This painful and dangerous condition is known as “the bends” and is avoided by the use of the helium - oxygen mixture. Helium is used in industry to provide an inert atmosphere in electric arc welding of metals. An electric arc welding is a type of welding operation whereby the heat source used for welding is created when current flows between an electrode held by the welder and the work, which is connected to the opposite side of the electric source.

In all electric arc processes, the electrode, the molten pool and the heat-affected metal parts must be protected from reaction (oxidation) with the surrounding air, hence a shielding gas must be introduced. Typical shielding gases consist of argon, helium or a mixture of the two. The addition of hydrogen or nitrogen can also be beneficial under certain conditions. In addition, the shielding gas affects the characteristics of the arc and the appearance of the weld. Helium is also used in chemical processes that must be carried out in the absence of air.

Helium has the lowest boiling point of any known substance, 4.1 K, therefore, its chief scientific use is in cryogenics (the science and art of producing very low temperatures).

Liquid Helium

Liquid helium has some remarkable properties. At 1 atmosphere pressure it is a normal liquid from its boiling point at 4.18 K to 2.18 K and is designated He-I . Below 2.18 K it is a liquid of very unique properties and is designated He-II. This strange form of liquid helium has no measurable viscosity, and it cannot be confined in an open container because it creeps over the walls and flows down the outside.

Its conductivity for heat and electricity is several hundred times as great as that which metallic copper has at room temperature. It is referred to as a superconductor. No fully satisfactory theory has been proposed to explain the unusual properties of He-II. At atmospheric pressure helium remains liquid at the lowest temperatures yet obtained. It is not known whether it would freeze without being under pressure if it were possible to cool it to absolute zero.

Neon

Neon is used in the familiar ‘neon sign” used in advertisement. The brilliant red glow is caused by the passage of electric current through neon gas under low pressure. The color of the light given off can be changed by mixing mercury vapor and argon with the neon.

Argon

Argon, like all the noble gases is chemically inert. It is used in filament bulbs because the metal filament will not burn in argon (i.e., argon prevents the oxidation of the hot filament, thereby prolonging its life). Its greater chemical inertness and lower thermal conductivity make argon superior to nitrogen for this purpose. Argon is also used in electric arc welding of metals as a shielding gas, to produce an inert atmosphere.

Due to its relative high abundance in air, argon is the cheapest of the noble gases to produce, and to be used wherever a cheap inert atmosphere is required, such as in electric arc welding of metals. Krypton krypton is used in fluorescent bulbs, flash bulbs and lasers. Lamps filled with krypton are used at some airports as approach lights since their light can penetrate dense fog unusually well.

Xenon

Xenon is used in fluorescent bulbs, flash bulbs and lasers. Xenon emits an instant, intense light when present in discharge tubes. This property of xenon is utilized in high-speed electronic flash bulbs used by photographers.

Radon

Radon is radioactive and is used in medicine as a source of gamma rays. The gas is sealed in small capsules, which are implanted in the body to destroy malignant (e.g., cancerous) growths.