Properties and Uses of Group 18 Elements
Senior 4 · Chemistry
Properties and Uses of Group 18 Elements
Lesson 3 of 3
25 min
Pass an electric current through a glass tube of neon at low pressure and it glows a fierce orange-red. Fill a balloon with helium and it pulls upwards on its string. Wrap an argon blanket round a glowing-hot weld and the metal does not burn. Every use of a noble gas rests on one of two kinds of property: its chemical inertness (it protects whatever it surrounds) or an unusual physical property (very low density, very low boiling point, low solubility, or a bright coloured glow). The skill this lesson builds is to match the use to the property, and to judge which gas is the best choice when several would work.
Definition
Inert atmosphere: a gas surrounding a substance or a process to keep out oxygen and moisture, so that nothing reacts with them. Example: argon around a hot weld.
Gas-discharge lamp: a sealed tube of gas at low pressure through which an electric current passes. Electrons collide with the gas atoms and excite them; as the atoms fall back to lower energy levels they emit light of particular wavelengths, giving each gas its own colour. Example: a neon sign.
Cryogenic: to do with very low temperatures, below about −150 °C (123 K). Example: liquid helium (b.p. 4.2 K) is a cryogenic coolant.
Table 13.4: Uses of the noble gases
| Gas | Use | Property used |
|---|---|---|
| Helium | filling weather balloons and airships | far less dense than air (0.18 against 1.29 g dm⁻³) and, unlike hydrogen, not flammable |
| Helium | cooling the superconducting magnets of MRI scanners | lowest boiling point of any substance (4.2 K) |
| Helium | breathing mixtures (heliox, trimix) for deep-sea divers | much less soluble in blood and body fat than nitrogen, so less nitrogen narcosis and less risk of "the bends" |
| Helium | carrier gas in gas chromatography; shielding gas in welding | inert |
| Neon | advertising signs ("neon lights"); indicator lamps | bright orange-red glow in a discharge tube |
| Argon | shielding gas in TIG and MIG welding | inert even at high temperature; denser than air, so it stays over the weld; the cheapest noble gas |
| Argon | filling incandescent light bulbs (mixed with nitrogen) | inert: stops the hot tungsten filament from burning and slows its evaporation |
| Argon | inert atmosphere for reactive metals (titanium, magnesium) and for growing silicon crystals | inert; nitrogen would react with these hot metals to form nitrides |
| Krypton | high-performance and halogen lamps | heavier atoms slow the evaporation of the filament more than argon does |
| Krypton | krypton fluoride (KrF) lasers for making computer chips | forms an excited KrF molecule that emits ultraviolet light of 248 nm |
| Xenon | arc lamps in cinema projectors and some car headlamps | a near-continuous, daylight-like spectrum in a discharge |
| Xenon | general anaesthetic | dissolves well in fats and leaves the body quickly |
| Xenon | ion engines that steer spacecraft | heavy, inert atoms that are easy to ionise |
| Radon | (formerly) radiotherapy; today mainly a health hazard to be measured and kept out of buildings | radioactive |
In a discharge tube, fast electrons strike the gas atoms and lift one of their electrons to a higher energy level. When it falls back, the atom gives out a photon whose energy equals the gap between the two levels, (Unit 2). Each element has its own set of levels, so each gas glows its own colour: neon orange-red, argon violet-blue, helium pinkish-white, xenon blue-white. "Neon" signs of other colours use other gases, or a coating of phosphor on the glass.
Radon-222 forms continuously in rocks and soils that contain uranium. It seeps into buildings through cracks in floors and can build up in rooms with poor ventilation. When breathed in, it and its radioactive decay products damage the cells lining the airways. The World Health Organization estimates that radon causes between 3% and 14% of all lung cancers in a country, and that the risk rises by about 16% for every 100 Bq m⁻³ increase in the long-term average radon concentration. Smokers are about 25 times more at risk from radon than non-smokers. Radon is easily measured with a small passive detector, and good ventilation and sealed floors keep it out.
Worked example 1: why a KrF laser can cut a chemical bond
A krypton fluoride laser emits ultraviolet light of wavelength 248 nm. Calculate the energy of one photon, and of one mole of photons, and compare it with the energy of a C–C bond (about 347 kJ mol⁻¹). , , .
Solution. Each photon carries more energy than a C–C bond needs to break (483 > 347 kJ mol⁻¹), so the laser can break bonds in polymers and tissue directly. This is why KrF and ArF (193 nm) lasers are used to pattern computer chips and in eye surgery.
Worked example 2: choosing a gas for light bulbs
A factory fills 10 000 incandescent bulbs, each with 0.20 dm³ of gas at 1 atm. In 2004, industrial argon cost about 5 US dollars per m³ and krypton about 450 US dollars per m³. Compare the cost of the gas, and evaluate which gas the factory should use.
Solution. Total volume . Both are inert, so both stop the filament burning. Krypton atoms are heavier and slow the evaporation of the tungsten more, so the filament can run hotter and brighter or last longer. But krypton costs 90 times as much, because there is only 1.1 ppm of it in air against 9340 ppm of argon. Judgement: argon (with some nitrogen) for ordinary bulbs; krypton only for premium lamps where the better light is worth the price.
Warning
Common mistakes. (1) Saying helium is used in balloons "because it is inert" only: hydrogen is lighter still; helium is chosen because it is light and does not burn. (2) Saying argon is used in welding because it is "light": it is denser than air, which is exactly why it settles over the weld. (3) Writing that all coloured "neon" signs contain neon: only the orange-red ones do. (4) Thinking that because radon is a noble gas it is harmless: it is radioactive.
Safety. Never breathe helium from balloons or cylinders to make your voice squeaky: it can starve the brain of oxygen within a few breaths, and gas from a pressurised cylinder can damage the lungs. Welding arcs give off intense ultraviolet light that burns the eyes and skin, so only trained welders with a proper visor and covered skin should weld, and others should not watch the arc unprotected.
Tip
Ask "inert or physical?" For any use, decide whether the gas is there to protect (inertness: welding, bulbs, reactive metals) or for a physical property (low density, low boiling point, low solubility, colour of its glow). Then argue cost: argon is cheapest because there is so much of it in air.
Periodic Videos: Neon (2 min), a short film from the University of Nottingham about the element neon. Watch for the colour of the light neon gives out when a current passes through it.
Practice
Key points
Further reading
Sources and credits
Quick check
Match each use to the noble gas chosen for it.
Tap one on the left, then its partner on the right.
