The Specifics of Noble Gases Content from the guide to life, the universe and everything

The Specifics of Noble Gases

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'Noble Gases' is the most common designation for the elements in the rightmost group (group 0, group VIII or group 18 depending on which nomenclature is being used) of The Periodic Table of the Elements. The commonly found designations 'inert gases' or 'rare gases' are somewhat misleading. Firstly, they are not inert in a strict sense. And secondly, they are not rare. Argon, for example, makes up almost one per cent of the atmosphere, and helium is the second most common element in the universe.

For a number of complicated reasons, which involve serious Quantum Mechanics, and electron filling rules for atomic shells and orbitals, these elements have many properties in common. This includes their extremely low chemical reactivity. For example, the Noble Gases are not flammable and are very unreactive when a chemical reaction is applied. This low reactivity is the reason why these elements are termed 'noble.' Or possibly their arrogant behaviour - as seen from a chemical perspective, these gases do not get together with other elements, similar to many 'noble' aristocrats. Some pieces of data on the noble gases are collected in the following table:

Some Data for the Noble Gases
Atomic number21018365486
Atomic weight [u]14.00160220.179739.94883.80131.29(222.02)2
%Vol. in air30.0005240.0018180.9340.0001140.00000876 x 10-18
Boiling point [°C]-268.9-246.08-185.9-153.2-108.0-61.7
Melting point [°C](-272.2°C)4-248.59-189.3-157.4-111.76-71
Inversion temperature5 [°C]-2336-34501000717008-
Critical temperature9 [°C]-267.9-228.7-122.3-63.816.6105
Critical pressure10 [bar]2.2626.948.354.357.662
Enthalpy of evaporation [kJ/mol]0.08121.7326.5189.02912.63518.096
Gas density [g/l]0.17850.899901.78373.7335.8879.73
Liquid density11 [g/ml]0.1251.2071.4002.4133.0574.4
Atomic radius [pm]93131174189209214
Colour of light
in a gas discharge tube
1st ionisation energy15 [eV]24.58621.56315.75913.99812.13010.75
2nd ionisation energy16 [eV]54.4140.9627.6224.3521.20-

Noble Gases: Yesterday and Today

One consequence of the aforementioned Noble Gases' low reactivity was that these elements were discovered and isolated only relatively late. The first person to actually isolate and note the presence of Noble Gases - however, without going into further details - was Henry Cavendish (1731-1810 chemistry weirdo and 'discoverer' of hydrogen) in 1785. The real 'discovery' of the Noble Gases, their isolation and proper characterisation took place between 1890 and 1900. A key figure in the process of discovering, isolating and characterising the gases was a chap called William Ramsay, who in the end became 'nobel' for the noble stuff (more details are in The History Around the Noble Gases).

All Noble Gases are widely used in industrial applications and in scientific research. With the exception of radon they are all used as luminescent gases, e.g. in lasers or in neon lighting. They are also widely used as protective gases, eg in semiconductor manufacturing. All the gases, except helium and radon, can be obtained from the fractional distillation of liquid air. Helium (originating from radioactive decay) is not held in the atmosphere by the earth's gravity. However, it is found sealed or trapped deep inside the earth, commonly along with gas and petroleum, from where it is extracted. Helium is the most well-known and most widely used Noble Gas, eg to fill balloons and dirigibles. Radon is very difficult to obtain as a pure gas, since it is not a stable element. Even so, in the past it has found uses as a radioactive tracer gas for medical applications.

For a long time it was thought that the Noble Gases were unable to perform any chemical reactions and form compounds. It has been questioned whether this was absolutely true. In the 1960s the first Noble Gas compound was synthesised using a very tough oxidizer (platinum hexafluoride) and xenon; the compound, a yellow solid, is xenon hexafluoroplatinate (XePtF6-). Other Noble Gas compounds were synthesised after this. However, they haven't found any broad applications and are mainly of academic interest.

11 u the 'atomic unit of mass' is sort of a mean value between the mass of a proton and the mass of the neutron, it is easier to calculate using this unit. One 'u' corresponds to 1.660566 x 10-27 kg2Radon is a radioactive element. The most stable isotope is 222Rn, with a half-life of 3.825d (alpha-emitter).3To get the picture: 5000 m3 of air (the volume of a small concert hall for example) contains: 47000 litres of argon, 80 litres of neon, 23 litres of helium, 5 litres of krypton, a half litre of xenon and some ten nanolitres of radon.4Helium does not solidify at room pressure, the given value is obtained at a pressure of 26atm.5the 'normal' effect of cooling when a gas expands takes place below that temperature, above that temperature it heats under expansion.6Note that helium is one of the few elements that have an inversion temperature that lies way below room temperature.7literature values vary8literature values vary9no liquefaction above this temperature10the pressure of the gas at the critical temperature11at boiling point12from red and blue emissions13or yellowish green14or blueish green15The first ionisation energy is the energy needed to remove one electron from the neutral element, thus ionising it.16The second ionisation energy is the energy needed to remove a second electron from the already charged element. To calculate the total energy needed to remove two electrons one should then add the first and the second ionisation energies.

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