8.8: Noble Gases

The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name “noble” because they were assumed to be nonreactive since they have filled valence shells. In 1962, Dr. Neil Bartlett at the University of British Columbia proved this assumption to be false.

These elements are present in the atmosphere in small amounts. Some natural gas contains 1–2% helium by mass. Helium is isolated from natural gas by liquefying the condensable components, leaving only helium as a gas. Radon comes from other radioactive elements. More recently, it was observed that this radioactive gas is present in very small amounts in soils and minerals. Its accumulation in well-insulated, tightly sealed buildings, however, constitutes a health hazard, primarily lung cancer.

The boiling points and melting points of the noble gases are extremely low relative to those of other substances of comparable atomic or molecular masses. This is because only weak London dispersion forces are present, and these forces can hold the atoms together only when molecular motion is very slight, as it is a very low temperature.

The full s and p orbitals of the valence shell add stability to the noble gases. These elements have the largest first ionization energies, indicating that the removal of an electron is difficult. Going down the group, atomic radius increases and ionization energy decreases. The positive electron affinity values of these elements reveal that they are unlikely to gain electrons as well. Table 1 summarizes the properties of the noble gases.

Table 1: Properties of the Noble Gases.

Argon is useful in the manufacture of gas-filled electric light bulbs, where its lower heat conductivity and chemical inertness made it preferable to nitrogen for inhibiting the vaporization of the tungsten filament and prolonging the life of the bulb. Fluorescent tubes commonly contain a mixture of argon and mercury vapor. Argon is the third most abundant gas in dry air.

Helium is used for filling balloons and lighter-than-air craft because it does not burn, making it safer to use than hydrogen. Liquid helium (boiling point, 4.2 K) is an important coolant to reach the low temperatures necessary for cryogenic research, and it is essential for achieving the low temperatures necessary to produce superconduction in traditional superconducting materials used in powerful magnets and other devices.

Neon is a component of neon lamps and signs. Passing an electric spark through a tube containing neon at low pressure generates the familiar red glow of neon. It is possible to change the color of the light by mixing argon or mercury vapor with the neon or by utilizing glass tubes of a special color.

Krypton-xenon flash tubes are used to take high-speed photographs. An electric discharge through such a tube gives a very intense light that lasts only 1/50,000 of a second. Krypton forms a difluoride, which is thermally unstable at room temperature.

Stable compounds of xenon form when xenon reacts with fluorine. Xenon difluoride, XeF₂, forms after heating an excess of xenon gas with fluorine gas and then cooling. The material forms colorless crystals, which are stable at room temperature in a dry atmosphere. Xenon tetrafluoride, XeF₄, and xenon hexafluoride, XeF₆, are prepared in an analogous manner, with a stoichiometric amount of fluorine and an excess of fluorine, respectively. Compounds with oxygen are prepared by replacing fluorine atoms in the xenon fluorides with oxygen.

When XeF₆ reacts with water, a solution of XeO₃ results and the xenon remains in the +6 oxidation state. Dry, solid xenon trioxide, XeO₃, is extremely explosive — it will spontaneously detonate.

Unstable compounds of argon form at low temperatures, but stable compounds of helium and neon are not known.

This text is adapted from Openstax, Chemistry 2e, Section 18.2: Occurrence, Preparation, and the Properties of Noble Gases.

The nonmetallic elements categorized under group 18 – helium, neon, argon, krypton, xenon, and radon – are called noble gases. These elements occur as monatomic species and exist as gases under room-temperature. Radon is the only radioactive element from group 18. 

Moving down the group, the elements exhibit an increase in boiling points, densities, and atomic radii, which consequently leads to the decline of ionization energies of each successive element. 

Yet, noble gases have high first ionization energies compared to all other elements in the periodic table. This is because these elements have stable electron configurations with complete octets. The removal of an electron requires the input of a large amount of energy, which is unfavorable. 

Noble gases also have positive electron affinity values. Meaning, energy is required to add an additional electron to a gaseous atom. Noble gases resist electron additions as their valence shells are already complete, and the incoming electron needs to enter a higher principal quantum shell. 

The high stability of noble gases attests to their chemical inertness, which finds many industrial applications. For instance, argon is used to manufacture gas-filled electric light bulbs to prevent the oxidation of tungsten filaments, prolonging the bulb’s life. Helium is used to create an inert atmosphere during the melting and welding of easily oxidizable metals.

Noble gases were Initially thought to be entirely chemically unreactive and were called inert gases. However, in the early sixties Neil Barlett discovered some exceptions. For example, xenon, with the lowest ionization energy from the noble gases, was found to react with the most electronegative element, fluorine. 

Xenon difluoride, obtained by heating an excess of xenon gas with fluorine gas, is a stable, crystalline material. Other compounds like xenon tetrafluoride and xenon hexafluoride can also be prepared similarly.

Xenon-compounds with the electronegative element, oxygen, can be produced by replacing fluorine atoms in xenon fluorides with oxygen. For instance, xenon hexafluoride reacts with water, yielding a solution of xenon trioxide.