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.