What is the heaviest noble gas?

The noble gases make up group 18 of elements of the periodic table (formerly group VIII-A). These elements are characterized by having a shell-filled electronic configuration in which the last energy level has its syp orbitals completely filled. This electronic configuration is particularly stable, which is why these elements do not have the need to form chemical bonds to share electrons seeking more stability. In fact, most of the chemical reactions that the other elements of the periodic table undergo do so to surround themselves with the same 8 electrons that surround the noble gases. This is known as the octet rule.

By the very fact that they are so stable, the group 18 elements are also extremely inert and do not combine with virtually any other element. Furthermore, these elements do not even have a tendency to bond with each other, and the only interactions that occur between two atoms are weak London dispersion forces. For this reason, these elements have very low boiling points and are generally in a gaseous state under normal conditions of temperature and pressure. Both physicochemical characteristics have earned these elements the name of noble gases.

In short, what makes noble gases noble gases is that they are in a gaseous state and that they are chemically inert. This is an important point when determining which is the heavier noble gas.

What does it mean to be the heaviest noble gas?

Let’s first define what we mean by “the heaviest noble gas.” This qualifier can actually have one of two interpretations: on the one hand, it can refer to the gaseous element with the highest atomic weight. On the other hand, we could refer to the denser gas.

Although density is proportional to the molar mass of a gas and the molar mass of gases increases as you go down a group on the periodic table, the answer to the question of which gas is heavier is not it’s as simple as scrolling down the list to the last item in the group.

In fact, there are two candidates for the heaviest noble gas, and neither is the last element in the group.

Oganese is not the heaviest noble gas.

As we mentioned a moment ago, contrary to initial intuition, the heaviest noble gas is not the last member of the group, that is, oganeson, chemical symbol Og. This is due to several reasons. For starters, oganeson is a synthetic transactinide element, which means that this element does not exist in nature, but was synthesized in a particle accelerator through nuclear fusion.

The problem with oganeson, and the main reason we can’t give it the title of heaviest noble gas, is that it has a very short lifetime; less than 1ms. Furthermore, synthetic elements are produced in extremely small quantities. For both reasons, it is nearly impossible to accumulate enough oganeson atoms over a long enough time to measure its physicochemical properties. Consequently, nothing is known for sure about the physical state of this element at normal temperature and pressure.

In fact, it is estimated that, if it lasted long enough, this element would be a solid at room temperature. This in itself disqualifies it as the heaviest “noble gas”, despite being the heaviest element known to man.

On the other hand, multiple theoretical calculations have also been carried out on the electronic structure that this element would have and the results are truly unexpected. It is hypothesized that the large nuclear charge would accelerate electrons to nearly the speed of light, causing them to behave very differently from other known elements. The clearest consequence of this is that we don’t really even know if it would have the same inert characteristics as the other members of the group.

Under certain conditions, xenon can take the trophy

Since gases, especially noble gases, behave like ideal gases under normal conditions of temperature and pressure, a relationship between the density and molar mass of a gas can easily be obtained. This relationship is given by:

where ρ is the density of the gas in g/L, P is the pressure in atmospheres, T is the absolute temperature, R is the ideal gas constant, and MM is the molar mass of the gas. As you can see, the density is directly proportional to the molar mass. If we consider that all noble gases are in the form of monatomic elements, the densest element should be radon.

However, under certain very special conditions (applying electrical discharges on a supersonic jet of gaseous xenon), it is possible to convert xenon into ionized dimers or into diatomic molecular ions of the formula Xe ₂ ⁺ . This new gas would have a molar mass of 263 g/mol, which is greater than the molar mass of radon, which is 222 g/mol. Having a higher molar mass, this gaseous form of Xe would be denser than gaseous radon, thus stealing the crown.

However, this would be highly speculative, since the conditions in which dimers are formed are difficult to maintain, thus the molecular species last for a very short time.

The heaviest noble gas is radon (Rn)

Given the above arguments, we conclude that the heaviest noble gas is radon. This element is an inert, colorless and odorless gas that is also radioactive.

Of all the elements in group 18, radon has the highest atomic weight (222 u) and, apart from the debatable exception of Xe ₂ , is also the densest of the noble gases, with a density of 9.074 g/L at a temperature of 25 °C and a pressure of 1 atm.

References

Dubé, P. (1991, December 1). Supersonic cooling of rare-gas excimers excited in dc discharges . Optical Publishing Group. https://www.osapublishing.org/ol/abstract.cfm?uri=ol-16-23-1887

Jerabek, P. (2018, January 31). Electron and Nucleon Localization Functions of Oganesson: Approaching the Thomas-Fermi Limit . Physical Review Letters 120, 053001. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.053001

Lomaev, MI, Tarasenko, V., & Schitz, D. (2006, June). A high-power xenon dimer excilamp . Technical Physics Letters 32(6):495–497. https://www.researchgate.net/publication/243533559_A_high-power_xenon_dimer_excilamp

National Institute of Standards and Technology. (2021). Xenon dimming . NIST. https://webbook.nist.gov/cgi/inchi/InChI%3D1S/Xe2/c1-2

Oganessian, YT, & Rykaczewski, KP (2015). A beachhead on the island of stability. Physics Today 68, 8, 32. https://physicstoday.scitation.org/doi/10.1063/PT.3.2880