Elements

Noble Gases – Periodic Table

What are the Noble Gases

The noble gases are a group of six inert (nonreactive) gases on the far right side of the periodic table. They are members of group 18, the last group on the periodic table.

All of the noble gases occur in the atmosphere. In fact, air is 0.934% argon, while the other group 18 elements are present in much smaller quantities. Most of this argon in the air arises from the decay of radioactive potassium-40. Neon makes up .0018% of air, helium .00052%, krypton .00011%, and xenon .000009%.

Properties & Uses of the Noble Gases

The noble gases are tasteless, odorless, colorless, nonflammable, and mostly nonreactive. These monatomic gases also conduct electricity and can fluoresce. Because of their nonreactivity, one organic chemist even referred to them as the “lazy elements.” However, Neil Bartlett showed us another side to these elements in 1962 by discovering a compound of xenon.

The noble gases all have very low melting and boiling points, due to their weak intermolecular forces. They are all very close to being ideal gases. Unlike the halogens, noble gases are monatomic, meaning that they do not form molecules of multiple atoms – each “molecule” of the gas is just a single atom. Helium is the only element that will not freeze at atmospheric pressure, no matter how low the temperature.

Helium’s uses include blimps and balloons, as well as deep-sea diving (aka technical diving), mixed with oxygen. Most helium comes from natural gas, and some people worry that we may run out of helium one day. Argon is used in incandescent lamps, and neon is used in signs. Both helium and neon are used as cryogenic refrigerants.

Xenon is an anesthetic, albeit a costly one. Radon is a product of the radioactive decomposition of radium compounds.

Noble gases are often used in fluorescent lighting and discharge lamps. This is because they easily emit electromagnetic radiation. If an electrical discharge passes through a noble gas at low pressure, the gas will glow, as seen in the image below. Each noble gas glows a different color: for example, neon is reddish orange, like in neon signs. Argon glows blue, and krypton glows a whitish purple.

File:Edelgase 1.jpg
Each of the noble gases (except radon) and their characteristic glow with an electrical discharge.

List of all Noble Gases

The list of all noble gases includes helium, neon, argon, krypton, xenon, and radon. A potential addition to the list appeared on the scene in 2006, when researchers at the Joint Institute for Nuclear Research in Russia synthesized Oganesson, atomic number 118. It may also be a noble gas, but because scientists can only synthesize it for brief periods of time its properties are still largely unknown.

History of the Noble Gases

When Lord Rayleigh and William Ramsay discovered argon in 1895, they surprised the scientific world. They had not predicted any elements could lie between the halogens and the alkali metals. In the following three years, Ramsay would go on to discover helium, neon, krypton, and xenon. German chemist Friedrich Ernst Dorn discovered radon soon after in 1900, initially naming it niton.

Originally these elements were called inert gases, or rare gases. The phrase noble gas comes from the German world Edelgas, used first in 1898 by Hugo Erdmann, the same year radon was first identified. We now know that several of these elements are quite abundant on earth. Helium is the second most abundant element in the universe, after hydrogen. It is also now known that many of the gases are not completely inert.

Noble gas inside a buckminsterfullerene molecule,
Noble gas atom held within a buckminsterfullerene molecule, which is 60 carbon atoms. Rendition by Hugo Alejandro Jimenez Vazquez

Noble Gas Chemistry

For a long time, chemists thought that these gases could not bond with other elements. We now know that is incorrect, in fact, the further down the group you go, the more reactive the noble gases get. Xenon can form several compounds with fluorine. However, it is quite difficult to get xenon and krypton to form compounds.

Linus Pauling predicted in 1933 that xenon and krypton could form compounds, and he was correct. In 1962, Neil Bartlett used platinum hexafluoride to form a compound between xenon, platinum, and fluorine – xenon hexafluoroplatinate, Xe[PtF6]. Interestingly, it was a mustard yellow solid compound. Since then, many compounds of xenon have been formed, along with some compounds of krypton, and with much difficulty, argon fluorohydride – HArF. XeF2, XeF4, and XeF6 all appeared shortly after the first xenon compound was announced.

Trapped in a Fullerene

It was found that noble gas atoms can be “caged” inside a buckminsterfullerene molecule. A fullerene molecule, depicted in the image above, consists of a large number of carbon atoms in a polyhedral shape – with a 60 carbon structure being particularly stable. The noble gas enters inside the molecule when heated to 6500 degrees Celsius under 3000 atmospheres pressure of one of the gases. With an atom trapped in the center, it becomes an “endohedral fullerene.”

Why are the Noble Gases Unreactive?

All of the noble gases have a full outer shell, with the maximum number of valence electrons. This electron configuration is extremely stable, and it takes a large amount of energy to remove an electron from this stable configuration.

Fun fact about the Noble Gases

  1. In Antarctica, it gets so cold that radon could be a liquid there.
  2. All of the noble gases are soluble in water, and radon is highly soluble
  3. Krypton gets its name from the Greek word “Kryptos”, which means “the hidden one.”
  4. Neon signs only contain pure neon if they are orange, otherwise they contain other gases
  5. Xenon is a more potent anesthetic than nitrous oxide and allows the patient to recover more quickly
  6. Because of the way we define electronegativity, it is undefined or close to zero for noble gases

Further Reading

What are the Halogens
Learning the periodic table
Functional groups in organic chemistry

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