Group 8A: The Noble Gases

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The noble gases consist of the elements helium, neon, argon, krypton, xenon, and radon. They comprise group 18 of the Periodic Table of the Elements. They are typically regarded as "inert" since they generally do not react with other elements or compounds. They do not tend to react because, in their atomic state, they have a full valence shell of electrons, giving them little reason to form bonds to other species. Noble gases serve a variety of practical uses; they are commonly used in lasers and in fluorescent lighting among other applications.

Contents

History/Applications

Helium

Helium was initially discovered in 1868 during a solar eclipse, hence the name (helios is the Greek word for "sun"). French astronomer Pierre Janssen and English astronomer Joseph Lockyer both found an unexplainable line in the spectral line of the sun during the eclipse. They quickly speculated that the line indicated a new element. Their theory was met with strong resistance since spectroscopy was still a relatively new technique, but nonetheless the name "helium" was given to the newly discovered element.

Nothing more was discovered with regard to helium until the late 1880s, when an unknown gas was extracted from various minerals by Sir William Ramsay. When the visible spectrum of the gas was taken, it was found to match with the one noted by Janssen and Lockyer, and thus the discovery was confirmed.

In the universe as a whole, helium is the second most abundant element and the most abundant noble gas. However, on earth it is less abundant than neon and argon. It is commonly found in natural gas deposits, where it is created by alpha decay in which the emitted alpha particles pick up electrons and become helium atoms.

Helium has many practical applications. One of the most common uses is in balloons, blimps, and other airborne devices. Since helium is lighter than air (which is mainly nitrogen and oxygen), it floats in air, and thus is commonly used to provide lift. Helium is also used to create an artificial atmosphere for divers, since the nitrogen in normal air is not conducive to the pressures associated with diving and resurfacing. Liquid helium, which has the lowest boiling point of any liquid, is commonly used as a coolant for magnetic resonance imaging equipment (NMR, MRI, etc.).

Neon

Neon, krypton, and xenon were all discovered in 1898 by Sir William Ramsay and Morris Travers. They used liquid samples of argon, eventually discovering a lighter colored fraction that contained distinct spectral lines. The new element was named "neon".

Neon is most commonly used in lights since it can fluoresce brightly. It is also used in lasers.

Argon

Argon was discovered by the work of Lord Rayleigh and Sir William Ramsay in 1895. Upon measuring the density of air samples, they noticed that a small fraction of the mass of the samples was unaccounted for. Upon taking the visible spectrum, they found it to be distinct from that of any other known element. However, they noted that if the newly discovered component was a pure element, it would have a mass of approximately 39.9 g/mol, putting it between potassium and calcium in the periodic table, where there was no open spot for a new element. Other researchers proposed numerous other possible structures for the unknown component, such as a trinitrogen molecule or a diatomic molecule of a yet unknown element with an atomic mass of around 20 g/mol. Although the name "argon" was proposed for the newfound compound, its status as an element was not cemented until after the discoveries of other elements in group 18, which led scientists to realize that there was, in fact, another group at the end of the periodic table.

On earth, Argon is the most abundant noble gas. It is commonly used in fluorescent tubes and incandescent light bulbs. Additionally, it is often used as an inert atmosphere for research purposes in cases of compounds that react with common constituents in air.

Krypton

Krypton, neon, and xenon were all discovered in 1898 by Sir William Ramsay and Morris Travers. They used liquid samples of air and allowed the known constituents to boil off slowly. After most of the components boiled off, the researchers found new spectral lines that were attributed to a new element. The newly discovered element was named "krypton".

Krypton is commonly used in fluorescent tubes and lasers, like other elements in the group.

Xenon

Xenon, neon, and krypton were all discovered in 1898 by Sir William Ramsay and Morris Travers. They used the same techniques that they had used in the past to discover neon and krypton, but for xenon they used a "liquid-air machine". They eventually discovered a new component of air that glowed bright blue when placed in a discharge tube, and named the new discovery "xenon".

Xenon is commonly used in fluorescent tubes and lasers, like other elements in the group.

Radon

Radon was discovered in conjunction with the work of Marie and Pierre Curie. They discovered that a radioactive gas was formed when radon was left out in the air. This gas was determined to be the sixth noble gas, and was given the name "radon".

Unlike the other noble gases, radon is not used for any practical purpose in today's society. This is mainly because it is highly carcinogenic, causing approximately 12% of all cases of lung cancer in the United States every year. Since radon is a decay product of the naturally occurring radium-226, many households have radon detectors in order to monitor levels of the toxic gas.

Chemical Properties

Physical Constants of Noble Gases, [1]

Noble gases all have fairly high ionization energies because their valence electrons experience a very high effective nuclear charge. As the atoms get bigger as one goes down the column, the ionization energies decrease because the valence electrons are further away from the nucleus and thus are shielded from the full nuclear charge by other electrons.

Because they have a full valence shell of electrons, noble gases do not tend to attract electrons to themselves. This is why they have no electronegativity values.

As one goes down the column, the polarizabilities of the atoms increase due to the increased size. This in turn increases the strength of the induced dipole-induced dipole interactions between the molecules, which in turn increases the melting and boiling points as one goes down the column.

Reactions/Compounds

Since the noble gases have a full valence shell of electrons in their atomic state, they do not tend to bond with other species. Helium, neon, and argon are currently classified as completely inert- they do not form any compounds whatsoever. A few compounds of krypton and radon have been isolated, and many compounds of xenon have been isolated.

Xenon can form compounds with fluorine- these are the most common compounds and are often used to make other compounds. Upon mixing xenon gas with difluorine gas under conditions of high pressure and temperature, the following reactions can occur depending on the specific pressure and temperature:

Xe(g) + F2(g) ---> XeF2(s)
Xe(g) + 2F2(g) ---> XeF4(s)
Xe(g) + 3F2(g) ---> XeF6(s)

Xenon difluoride is a strong oxidizing agent, and xenon hexafluoride functions as both a fluoride donor and a floride acceptor. The three xenon fluorides will react violently with water in various manners. XeF2 will form xenon gas, whereas XeF4 and XeF6 will produce xenon trioxide.

2XeF2 + 2H2O ---> 2Xe + 4HF + O2
6XeF4 + 12H2O ---> 2XeO3 + 4Xe + 3O2 + 24HF
XeF6 + 3H2O ---> XeO3 + 6HF

These four compounds, xenon difluoride (XeF2), xenon tetrafluoride (XeF4), xenon hexafluoride (XeF6), and xenon trioxide (XeO3), are the most common noble gas compounds. Other xenon compounds also exist, such as XeF2, XeO4, XeO2F2, and XeOF4.

While xenon forms the majority of the noble gas compounds, it is not the only reactive noble gas. Krypton difluoride and radon difluoride have also been synthesized. It is possible that radon forms many more compounds, but since it is so radioactive it is too dangerous to work with in many cases.

Additional Information

Wikipedia, [2]

Chemical Elements.com, [3]

References

Rodgers, Glen E. Descriptive Inorganic, Coordination, and Solid-State Chemistry. Canada: Thomas Learning, 2002.