Structural Biochemistry/Noble Gases

Background Information

The Noble Gases are:

• Helium (He)
• Neon (Ne)
• Argon (Ar)
• Krypton (Kr)
• Xenon (Xe)
• Radon (Rn)

The noble gases are the 18th group in the periodic table. They are also sometimes called the "inert gasses" due to their lack of reactivity with other chemicals. The elements in this group are typically inert because they possess a full valence shell. All of the noble gases are monatomic, as opposed to other gases (ie. H₂, O₂, N₂, F₂, Cl₂) which exist as diatomic at room temperature and atmospheric pressure. The inert gas helium has been employed in respiratory obstruction, in investigative and diagnostic testing, and in hyperbaric applications[2].

The noble gases have weak interatomic force, and consequently have very low melting and boiling points. They are all monatomic gases under standard conditions, including the elements with larger atomic masses than many normally solid elements. Helium has several unique qualities when compared with other elements: its boiling and melting points are lower than those of any other known substance; it is the only element known to exhibit superfluidity; it is the only element that cannot be solidified by cooling under standard conditions—a pressure of 25 standard atmospheres (2,500 kPa; 370 psi) must be applied at a temperature of 0.95 K (−272.200 °C; −457.960 °F) to convert it to a solid. The noble gases up to xenon have multiple stable isotopes. Radon has no stable isotopes; its longest-lived isotope, 222Rn, has a half-life of 3.8 days and decays to form helium and polonium, which ultimately decays to lead.

The noble gases are colorless, odorless, tasteless, and nonflammable under standard conditions. They were once labeled group 0 in the periodic table because it was believed they had a valence of zero, meaning their atoms cannot combine with those of other elements to form compounds. However, it was later discovered some do indeed form compounds, causing this label to fall into disuse. Very little is known about the properties of the most recent member of group 18, ununoctium (Uuo).

The noble gases show extremely low chemical reactivity; consequently, only a few hundred noble gas compounds have been formed. Neutral compounds in which helium and neon are involved in chemical bonds have not been formed (although there are some theoretical evidence for a few helium compounds), while xenon, krypton, and argon have shown only minor reactivity. The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn.

Neon, argon, krypton, and xenon are obtained from air using the methods of liquefaction of gases, to convert elements to a liquid state, and fractional distillation, to separate mixtures into component parts. Helium is typically produced by separating it from natural gas, and radon is isolated from the radioactive decay of radium compounds.[11] The prices of the noble gases are influenced by their natural abundance, with argon being the cheapest and xenon the most expensive. As an example, the table to the right lists the 2004 prices in the United States for laboratory quantities of each gas.

Helium

Helium is colorless and odorless. Among all the elements in the periodic table, its boiling and melting points are very low. It only exists in gas form at extreme environmental conditions. Pierre Janssen and Norman Lockyer first observed this element in 1868 when they found there existed a yellow line of light in a solar eclipse. Helium is the second light element and is very abundant on earth. This is due to the high binding energy of helium to lithium, beryllium, and boron. Helium has two electrons in the orbital around the nucleus with two protons.

Krypton

Krypton, found often times in fluorescent lamps, is also colorlesss and odorless and is isolated for research or industrial purposes by distilling air. Krypton can be used to construct high power lasers or krypton fluoride lasers. It has very unique spectral signatures because of its strong spectrum lines. The amount of krypton available in the history of the universe was derived from meteors and solar wind. It acts as a great light source for high end photography.

Argon

Argon is more soluble in water than nitrogen gas. During room temperature, argon will not form any stable formations. Argon was isolated in 1894 by William Ramsay when he removed oxygen, carbon dioxide, water, and nitrogen from clean air. Earth's atmosphere is composed of 1.29% of argon. Argon exists in the most common isotopes as Argon 40, Argon 36, Argon 38, and Argon 40.

Xenon

Xenon served as an important tool to gain insight of the solar system. Xenon exists in nine stable isotopes but there are actually over 40 stable isotopes that undergo half life and decay. Xenon is used in thrusters and spacecraft. Xenon exists in trace amounts in the earth's atmosphere and in mineral springs. The light properties of xenon have a broad spectrum of visible light and emits bluish light in a gas tube. Nuclear reactors often times expel xenon as well.

Neon

Neon emits red light in neon lamps and in discharge tubes. It exists in trace amounts in the earth's atmosphere and in air. It is the second lightest noble gas. Neon emits the strongest discharge of light at normal conditions. Glow-discharge lamps are typically very small but give off a good amount of light. Although all noble gases are unreactive, neon is considered the least reactive. The rarity of neon gas makes attaining small quantities for research very expensive.

Radon

Radon is a very dense gas at room temperature and is a health hazard. It has high radioactivity therefore it's hard to study it. Radon is formed from the decay of a chain or uranium. High concentrations of radon can cause lung cancer therefore it is considered a very toxic air contaminant and facilities must be evacuated if radon is released. Miners are most exposed to radon. Lung cancer and bad ventilation occurred among many miners in Schneeberg, Germany during the early years of the Cold War.

References

1. Oxtoby, David. (2008). Principles of Modern Chemistry, 6th Ed., ISBN0-534-49366-1.

2. Goodman, Louis S, and Alfred Goodman Gilman. The Pharmacological Basis of Therapeutics. 7th ed. New York, N.Y.: Macmillan, 1985.