Group 8A (or VIIIA) of the periodic table are the noble gases or inert gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). The name comes from the fact that these elements are virtually unreactive towards other elements or compounds. They are found in trace amounts in the atmosphere (in fact, 1% of the atmosphere is argon); helium is also found in natural gas deposits. In their elemental form at room temperature, the Group 8A elements are all colorless, odorless, monatomic gases.
The Group 8A elements have a full octet of eight valence electrons in their highest-energy orbitals (ns2np6), so these elements have very little tendency to gain or lose electrons to form ions, or share electrons with other elements in covalent bonds. They can be forced to form compounds with other elements, but require special conditions to do so. Xenon, krypton compounds are known (this was first accomplished by Neil Bartlett in 1962), including XeF2, XeF4, XeF6, XeOF2, XeOF4, XeO2F2, XeO3F2, XeO2F4, XeO3, XeO4, KrF2, RnF2, among others.
Helium (He, Z=2).
Helium is a colorless, odorless, unreactive gas which liquefies at -268.97°C (4.18 K). The name "helium" is derived from the Greek word for the Sun, helios. Helium was originally discovered on the Sun in 1868 by Pierre Janssen: his analysis of the light emitted from the Sun's corona during a solar eclipse showed that there were lines in the Sun's spectrum that were produced by a previously unknown element. When helium was found on Earth in the late 1890's (in association with uranium ores), it was found to have the same emission spectrum as the element previously found in the Sun, indicating that it was indeed the same element. The concentration of helium in the Earth's crust is about 8 ppb, making it the 71st most abundant element; it is found in the atmosphere at a concentration of 5 ppm (by volume). It is found in some minerals, where it is produced by radioactive elements that decay by alpha particle emission (see below).
Helium is the second most abundant element in the universe (23% by mass); hydrogen and helium together make up 99% of the "normal" matter of the universe. (Of course, there's also "dark matter" and "dark energy" to worry about, but that's another story.) Hydrogen, helium, and trace amounts of lithium were produced at the beginning of the Universe in the Big Bang, and became concentrated into stars by the force of gravity. The fusion of hydrogen atoms in stars to produce helium produces huge amounts of energy; the energy in sunlight is captured by plants in photosynthesis, and drives most of the chemistry of living organisms.
The most common isotope of helium, helium-4, consists of two protons and two neutrons in the nucleus, surrounded by two electrons. An alpha particle is a type of radiation emitted by some radioactive nuclei, which consists of two protons and two neutrons, having a 2+ charge; when an alpha particle is emitted by a radioactive element, it quickly picks up two electrons from its environment, becoming a helium atom. Helium is found along with some radioactive minerals that decay by alpha particle emission, however the concentration of helium in these minerals is very low. The main source of helium is from natural gas, where it is separated out from the other components by fractional distillation.
Helium liquefies at 4.18 K (-268.97°C); at 2.17 K it becomes a superfluid, in which all viscosity (resistance to flow) is lost. Some materials become superconductors when immersed in liquid helium; in these materials, there is no resistance to the flow of electricity. Superconducting magnets are used for an important medical imaging technology called Magnetic Resonance Imaging (MRI), which allows images of tissues and organs to be obtained. A related technology is used by chemists to obtain information that can be used to deduce the structures of molecules; this technique is called Nuclear Magnetic Resonance (NMR); the two technologies are basically the same, but the term "nuclear" is avoided in the medical application because of its unpleasant associations (even though there is no nuclear radiation involved).
Helium is also used in blimps, where the combination of being lighter than air and nonflammable makes it much safer than hydrogen (as demonstrated in the explosion of the Hindenburg). It is also used in deep-sea diving, since helium is less soluble in the bloodstream than nitrogen, and reduces the risk of the diver suffering nitrogen narcosis, high pressure nervous syndrome (HPNS), or the bends. Breathing helium gas causes the voice to become higher pitched because helium is less dense than air, changing the frequency of the sound waves generated in the larynx.
Neon (Ne, Z=10).
Neon is a colorless, odorless, unreactive gas which liquefies at -246.1°C (27.1 K). Neon was discovered in 1898 by Sir William Ramsay (see entry on Argon below) and his assistant Morris Travers while working with a sample of krypton. The name comes from the Greek word for "new," neos. It is found in the Earth's crust at a concentration of 70 ppt, making it the 80th most abundant element; it is found in the atmosphere at a concentration of 18 ppm (by volume). Neon which is used commercially is extracted from liquid air by fractional distillation.
Neon is used primarily in neon lights; stimulating neon with electricity causes it to emit an intense red light. Other noble gases are used in "neon lights" as well, to produce different colors. Neon is also used in some underwater breathing mixtures; liquid neon is used in low-temperature cooling.
Argon (Ar, Z=18).
Argon is a colorless, odorless, unreactive gas which liquefies at -185.8°C (87.3 K). It is found in the Earth's crust at a concentration of 1.2 ppm (produced by the radioactive decay of potassium-40), making it the 56th most abundant element; it is found in the atmosphere at a concentration of 0.93% (by volume). Argon which is used commercially is extracted from liquid air by fractional distillation.
Even though argon makes up almost 1% of the Earth's atmosphere, it was not identified until 1894 by John William Strutt, 3rd Baron Rayleigh (Nobel Prize in Physics, 1904) and Sir William Ramsay (Nobel Prize in Chemistry, 1904). Argon had been observed before (by Henry Cavendish in 1785), but it had not been identified as an element. Because of its unreactivity, it was named for the Greek word for "idle," argos.
When argon was discovered, it didn't fit with the reactivities of any other element on the periodic table, and chemists realized that there might be a whole family of previously-unrecognized elements, which we now recognize as Group 8A.
Argon is used in light bulbs because it is inert to the hot metal of the bulb filament; the filament would burn out much faster in air or even pure nitrogen. It is also used in metal refining, some "neon" lights (argon produces a blue light), some food packaging (to provide an inert atmosphere to reduce spoilage), in the tires of some luxury cars in place of air, and in lasers used in eye surgery.
Argon is produced by the radioactive decay of potassium-40; the potassium-40 nucleus undergoes electron capture to produce the stable argon-40 nucleus. The ratio of potassium-40 to argon-40 in some rocks can be used to determine the age of the rock.
Recently, the first argon compound, argon fluorohydride (HArF), was reported, which is stable up to 27 K. (I'm waiting for hydrogen to be replaced by boron: I think a molecule that spells out "BArF" would be worth investigating. Or not.)
Krypton (Kr, Z=36).
Krypton is a colorless, odorless, unreactive gas which liquefies at -153.2°C (119.9 K). It is found in the Earth's crust at a concentration of 10 ppt, making it the 81st most abundant element; it is found in the atmosphere at a concentration of 1 ppm (by volume). Krypton which is used commercially is extracted from liquid air by fractional distillation.
Krypton was discovered in 1898 by Sir William Ramsay and his assistant, Morris Travers, in a sample of argon gas extracted from the atmosphere. The name "krypton" is derived from the Greek word kryptos, meaning "hidden" (which has nothing to do with the home planet of Superman).
Krypton is used in some "neon" lights, producing a violet color when electricity is passed through it. Radioactive kyrypton-85 is produced in nuclear reactors and nuclear processing plants; monitoring krypton-85 in the atmosphere allowed the United States to monitor how much nuclear material the Soviet Union was using.
Xenon (Ze, Z=54).
Xenon is a colorless, odorless, unreactive gas which liquefies at -111.7°C (161.4 K). It is found in the Earth's crust at a concentration of 2 ppt, making it the 83rd most abundant element; it is found in the atmosphere at a concentration of 90 ppb (by volume). Xenon which is used commercially is extracted from liquid air by fractional distillation.
Xenon was also discovered by Ramsay and Travers in 1898. The name is derived from the Greek word for "stranger," xenos. Xenon lights glow with a blue light, and also emits some low-frequency ultraviolet light. Xenon lights are used in tanning beds, biocidal lamps used in food preparation, car headlights, flash lighting on cameras, strobe lights used in high-speed photography, and has been tested for use in space travel in ion-propulsion engines (such as the experimental NASA spacecraft Deep Space 1).
Radon (Rn, Z=86).
Radon is a colorless, odorless, radioactive gas which liquefies at -61.8°C (211.3 K). It is found in the Earth's crust in only trace amounts, and is one of the ten least abundant elements; it is found in the atmosphere at a concentration of 10-9 ppt (by volume).
Radon was discovered in 1900 by Friederich Ernst Dorn while investigating "radium emanation" (first observed by Marie and Pierre Curie), a gas found in sealed vessels of radium. The name is a variation on the name of the element radium. It is produced in the radioactive decay of uranium and thorium, which in a series of steps decay into radium, which then undergoes alpha-decay to produce radon. The most common isotopes of radon are radon-220 and radon-222, which have half-lives of 55.6 seconds and 3.82 days, respectively. Although chemically unreactive, when radon is produced, it can escape from the ground in the gas form, and collect in basements, or other contained areas. Radon is an alpha-particle emitter; while alpha particles can be blocked easily, if the radon gas is inhaled, and undergoes alpha decay in the lungs, it produces an isotope of polonium, which is a solid, as are all of its decay products, some of which have half-lives of over 20 years. Having alpha-particle emitters inside the lungs leads to a greatly increased risk of lung cancer, accounting for about 10% of all reported cases of lung cancer.
John Emsley, The Elements, 3rd edition. Oxford: Clarendon Press, 1998.
John Emsley, Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2001.
David L. Heiserman, Exploring Chemical Elements and their Compounds. New York: TAB Books, 1992.