Noble gases

The noble gases are the elements in group 18 (also sometimes Group 0 IUPAC Style, or Group 8) of the periodic table. It is also called helium family or neon family. Chemically, they are very stable due to having the maximum number of valence electrons their outer shell can hold. A thorough explanation requires an understanding of electronic configuration, with references to quantum mechanics. Noble gases rarely react with other elements since they are already stable. Under normal conditions, they occur as odorless, colorless, monatomic gases. Each of them has its melting and boiling point close together, so that only a small temperature range exists for each noble gas in which it is a liquid. Noble gases have numerous important applications in lighting, welding and space technology. The seven noble gasses are: helium, neon, argon, krypton, xenon, radon, and ununoctium.

"Noble gas" is the translation of the German Edelgas, which was in use as early as 1898. The term edelgas is literally translated as immaculate gas. This refers to the extremely low level of reactivity under normal conditions. The noble gases have also been referred to as inert gases, but these terms are not strictly accurate because several of them do take part in chemical reactions. Another old term is rare gases, although in fact argon forms a considerable part (0.93% by volume, 1.29% by mass) of the Earth's atmosphere.

The existence of noble gases was not known until after the advent of the periodic table. In the late nineteenth century, Lord Rayleigh discovered that some samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. He and Scottish scientist William Ramsay theorized that the nitrogen extracted from air was associated with another gas, argon. With this discovery, they realized that a whole class of gases was missing from the periodic table. Eventually all the known noble gases except for helium were discovered in the air, with argon being much more common than the others, and the table was completed. Helium was detected spectrographically in the Sun in 1868. The isolation of helium on Earth had to wait until 1895. Under standard conditions, the noble gases all occur as monatomic gases.

One of the most commonly encountered uses of the noble gases in everyday life is in lighting. Argon is often used as a suitable safe and inert atmosphere for the inside of filament light bulbs, and is also used as an inert atmosphere in the synthesis of air and moisture sensitive compounds (as an alternative for nitrogen). Some of the noble gases glow distinctive colors when used inside lighting tubes (neon lights). Helium, due to its nonreactivity (compared with flammable hydrogen) and lightness, is often used in blimps and balloons. Helium and Argon are very commonly used to shield a welding arc, and the surrounding base metal from the atmosphere during welding. Krypton is also used in lasers, which are used by doctors for eye surgery.

Chemical series of the periodic table

	Alkali metals
	Alkaline earth metals
	Lanthanides
	Actinides
	Transition metals

	Poor metals
	Metalloids
	Nonmetals
	Halogens
	Noble gases

The periodic table of the chemical elements is a tabular method of displaying the chemical elements. Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869. Mendeleev intended the table to illustrate recurring ('periodic') trends in the properties of the elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.

The periodic table is now ubiquitous within the academic discipline of chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of chemical behavior. The table has also found wide application in physics, biology, engineering, and industry. The current standard table contains 117 confirmed elements as of October 16, 2006 (while element 118 has been synthesized, element 117 has not).