Noble+Gas

For the first six periods of the periodic table, the noble gases are exactly the members of **group 18** of the periodic table. However, this may no longer hold in the seventh period ; the next member of group 18 after radon, ununoctium, is probably not a noble gas. Instead, group 14 member ununguiadim likely exhibits noble-gas-like properties. The properties of the noble gases can be well explained by modern theories of atomic structure: their outer shell of valence electrons considered to be "full", giving them little tendency to participate in chemical reactions, and it has only been possible to prepare a few hundred noble gas compunds. The melting and boiling points or each noble gas are close together, differing by less than 10 °C (18 °F); consequently, they are liquids over only a small temperature range. Neon, argon, krypton, and xenon are obtained from air using the methods of liquefaction of gases and fractional distillation. Helium is typically separated from natural gas, and radon is usually isolated from the radioactive decay of dissolved radium compounds. Noble gases have several important applications in industries such as lighting, welding, and space exploration. A helium-oxygen breathing gas is often used by deep-sea divers at depths of seawater over 55 m (180 ft) to keep the diver from experiencing oxygen toxemia, the lethal effect of high-pressure oxygen, and nitrogen narcosis, the distracting narcotic effect of the nitrogen in air beyond this partial-pressure threshold. After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and ballons.
 * Noble gases** are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity. The six noble gases that occur naturally are helium (HE0, neon (Ne), argon (Ar), krypton (Kr), xeon (Xe), and the radioactive radon (Rn).

A group of chemical elements with very similar properties. These gases are odorless, colorless, monatomic with very low chemical reactivity. The six noble gases that occur naturally are Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xn), and radioactive Radon (Rn). The outer shell of the valence electrons have 8 electrons and are considered "full", which gives them little tendency to participate in chemical reactions. The noble gases can be found on the right-hand side of the periodic table as the last column.

Helium (He)

Neon (Ne)

Argon (Ar)

Krypton (Kr)

Xenon (Xe)

Chemical properties
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. [|[11]] Like other groups, the members of this family show patterns in its [|electron configuration], especially the outermost shells resulting in trends in chemical behavior: The noble gases have full valence [|electron shells]. [|Valence electrons] are the outermost [|electrons] of an atom and are normally the only electrons that participate in [|chemical bonding]. Atoms with full valence electron shells are extremely stable and therefore do not tend to form chemical bonds and have little tendency to gain or lose electrons. [|[30]] However, heavier noble gases such as radon are held less firmly together by [|electromagnetic force] than lighter noble gases such as helium, making it easier to remove outer electrons from heavy noble gases.
 * ~ [|Z] ||~ [|Element] ||~ No. of electrons/ [|shell] ||
 * 2 || [|helium] || 2 ||
 * 10 || [|neon] || 2, 8 ||
 * 18 || [|argon] || 2, 8, 8 ||
 * 36 || [|krypton] || 2, 8, 18, 8 ||
 * 54 || [|xenon] || 2, 8, 18, 18, 8 ||
 * 86 || [|radon] || 2, 8, 18, 32, 18, 8 ||



Noble gas notation
As a result of a full shell, the noble gases can be used in conjunction with the [|electron configuration] notation to form the //noble gas notation//. To do this, the nearest noble gas that precedes the element in question is written first, and then the electron configuration is continued from that point forward. For example, the electron notation of [|carbon] is 1s22s22p2, and the noble gas notation is [He]2s22p2. This notation makes it easier to identify elements, and is shorter than writing out the full notation of [|atomic orbitals]. [|[31]] Although there is evidence that [|ununquadium] is the next noble gas after radon, [|ununoctium] is still used for writing theoretical electron configurations for [|period 8 elements] because it is predicted to have a full shell. Element 120, for example, is predicted to have the electron configuration [Uuo]8s2.

Compounds
Structure of XeF4, one of the first noble gas compounds to be discovered 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 is some theoretical evidence for a few helium compounds), while xenon, krypton, and argon have shown only minor reactivity. [|[32]] The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn. In 1933, [|Linus Pauling] predicted that the heavier noble gases could form compounds with fluorine and oxygen. He predicted the existence of krypton hexafluoride ( KrF6 ) and [|xenon hexafluoride] ( XeF6 ), speculated XeF8 might exist as an unstable compound, and suggested [|xenic acid] could form [|perxenate] salts. [|[33]][|[34]] These predictions were shown to be generally accurate, except XeF8 is now thought to be both [|thermodynamically] and [|kinetically] unstable. [|[35]] Xenon compounds are the most numerous of the noble gas compounds that have been formed. [|[36]] Most of them have the xenon atom in the [|oxidation state] of +2, +4, +6, or +8 bonded to highly [|electronegative] atoms such as fluorine or oxygen, as in [|xenon difluoride] ( XeF2 ), [|xenon tetrafluoride] ( XeF4 ), [|xenon hexafluoride] ( XeF6 ), [|xenon tetroxide] ( XeO4 ), and sodium perxenate ( Na4XeO6 ). Some of these compounds have found use in [|chemical synthesis] as [|oxidizing agents] ; XeF2, in particular, is commercially available and can be used as a [|fluorinating] agent. [|[37]] As of 2007, about five hundred compounds of xenon bonded to other elements have been identified, including organoxenon compounds (those bonded to carbon), and xenon bonded to nitrogen, chlorine, gold, mercury, and xenon itself. [|[32]][|[38]] Compounds of xenon bound to boron, hydrogen, bromine, iodine, beryllium, sulphur, titanium, copper, and silver have also been observed but only at low temperatures in noble gas [|matrices], or in supersonic noble gas jets. [|[32]] In theory, radon is more reactive than xenon, and therefore should form chemical bonds more easily than xenon does. However, due to the high radioactivity and short half-life of [|radon isotopes], only a few [|fluorides] and [|oxides] of radon have been formed in practice. [|[39]] Krypton is less reactive than xenon, but several compounds have been reported with krypton in the [|oxidation state] of +2. [|[32]] [|Krypton difluoride] is the most notable and easily characterized. Compounds in which krypton forms a single bond to nitrogen and oxygen have also been characterized, [|[40]] but are only stable below −60 °C (−76 °F) and −90 °C(−130 °F) respectively). [|[32]]  Krypton atoms chemically bound to other nonmetals (hydrogen, chlorine, carbon) as well as some late [|transition metals] (copper, silver, gold) have also been observed, but only either at low temperatures in noble gas matrices, or in supersonic noble gas jets. [|[32]] Similar conditions were used to obtain the first few compounds of argon in 2000, such as [|argon fluorohydride] (HArF), and some bound to the late transition metals copper, silver, and gold. [|[32]] As of 2007, no stable neutral molecules involving covalently bound helium or neon are known. [|[32]] The noble gases—including helium—can form stable [|molecular ions] in the gas phase. The simplest is the [|helium hydride molecular ion], HeH+, discovered in 1925. [|[41]] Because it is composed of the two most abundant elements in the universe, hydrogen and helium, it is believed to occur naturally in the [|interstellar medium], although it has not been detected yet. [|[42]] In addition to these ions, there are many known neutral [|excimers] of the noble gases. These are compounds such as ArF and KrF that are stable only when in an [|excited electronic state] ; some of them find application in [|excimer lasers]. In addition to the compounds where a noble gas atom is involved in a [|covalent bond], noble gases also form [|non-covalent] compounds. The [|clathrates], first described in 1949, [|[43]] consist of a noble gas atom trapped within cavities of [|crystal lattices] of certain organic and inorganic substances. The essential condition for their formation is that the guest (noble gas) atoms must be of appropriate size to fit in the cavities of the host crystal lattice. For instance, argon, krypton, and xenon form clathrates with [|hydroquinone], but helium and neon do not because they are too small or insufficiently [|polarizable] to be retained. [|[44]] Neon, argon, krypton, and xenon also form clathrate hydrates, where the noble gas is trapped in ice. [|[45]]

Noble gases can form [|endohedral fullerene] compounds, in which the noble gas atom is trapped inside a [|fullerene] molecule. In 1993, it was discovered that when C60, a spherical molecule consisting of 60 [|carbon] atoms, is exposed to noble gases at high pressure, [|complexes] such as He@C60 can be formed (the //@// notation indicates He is contained inside C60 but not covalently bound to it). [|[46]] As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been obtained. [|[47]] These compounds have found use in the study of the structure and reactivity of fullerenes by means of the [|nuclear magnetic resonance] of the noble gas atom. [|[48]]

Noble gas compounds such as [|xenon difluoride] ( XeF2 ) are considered to be [|hypervalent] because they violate the [|octet rule]. Bonding in such compounds can be explained using a [|3-center-4-electron bond] model. [|[49]][|[50]] This model, first proposed in 1951, considers bonding of three collinear atoms. For example, bonding in XeF2 is described by a set of three [|molecular orbitals] (MOs) derived from [|p-orbitals] on each atom. Bonding results from the combination of a filled p-orbital from Xe with one half-filled p-orbital from each [|F] atom, resulting in a filled bonding orbital, a filled non-bonding orbital, and an empty [|antibonding] orbital. The [|highest occupied molecular orbital] is localized on the two terminal atoms. This represents a localization of charge which is facilitated by the high electronegativity of fluorine. [|[51]] The chemistry of heavier noble gases, krypton and xenon, are well established. The chemistry of the lighter ones, argon and helium, is still at an early stage, while a neon compound is still yet to be identified.