Noble gas

The noble gases are the members of group 18 of the periodic table: helium, neon, argon, krypton, xenon, radon and, in some cases, oganesson. Under standard conditions, the first six of these elements are odorless, colorless, monatomic gases with very low chemical reactivity and cryogenic boiling points. The properties of oganesson are uncertain.
The intermolecular force between noble gas atoms is the very weak London dispersion force, so their boiling points are all cryogenic, below.
The noble gases' inertness, or tendency not to react with other chemical substances, results from their electron configuration: their outer shell of valence electrons is "full", giving them little tendency to participate in chemical reactions. Only a few hundred noble gas compounds are known to exist. The inertness of noble gases makes them useful whenever chemical reactions are unwanted. For example, argon is used as a shielding gas in welding and as a filler gas in incandescent light bulbs. Helium is used to provide buoyancy in blimps and balloons. Helium and neon are also used as refrigerants due to their low boiling points. Industrial quantities of the noble gases, except for radon, are obtained by separating them from air using the methods of liquefaction of gases and fractional distillation. Helium is also a byproduct of the mining of natural gas. Radon is usually isolated from the radioactive decay of dissolved radium, thorium, or uranium compounds.
The seventh member of group 18 is oganesson, an unstable synthetic element whose chemistry is still uncertain because only five very short-lived atoms have ever been synthesized. IUPAC uses the term "noble gas" interchangeably with "group 18" and thus includes oganesson; however, due to relativistic effects, oganesson is predicted to be a solid under standard conditions and reactive enough not to qualify functionally as "noble".

History

Noble gas is translated from the German noun Edelgas, first used in 1900 by Hugo Erdmann to indicate their extremely low level of reactivity. The name makes an analogy to the term "noble metals", which also have low reactivity. The noble gases have also been referred to as inert gases, but this label is deprecated as many noble gas compounds are now known. Rare gases is another term that was used, but this is also inaccurate because argon forms a fairly considerable part of the Earth's atmosphere due to decay of radioactive potassium-40.
Pierre Janssen and Joseph Norman Lockyer had discovered a new element on 18 August 1868 while looking at the chromosphere of the Sun, and named it helium after the Greek word for the Sun, ἥλιος. No chemical analysis was possible at the time, but helium was later found to be a noble gas. Before them, in 1784, the English chemist and physicist Henry Cavendish had discovered that air contains a small proportion of a substance less reactive than nitrogen. A century later, in 1895, Lord Rayleigh discovered that samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. Along with Scottish scientist William Ramsay at University College, London, Lord Rayleigh theorized that the nitrogen extracted from air was mixed with another gas, leading to an experiment that successfully isolated a new element, argon, from the Greek word ἀργός. With this discovery, they realized an entire class of gases was missing from the periodic table. During his search for argon, Ramsay also managed to isolate helium for the first time while heating cleveite, a mineral. In 1902, having accepted the evidence for the elements helium and argon, Dmitri Mendeleev included these noble gases as group 0 in his arrangement of the elements, which would later become the periodic table.
Ramsay continued his search for these gases using the method of fractional distillation to separate liquid air into several components. In 1898, he discovered the elements krypton, neon, and xenon, and named them after the Greek words κρυπτός, νέος, and ξένος, respectively. Radon was first identified in 1898 by Friedrich Ernst Dorn, and was named radium emanation, but was not considered a noble gas until 1904 when its characteristics were found to be similar to those of other noble gases. Rayleigh and Ramsay received the 1904 Nobel Prizes in Physics and in Chemistry, respectively, for their discovery of the noble gases; in the words of J. E. Cederblom, then president of the Royal Swedish Academy of Sciences, "the discovery of an entirely new group of elements, of which no single representative had been known with any certainty, is something utterly unique in the history of chemistry, being intrinsically an advance in science of peculiar significance".
The discovery of the noble gases aided in the development of a general understanding of atomic structure. In 1895, French chemist Henri Moissan attempted to form a reaction between fluorine, the most electronegative element, and argon, one of the noble gases, but failed. Scientists were unable to prepare compounds of argon until the end of the 20th century, but these attempts helped to develop new theories of atomic structure. Learning from these experiments, Danish physicist Niels Bohr proposed in 1913 that the electrons in atoms are arranged in shells surrounding the nucleus, and that for all noble gases except helium the outermost shell always contains eight electrons. In 1916, Gilbert N. Lewis formulated the octet rule, which concluded an octet of electrons in the outer shell was the most stable arrangement for any atom; this arrangement caused them to be unreactive with other elements since they did not require any more electrons to complete their outer shell.
In 1962, Neil Bartlett discovered the first chemical compound of a noble gas, xenon hexafluoroplatinate. Compounds of other noble gases were discovered soon after: in 1962 for radon, radon difluoride, which was identified by radiotracer techniques and in 1963 for krypton, krypton difluoride. The first stable compound of argon was reported in 2000 when argon fluorohydride was formed at a temperature of.
In October 2006, scientists from the Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory successfully created synthetically oganesson, the seventh element in group 18, by bombarding californium with calcium.

Physical and atomic properties

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 point at 1 atm is lower than those of any other known substance; it is the only element known to exhibit superfluidity; and, it is the only element that cannot be solidified by cooling at atmospheric pressure – a pressure of must be applied at a temperature of to convert it to a solid while a pressure of about is required at room temperature. The noble gases up to xenon have multiple stable isotopes; krypton and xenon also have naturally occurring radioisotopes, namely ⁷⁸Kr, ¹²⁴Xe, and ¹³⁶Xe, all have very long lives and can undergo double electron capture or double beta decay. 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. Oganesson also has no stable isotopes, and its only known isotope ²⁹⁴Og is very short-lived. Melting and boiling points increase going down the group.
File:First Ionization Energy blocks.svg|left|thumb|upright=3|This is a plot of ionization potential versus atomic number. The noble gases have the largest ionization potential for each period, although period 7 is expected to break this trend because the predicted first ionization energy of oganesson is lower than those of elements 110–112.|alt=A graph of ionization energy vs. atomic number showing sharp peaks for the noble gas atoms.
The noble gas atoms, like atoms in most groups, increase steadily in atomic radius from one period to the next due to the increasing number of electrons. The size of the atom is related to several properties. For example, the ionization potential decreases with an increasing radius because the valence electrons in the larger noble gases are farther away from the nucleus and are therefore not held as tightly together by the atom. Noble gases have the largest ionization potential among the elements of each period, which reflects the stability of their electron configuration and is related to their relative lack of chemical reactivity. Some of the heavier noble gases, however, have ionization potentials small enough to be comparable to those of other elements and molecules. It was the insight that xenon has an ionization potential similar to that of the oxygen molecule that led Bartlett to attempt oxidizing xenon using platinum hexafluoride, an oxidizing agent known to be strong enough to react with oxygen. Noble gases cannot accept an electron to form stable anions; that is, they have a negative electron affinity.
The macroscopic physical properties of the noble gases are dominated by the weak van der Waals forces between the atoms. The attractive force increases with the size of the atom as a result of the increase in polarizability and the decrease in ionization potential. This results in systematic group trends: as one goes down group 18, the atomic radius increases, and with it the interatomic forces increase, resulting in an increasing melting point, boiling point, enthalpy of vaporization, and solubility. The increase in density is due to the increase in atomic mass.
The noble gases are nearly ideal gases under standard conditions, but their deviations from the ideal gas law provided important clues for the study of intermolecular interactions. The Lennard-Jones potential, often used to model intermolecular interactions, was deduced in 1924 by John Lennard-Jones from experimental data on argon before the development of quantum mechanics provided the tools for understanding intermolecular forces from first principles. The theoretical analysis of these interactions became tractable because the noble gases are monatomic and the atoms spherical, which means that the interaction between the atoms is independent of direction, or isotropic.