The noble gases make up a group of chemical elements with similar properties. The six noble gases that occur are helium, argon, krypton and the radioactive radon. Oganesson is variously predicted to be a noble gas as well or to break the trend due to relativistic effects. For the first six periods of the periodic table, the noble gases are the members of group 18. Noble gases are highly unreactive except when under particular extreme conditions; the inertness of noble gases makes them suitable in applications where reactions are not wanted. For example, argon is used in incandescent lamps to prevent the hot tungsten filament from oxidizing; the properties of the noble gases can be well explained by modern theories of atomic structure: their outer shell of valence electrons is considered to be "full", giving them little tendency to participate in chemical reactions, it has been possible to prepare only a few hundred noble gas compounds. The melting and boiling points for a given noble gas are close together, differing by less than 10 °C.
Neon, argon and xenon are obtained from air in an air separation unit using the methods of liquefaction of gases and fractional distillation. Helium is sourced from natural gas fields that have high concentrations of helium in the natural gas, using cryogenic gas separation techniques, radon is isolated from the radioactive decay of dissolved radium, thorium, or uranium compounds. Noble gases have several important applications in industries such as lighting and space exploration. A helium-oxygen breathing gas is used by deep-sea divers at depths of seawater over 55 m. After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and balloons. Noble gas is translated from the German noun Edelgas, first used in 1898 by Hugo Erdmann to indicate their low level of reactivity; the name makes an analogy to the term "noble metals", which have low reactivity. The noble gases have been referred to as inert gases, but this label is deprecated as many noble gas compounds are now known.
Rare gases is another term, used, but this is inaccurate because argon forms a considerable part of the Earth's atmosphere due to decay of radioactive potassium-40. Pierre Janssen and Joseph Norman Lockyer discovered a new element on August 18, 1868 while looking at the chromosphere of the Sun, named it helium after the Greek word for the Sun, ἥλιος. No chemical analysis was possible at the time, but helium was 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 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, Lord Rayleigh theorized that the nitrogen extracted from air was mixed with another gas, leading to an experiment that isolated a new element, from the Greek word ἀργός. With this discovery, they realized.
During his search for argon, Ramsay 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 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 and xenon, named them after the Greek words κρυπτός, νέος, ξένος, respectively. Radon was first identified in 1898 by Friedrich Ernst Dorn, 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 for their discovery of the noble gases; 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, 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, that for all noble gases except helium the outermost shell always contains eight electrons. In 1916, Gilbert N. Lewis formulated the octet rule, which conc
Neon is a chemical element with symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, inert monatomic gas under standard conditions, with about two-thirds the density of air, it was discovered in 1898 as one of the three residual rare inert elements remaining in dry air, after nitrogen, oxygen and carbon dioxide were removed. Neon was the second of these three rare gases to be discovered and was recognized as a new element from its bright red emission spectrum; the name neon is derived from the Greek νέον, neuter singular form of νέος, meaning new. Neon is chemically inert, no uncharged neon compounds are known; the compounds of neon known include ionic molecules, molecules held together by van der Waals forces and clathrates. During cosmic nucleogenesis of the elements, large amounts of neon are built up from the alpha-capture fusion process in stars. Although neon is a common element in the universe and solar system, it is rare on Earth, it composes about 18.2 ppm of air by a smaller fraction in Earth's crust.
The reason for neon's relative scarcity on Earth and the inner planets is that neon is volatile and forms no compounds to fix it to solids. As a result, it escaped from the planetesimals under the warmth of the newly ignited Sun in the early Solar System; the outer atmosphere of Jupiter is somewhat depleted of neon, although for a different reason. It is lighter than air, causing it to escape from Earth's atmosphere. Neon gives a distinct reddish-orange glow when used in low-voltage neon glow lamps, high-voltage discharge tubes and neon advertising signs; the red emission line from neon causes the well known red light of helium–neon lasers. Neon has few other commercial uses, it is commercially extracted by the fractional distillation of liquid air. Since air is the only source, it is more expensive than helium. Neon was discovered in 1898 by the British chemists Sir William Ramsay and Morris W. Travers in London. Neon was discovered when Ramsay chilled a sample of air until it became a liquid warmed the liquid and captured the gases as they boiled off.
The gases nitrogen and argon had been identified, but the remaining gases were isolated in their order of abundance, in a six-week period beginning at the end of May 1898. First to be identified was krypton; the next, after krypton had been removed, was a gas which gave a brilliant red light under spectroscopic discharge. This gas, identified in June, was named "neon", the Greek analogue of the Latin novum suggested by Ramsay's son; the characteristic brilliant red-orange color emitted by gaseous neon when excited electrically was noted immediately. Travers wrote: "the blaze of crimson light from the tube told its own story and was a sight to dwell upon and never forget."A second gas was reported along with neon, having the same density as argon but with a different spectrum – Ramsay and Travers named it metargon. However, subsequent spectroscopic analysis revealed it to be argon contaminated with carbon monoxide; the same team discovered xenon by the same process, in September 1898. Neon's scarcity precluded its prompt application for lighting along the lines of Moore tubes, which used nitrogen and which were commercialized in the early 1900s.
After 1902, Georges Claude's company Air Liquide produced industrial quantities of neon as a byproduct of his air-liquefaction business. In December 1910 Claude demonstrated modern neon lighting based on a sealed tube of neon. Claude tried to sell neon tubes for indoor domestic lighting, due to their intensity, but the market failed because homeowners objected to the color. In 1912, Claude's associate began selling neon discharge tubes as eye-catching advertising signs and was more successful. Neon tubes were introduced to the U. S. in 1923 with two large neon signs bought by a Los Angeles Packard car dealership. The glow and arresting red color made neon advertising different from the competition; the intense color and vibrancy of neon equated with American society at the time, suggesting a "century of progress" and transforming cities into sensational new environments filled with radiating advertisements and "electro-graphic architecture". Neon played a role in the basic understanding of the nature of atoms in 1913, when J. J. Thomson, as part of his exploration into the composition of canal rays, channeled streams of neon ions through a magnetic and an electric field and measured the deflection of the streams with a photographic plate.
Thomson observed two separate patches of light on the photographic plate, which suggested two different parabolas of deflection. Thomson concluded that some of the atoms in the neon gas were of higher mass than the rest. Though not understood at the time by Thomson, this was the first discovery of isotopes of stable atoms. Thomson's device was a crude version of the instrument. Neon is the second lightest inert gas. Neon has three stable isotopes: 21Ne and 22Ne. 21Ne and 22Ne are primordial and nucleogenic and their variations in natural abundance are well understood. In contrast, 20Ne is not known to be radiogenic; the causes of the variation of 20Ne in the Earth have thus been hotly debated. The princ
Simplified molecular-input line-entry system
The simplified molecular-input line-entry system is a specification in the form of a line notation for describing the structure of chemical species using short ASCII strings. SMILES strings can be imported by most molecule editors for conversion back into two-dimensional drawings or three-dimensional models of the molecules; the original SMILES specification was initiated in the 1980s. It has since been extended. In 2007, an open standard called. Other linear notations include the Wiswesser line notation, ROSDAL, SYBYL Line Notation; the original SMILES specification was initiated by David Weininger at the USEPA Mid-Continent Ecology Division Laboratory in Duluth in the 1980s. Acknowledged for their parts in the early development were "Gilman Veith and Rose Russo and Albert Leo and Corwin Hansch for supporting the work, Arthur Weininger and Jeremy Scofield for assistance in programming the system." The Environmental Protection Agency funded the initial project to develop SMILES. It has since been modified and extended by others, most notably by Daylight Chemical Information Systems.
In 2007, an open standard called "OpenSMILES" was developed by the Blue Obelisk open-source chemistry community. Other'linear' notations include the Wiswesser Line Notation, ROSDAL and SLN. In July 2006, the IUPAC introduced the InChI as a standard for formula representation. SMILES is considered to have the advantage of being more human-readable than InChI; the term SMILES refers to a line notation for encoding molecular structures and specific instances should be called SMILES strings. However, the term SMILES is commonly used to refer to both a single SMILES string and a number of SMILES strings; the terms "canonical" and "isomeric" can lead to some confusion when applied to SMILES. The terms are not mutually exclusive. A number of valid SMILES strings can be written for a molecule. For example, CCO, OCC and CC all specify the structure of ethanol. Algorithms have been developed to generate the same SMILES string for a given molecule; this SMILES is unique for each structure, although dependent on the canonicalization algorithm used to generate it, is termed the canonical SMILES.
These algorithms first convert the SMILES to an internal representation of the molecular structure. Various algorithms for generating canonical SMILES have been developed and include those by Daylight Chemical Information Systems, OpenEye Scientific Software, MEDIT, Chemical Computing Group, MolSoft LLC, the Chemistry Development Kit. A common application of canonical SMILES is indexing and ensuring uniqueness of molecules in a database; the original paper that described the CANGEN algorithm claimed to generate unique SMILES strings for graphs representing molecules, but the algorithm fails for a number of simple cases and cannot be considered a correct method for representing a graph canonically. There is no systematic comparison across commercial software to test if such flaws exist in those packages. SMILES notation allows the specification of configuration at tetrahedral centers, double bond geometry; these are structural features that cannot be specified by connectivity alone and SMILES which encode this information are termed isomeric SMILES.
A notable feature of these rules is. The term isomeric SMILES is applied to SMILES in which isotopes are specified. In terms of a graph-based computational procedure, SMILES is a string obtained by printing the symbol nodes encountered in a depth-first tree traversal of a chemical graph; the chemical graph is first trimmed to remove hydrogen atoms and cycles are broken to turn it into a spanning tree. Where cycles have been broken, numeric suffix labels are included to indicate the connected nodes. Parentheses are used to indicate points of branching on the tree; the resultant SMILES form depends on the choices: of the bonds chosen to break cycles, of the starting atom used for the depth-first traversal, of the order in which branches are listed when encountered. Atoms are represented by the standard abbreviation of the chemical elements, in square brackets, such as for gold. Brackets may be omitted in the common case of atoms which: are in the "organic subset" of B, C, N, O, P, S, F, Cl, Br, or I, have no formal charge, have the number of hydrogens attached implied by the SMILES valence model, are the normal isotopes, are not chiral centers.
All other elements must be enclosed in brackets, have charges and hydrogens shown explicitly. For instance, the SMILES for water may be written as either O or. Hydrogen may be written as a separate atom; when brackets are used, the symbol H is added if the atom in brackets is bonded to one or more hydrogen, followed by the number of hydrogen atoms if greater than 1 by the sign + for a positive charge or by - for a negative charge. For example, for ammonium. If there is more than one charge, it is written as digit.
Half-life is the time required for a quantity to reduce to half its initial value. The term is used in nuclear physics to describe how unstable atoms undergo, or how long stable atoms survive, radioactive decay; the term is used more to characterize any type of exponential or non-exponential decay. For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the human body; the converse of half-life is doubling time. The original term, half-life period, dating to Ernest Rutherford's discovery of the principle in 1907, was shortened to half-life in the early 1950s. Rutherford applied the principle of a radioactive element's half-life to studies of age determination of rocks by measuring the decay period of radium to lead-206. Half-life is constant over the lifetime of an exponentially decaying quantity, it is a characteristic unit for the exponential decay equation; the accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed.
A half-life describes the decay of discrete entities, such as radioactive atoms. In that case, it does not work to use the definition that states "half-life is the time required for half of the entities to decay". For example, if there is just one radioactive atom, its half-life is one second, there will not be "half of an atom" left after one second. Instead, the half-life is defined in terms of probability: "Half-life is the time required for half of the entities to decay on average". In other words, the probability of a radioactive atom decaying within its half-life is 50%. For example, the image on the right is a simulation of many identical atoms undergoing radioactive decay. Note that after one half-life there are not one-half of the atoms remaining, only because of the random variation in the process; when there are many identical atoms decaying, the law of large numbers suggests that it is a good approximation to say that half of the atoms remain after one half-life. There are various simple exercises that demonstrate probabilistic decay, for example involving flipping coins or running a statistical computer program.
An exponential decay can be described by any of the following three equivalent formulas: where N0 is the initial quantity of the substance that will decay, N is the quantity that still remains and has not yet decayed after a time t, t1⁄2 is the half-life of the decaying quantity, τ is a positive number called the mean lifetime of the decaying quantity, λ is a positive number called the decay constant of the decaying quantity. The three parameters t1⁄2, τ, λ are all directly related in the following way: where ln is the natural logarithm of 2; some quantities decay by two exponential-decay processes simultaneously. In this case, the actual half-life T1⁄2 can be related to the half-lives t1 and t2 that the quantity would have if each of the decay processes acted in isolation: 1 T 1 / 2 = 1 t 1 + 1 t 2 For three or more processes, the analogous formula is: 1 T 1 / 2 = 1 t 1 + 1 t 2 + 1 t 3 + ⋯ For a proof of these formulas, see Exponential decay § Decay by two or more processes. There is a half-life describing any exponential-decay process.
For example: As noted above, in radioactive decay the half-life is the length of time after which there is a 50% chance that an atom will have undergone nuclear decay. It varies depending on the atom type and isotope, is determined experimentally. See List of nuclides; the current flowing through an RC circuit or RL circuit decays with a half-life of lnRC or lnL/R, respectively. For this example the term half time tends to be used, rather than "half life", but they mean the same thing. In a chemical reaction, the half-life of a species is the time it takes for the concentration of that substance to fall to half of its initial value. In a first-order reaction the half-life of the reactant is ln/λ, where λ is the reaction rate constant; the term "half-life" is exclusively used for decay processes that are exponential, or exponential. In a decay process, not close to exponential, the half-life will change while the decay is happening. In this situation it is uncommon to talk about half-life in the first place, but sometimes people will describe the decay in terms of its "first half-life", "second half-life", etc. where the first half-life is defined as the time required for decay from the initial value to 50%, the second half-life is from 50% to 25%, so on.
A biological half-life or elimination half-life is the time it takes for a substance to lose one-half of its pharmacologic, physiologic, or radiological activity. In a medical context, the half-life may describe the time that it takes for the concentration of a substance in blood plasma to reach one-half of its steady-state value; the relationship between the biological and plasma half-lives of a subs
The Jmol applet, among other abilities, offers an alternative to the Chime plug-in, no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, the Sculpt mode. Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS 9. Jmol operates on a wide variety of platforms. For example, Jmol is functional in Mozilla Firefox, Internet Explorer, Google Chrome, Safari. Chemistry Development Kit Comparison of software for molecular mechanics modeling Jmol extension for MediaWiki List of molecular graphics systems Molecular graphics Molecule editor Proteopedia PyMOL SAMSON Official website Wiki with listings of websites and moodles Willighagen, Egon. "Fast and Scriptable Molecular Graphics in Web Browsers without Java3D". Doi:10.1038/npre.2007.50.1
Helium is the most unreactive element, so it was believed that helium compounds do not exist at all. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium has a complete shell of electrons, in this form the atom does not accept any extra electrons or join with anything to make covalent compounds; the electron affinity is 0.080 eV, close to zero. The helium atom is small with the radius of the outer electron shell at 0.29 Å. The atom is hard with a Pearson's hardness of 12.3 eV. It has the lowest polarizability of any kind of atom; however weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces. So at low temperatures helium may form van der Waals molecules. Repulsive forces between helium and other atoms may be overcome by high pressures. Helium has been shown to form a crystalline compound with sodium under pressure. Suitable pressures to force helium into solid combinations could be found inside planets. Clathrates are possible with helium under pressure in ice, other small molecules such as nitrogen.
Other ways to make helium reactive are: to convert it into an ion, or to excite an electron to a higher level, allowing it to form excimers. Ionised helium known as He II, is a high energy material able to extract an electron from any other atom. He+ has an electron configuration like hydrogen, so as well as being ionic it can form covalent bonds. Excimers do not last for long, as the molecule containing the higher energy level helium atom can decay back to a repulsive ground state, where the two atoms making up the bond repel. However, in some locations such as helium white dwarfs, conditions may be suitable to form excited helium atoms; the excited helium atom has a 1s electron promoted to 2s. This requires 1,900 kilojoules per gram of helium, which can be supplied by electron impact, or electric discharge; the 2s excited electron state resembles that of the lithium atom. Most solid combinations of helium with other substances require high pressure. Helium does not bond with the other atoms, but the substances can have a well defined crystal structure.
Disodium helide is a compound of helium and sodium, stable at high pressures above 113 gigapascals. Disodium helide was first predicted using USPEX code and was first synthesised in 2016, it was predicted to be dynamically stable over 100 GPa. Na2He has a cubic crystal structure, resembling fluorite. At 300 GPa the edge of a unit cell of the crystal has a = 3.95 Å. Each unit cell contains four helium atoms on the centre of the cube faces and corners, eight sodium atoms at coordinates a quarter cell in from each face. Double electrons are positioned on the centre of the unit cell; each pair of electrons is spin paired. The presence of these isolated electrons makes this an electride; the helium atoms do not participate in any bonding. However the electron pairs can be considered as an eight-centre two-electron bond. Disodium helide is predicted to be an insulator and transparent. Helium was first observed to enter into a silicate in 2007; the mineral melanophlogite is a natural silica clathrate that would contain carbon dioxide, methane or nitrogen.
When compressed with helium, a new clathrate forms. This has a much higher bulk modulus, resists amorphization. Helium was taken up around 17 GPa, enlarging the unit cell, given off again when pressure dropped to 11 GPa. Cristobalite He II is stable between 1.7 and 6.4 GPa. It has a rhombohedral space group R-3c with unit cell dimensions a = 9.080 Å, α = 31.809° and V = 184.77 Å3 at 4 GPa. Cristobalite He I can be formed under higher helium pressures over 6.4 GPa. It has a monoclinic space group P21/C with unit cell dimensions a = 8.062 Å, b = 4.797 Å, c = 9.491 Å, β = 120.43° and V = 316.47 Å3 at 10 GPa. Helium penetrates into fused silica at high pressure. Chibaite, another natural silica clathrate has its structure penetrated by helium under pressures higher than 2.5 GPa. The presence of guest hydrocarbons does not prevent this happening. Neon requires a higher pressure, 4.5 GPa to penetrate, unlike helium shows hysterisis. Linde-type A zeolites are renedered less compressible when penetrated by helium between 2 and 7 GPa.
Arsenolite helium inclusion compound As4O6·2He is stable from pressures over 3 GPa and up to at least 30 GPa. Arsenolite is one of most compressible minerals. Helium prevents amorphization; the solid containing helium is stronger and harder, with a higher sound velocity than plain arsenolite. The helium, included into the crystal causes a more uniform stress on the As4O6 molecules. No actual bond is formed from arsenic to helium despite the lone pairs of electrons available; the diffusion of helium into arsenolite is a slow process taking days at a pressure around 3 GPa. However if the pressure on the crystal is too high helium penetration does not take place, as the gaps between arsenolite molecules become too small. Neon does not diffuse into arsenolite. Helium can be inserted into the A sites of negative thermal expansion perovskites that otherwise have defects at the A site. At room temperature and 350 Mpa helium is included into CaZrF6 to expand its unit cell yielding HeCaZrF6. About half of the A sites are filled by helium atoms.
This substance loses helium over several minutes on depressurisation at ambient temperature, but below 130K it retains helium when depressurised. Under pressure helium penetrates dimethylammonium iron formate 2NH2Fe3, it affects this by causing a change to a monoclinic ordered state at a