Lithium is a chemical element with symbol Li and atomic number 3. It is a silvery-white alkali metal. Under standard conditions, it is the lightest solid element. Like all alkali metals, lithium is reactive and flammable, is stored in mineral oil; when cut, it exhibits a metallic luster, but moist air corrodes it to a dull silvery gray black tarnish. It never occurs in nature, but only in compounds, such as pegmatitic minerals, which were once the main source of lithium. Due to its solubility as an ion, it is present in ocean water and is obtained from brines. Lithium metal is isolated electrolytically from a mixture of lithium chloride and potassium chloride; the nucleus of the lithium atom verges on instability, since the two stable lithium isotopes found in nature have among the lowest binding energies per nucleon of all stable nuclides. Because of its relative nuclear instability, lithium is less common in the solar system than 25 of the first 32 chemical elements though its nuclei are light: it is an exception to the trend that heavier nuclei are less common.
For related reasons, lithium has important uses in nuclear physics. The transmutation of lithium atoms to helium in 1932 was the first man-made nuclear reaction, lithium deuteride serves as a fusion fuel in staged thermonuclear weapons. Lithium and its compounds have several industrial applications, including heat-resistant glass and ceramics, lithium grease lubricants, flux additives for iron and aluminium production, lithium batteries, lithium-ion batteries; these uses consume more than three quarters of lithium production. Lithium is present in biological systems in trace amounts. Lithium salts have proven to be useful as a mood-stabilizing drug in the treatment of bipolar disorder in humans. Like the other alkali metals, lithium has a single valence electron, given up to form a cation; because of this, lithium is a good conductor of heat and electricity as well as a reactive element, though it is the least reactive of the alkali metals. Lithium's low reactivity is due to the proximity of its valence electron to its nucleus.
However, molten lithium is more reactive than its solid form. Lithium metal is soft enough to be cut with a knife; when cut, it possesses a silvery-white color that changes to gray as it oxidizes to lithium oxide. While it has one of the lowest melting points among all metals, it has the highest melting and boiling points of the alkali metals. Lithium has a low density, comparable with pine wood, it is the least dense of all elements. Furthermore, apart from helium and hydrogen, it is less dense than any liquid element, being only two thirds as dense as liquid nitrogen. Lithium can float on the lightest hydrocarbon oils and is one of only three metals that can float on water, the other two being sodium and potassium. Lithium's coefficient of thermal expansion is twice that of aluminium and four times that of iron. Lithium is superconductive below 400 μK at standard pressure and at higher temperatures at high pressures. At temperatures below 70 K, like sodium, undergoes diffusionless phase change transformations.
At 4.2 K it has a rhombohedral crystal system. At liquid-helium temperatures the rhombohedral structure is prevalent. Multiple allotropic forms have been identified for lithium at high pressures. Lithium has a mass specific heat capacity of 3.58 kilojoules per kilogram-kelvin, the highest of all solids. Because of this, lithium metal is used in coolants for heat transfer applications. Lithium reacts with water but with noticeably less vigor than other alkali metals; the reaction forms hydrogen lithium hydroxide in aqueous solution. Because of its reactivity with water, lithium is stored in a hydrocarbon sealant petroleum jelly. Though the heavier alkali metals can be stored in more dense substances, such as mineral oil, lithium is not dense enough to be submerged in these liquids. In moist air, lithium tarnishes to form a black coating of lithium hydroxide, lithium nitride and lithium carbonate; when placed over a flame, lithium compounds give off a striking crimson color, but when it burns the flame becomes a brilliant silver.
Lithium will burn in oxygen when exposed to water or water vapors. Lithium is flammable, it is explosive when exposed to air and to water, though less so than the other alkali metals; the lithium-water reaction at normal temperatures is brisk but nonviolent because the hydrogen produced does not ignite on its own. As with all alkali metals, lithium fires are difficult to extinguish, requiring dry powder fire extinguishers. Lithium is one of the few metals. Lithium has a diagonal relationship with an element of similar atomic and ionic radius. Chemical resemblances between the two metals include the formation of a nitride by reaction with N2, the formation of an oxide and peroxide when burnt in O2, salts with similar solubilities, thermal instability of the carbonates and nitrides; the metal reacts with hy
Francium is a chemical element with symbol Fr and atomic number 87. It used to be known as eka-caesium, it is radioactive. It is the second-most electropositive element, behind only caesium, is the second rarest occurring element; the isotopes of francium decay into astatine and radon. The electronic structure of a francium atom is 7s1, so the element is classed as an alkali metal. Bulk francium has never been viewed; because of the general appearance of the other elements in its periodic table column, it is assumed that francium would appear as a reactive metal, if enough could be collected together to be viewed as a bulk solid or liquid. Obtaining such a sample is improbable, since the extreme heat of decay caused by its short half-life would vaporize any viewable quantity of the element. Francium was discovered by Marguerite Perey in France in 1939, it was the last element first discovered in nature, rather than by synthesis. Outside the laboratory, francium is rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 continually forms and decays.
As little as 20–30 g exists at any given time throughout the Earth's crust. The largest amount produced in the laboratory was a cluster of more than 300,000 atoms. Francium is one of the most unstable of the occurring elements: its longest-lived isotope, francium-223, has a half-life of only 22 minutes; the only comparable element is astatine, whose most stable natural isotope, astatine-219, has a half-life of 56 seconds, although synthetic astatine-210 is much longer-lived with a half-life of 8.1 hours. All isotopes of francium decay into radium, or radon. Francium-223 has a shorter half-life than the longest-lived isotope of each synthetic element up to and including element 105, dubnium. Francium is an alkali metal whose chemical properties resemble those of caesium. A heavy element with a single valence electron, it has the highest equivalent weight of any element. Liquid francium—if created—should have a surface tension of 0.05092 N/m at its melting point. Francium's melting point was calculated to be around 27 °C.
The melting point is uncertain because of the element's extreme radioactivity. The estimated boiling point of 677 °C is uncertain. Linus Pauling estimated the electronegativity of francium at 0.7 on the Pauling scale, the same as caesium. Francium has a higher ionization energy than caesium, 392.811 kJ/mol as opposed to 375.7041 kJ/mol for caesium, as would be expected from relativistic effects, this would imply that caesium is the less electronegative of the two. Francium should have a higher electron affinity than caesium and the Fr− ion should be more polarizable than the Cs− ion; the CsFr molecule is predicted to have francium at the negative end of the dipole, unlike all known heterodiatomic alkali metal molecules. Francium superoxide is expected to have a more covalent character than its lighter congeners. Francium coprecipitates with several caesium salts, such as caesium perchlorate, which results in small amounts of francium perchlorate; this coprecipitation can be used to isolate francium, by adapting the radiocaesium coprecipitation method of Lawrence E. Glendenin and Nelson.
It will additionally coprecipitate with many other caesium salts, including the iodate, the picrate, the tartrate, the chloroplatinate, the silicotungstate. It coprecipitates with silicotungstic acid, with perchloric acid, without another alkali metal as a carrier, which provides other methods of separation. Nearly all francium salts are water-soluble. There are 34 known isotopes of francium ranging in atomic mass from 199 to 232. Francium has seven metastable nuclear isomers. Francium-223 and francium-221 are the only isotopes that occur in nature, though the former is far more common. Francium-223 is the most stable isotope, with a half-life of 21.8 minutes, it is unlikely that an isotope of francium with a longer half-life will be discovered or synthesized. Francium-223 is the fifth product of the actinium decay series as the daughter isotope of actinium-227. Francium-223 decays into radium-223 by beta decay, with a minor alpha decay path to astatine-219. Francium-221 has a half-life of 4.8 minutes.
It is the ninth product of the neptunium decay series as a daughter isotope of actinium-225. Francium-221 decays into astatine-217 by alpha decay; the least stable ground state isotope is francium-215, with a half-life of 0.12 μs: it undergoes a 9.54 MeV alpha decay to astatine-211. Its metastable isomer, francium-215m, is less stable still, with a half-life of only 3.5 ns. Due to its instability and rarity, there are no commercial applications for francium, it has been used of atomic structure. Its use as a potential diagnostic aid for various cancers has been explored, but this application has been deemed impractical. Francium's ability to be synthesized and cooled, along with its simple atomic structure, has made it the subject of specialized sp
Mark 50 torpedo
The Mark 50 torpedo is a U. S. Navy advanced lightweight torpedo for use against deep-diving submarines; the Mk 50 can be launched from all anti-submarine aircraft and from torpedo tubes aboard surface combatant ships. The Mk 50 was intended to replace the Mk 46 as the fleet's lightweight torpedo. Instead the Mark 46 will be replaced with the Mark 54 LHT; the torpedo's stored chemical energy propulsion system uses a small tank of sulfur hexafluoride gas, sprayed over a block of solid lithium, which generates enormous quantities of heat, which generates steam. The steam propels the torpedo in a closed Rankine cycle; this propulsion system offers the important deep-water performance advantage in that the combustion products—sulfur and lithium fluoride—occupy less volume than the reactants, so the torpedo does not have to force these out against increasing water pressure as it approaches a deep-diving submarine. Primary function: air and ship-launched lightweight torpedo Contractor: Alliant Techsystems, Westinghouse Length: 9.5 ft Weight: approx.
800 lb Diameter: 12.75 in Speed: > 40 kn Power Plant: Stored Chemical Energy Propulsion System Propulsion: Pump Jet Guidance system: Active/passive acoustic homing Warhead: 100 lb high explosive Stingray torpedo MU90 Impact MK-50 Advanced Lightweight Torpedo via FAS USA Torpedoes since World War II - navweaps.com Issues Related to the Navy's Mark-50 Torpedo Propulsion System, General Accounting Office, January 1989 - has diagrams showing internal general arrangement, retrieved December 18, 2012
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter; the smallest group of particles in the material that constitutes this repeating pattern is the unit cell of the structure. The unit cell reflects the symmetry and structure of the entire crystal, built up by repetitive translation of the unit cell along its principal axes; the translation vectors define the nodes of the Bravais lattice. The lengths of the principal axes, or edges, of the unit cell and the angles between them are the lattice constants called lattice parameters or cell parameters; the symmetry properties of the crystal are described by the concept of space groups. All possible symmetric arrangements of particles in three-dimensional space may be described by the 230 space groups.
The crystal structure and symmetry play a critical role in determining many physical properties, such as cleavage, electronic band structure, optical transparency. Crystal structure is described in terms of the geometry of arrangement of particles in the unit cell; the unit cell is defined as the smallest repeating unit having the full symmetry of the crystal structure. The geometry of the unit cell is defined as a parallelepiped, providing six lattice parameters taken as the lengths of the cell edges and the angles between them; the positions of particles inside the unit cell are described by the fractional coordinates along the cell edges, measured from a reference point. It is only necessary to report the coordinates of a smallest asymmetric subset of particles; this group of particles may be chosen so that it occupies the smallest physical space, which means that not all particles need to be physically located inside the boundaries given by the lattice parameters. All other particles of the unit cell are generated by the symmetry operations that characterize the symmetry of the unit cell.
The collection of symmetry operations of the unit cell is expressed formally as the space group of the crystal structure. Vectors and planes in a crystal lattice are described by the three-value Miller index notation; this syntax uses the indices ℓ, m, n as directional orthogonal parameters, which are separated by 90°. By definition, the syntax denotes a plane that intercepts the three points a1/ℓ, a2/m, a3/n, or some multiple thereof; that is, the Miller indices are proportional to the inverses of the intercepts of the plane with the unit cell. If one or more of the indices is zero, it means. A plane containing a coordinate axis is translated so that it no longer contains that axis before its Miller indices are determined; the Miller indices for a plane are integers with no common factors. Negative indices are indicated with horizontal bars, as in. In an orthogonal coordinate system for a cubic cell, the Miller indices of a plane are the Cartesian components of a vector normal to the plane. Considering only planes intersecting one or more lattice points, the distance d between adjacent lattice planes is related to the reciprocal lattice vector orthogonal to the planes by the formula d = 2 π | g ℓ m n | The crystallographic directions are geometric lines linking nodes of a crystal.
The crystallographic planes are geometric planes linking nodes. Some directions and planes have a higher density of nodes; these high density planes have an influence on the behavior of the crystal as follows: Optical properties: Refractive index is directly related to density. Adsorption and reactivity: Physical adsorption and chemical reactions occur at or near surface atoms or molecules; these phenomena are thus sensitive to the density of nodes. Surface tension: The condensation of a material means that the atoms, ions or molecules are more stable if they are surrounded by other similar species; the surface tension of an interface thus varies according to the density on the surface. Microstructural defects: Pores and crystallites tend to have straight grain boundaries following higher density planes. Cleavage: This occurs preferentially parallel to higher density planes. Plastic deformation: Dislocation glide occurs preferentially parallel to higher density planes; the perturbation carried by the dislocation is along a dense direction.
The shift of one node in a more dense direction requires a lesser distortion of the crystal lattice. Some directions and planes are defined by symmetry of the crystal system. In monoclinic, rhombohedral and trigonal/hexagonal systems there is one unique axis which has higher rotational symmetry than the other two axes; the basal plane is the plane perpendicular to the principal axis in these crystal systems. For triclinic and cubic crystal systems the axis designation is arbitrary and there is no principal axis. For the special case of simple cubic crystals, the lattice vectors are orthogonal and of equal length. So, in this common case, the Miller indices and both denote normals/directions in Cartesian coordinates. For cubic crystals with lattice constant a, the spacing d between adjacent l
Sodium fluoride is an inorganic compound with the formula NaF. It is a colorless or white solid, soluble in water, it is a common source of fluoride in the production of pharmaceuticals and is used to prevent cavities. In 2016 it was the 215 most prescribed medication in the United States with more than 2 million prescriptions. Fluoride salts are added to municipal drinking water for the purposes of maintaining dental health; the fluoride enhances the strength of teeth by the formation of fluorapatite, a occurring component of tooth enamel. Although sodium fluoride is used to fluoridate water and, indeed, is the standard by which other water-fluoridation compounds are gauged, hexafluorosilicic acid and its salt sodium hexafluorosilicate are more used additives in the U. S. Fluoride supplementation has been extensively studied for the treatment of postmenopausal osteoporosis; this supplementation does not appear to be effective. In medical imaging, fluorine-18-labelled sodium fluoride is one of the oldest tracers used in positron emission tomography, having been in use since the 1960s.
Relative to conventional bone scintigraphy carried out with gamma cameras or SPECT systems, PET offers more sensitivity and spatial resolution. Fluorine-18 has a half-life of 110 min; however fluorine-18 is considered to be a superior radiopharmaceutical for skeletal imaging. In particular it has a high and rapid bone uptake accompanied by rapid blood clearance, which results in a high bone-to-background ratio in a short time. Additionally the annihilation photons produced by decay of 18F have a high energy of 511-keV compared to 140-keV photons of 99mTc. Sodium fluoride has a variety of specialty chemical applications in synthesis and extractive metallurgy, it reacts with electrophilic chlorides including acyl chlorides, sulfur chlorides, phosphorus chloride. Like other fluorides, sodium fluoride finds use in desilylation in organic synthesis. Sodium fluoride can be used to produce fluorocarbons via the Finkelstein reaction. Sodium fluoride is used as a cleaning agent. Sodium fluoride is used as a stomach poison for plant-feeding insects.
Inorganic fluorides such as fluorosilicates and sodium fluoride complex magnesium ions as magnesium fluorophosphate. They inhibit enzymes such as enolase. Thus, fluoride poisoning prevents phosphate transfer in oxidative metabolism. Fluorides aqueous solutions of sodium fluoride and quite extensively absorbed by the human body. Fluorides interfere with electron calcium metabolism. Calcium is essential in regulating coagulation. Large ingestion of fluoride salts or hydrofluoric acid may result in fatal arrhythmias due to profound hypocalcemia. Chronic over-absorption can cause hardening of bones, calcification of ligaments, buildup on teeth. Fluoride can cause irritation or corrosion to eyes and nasal membranes; the lethal dose for a 70 kg human is estimated at 5–10 g. Sodium fluoride is classed as toxic by both inhalation and ingestion. In high enough doses, it has been shown to affect the circulatory system. For occupational exposures, the Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health have established occupational exposure limits at 2.5 mg/m3 over an eight-hour time-weighted average.
In the higher doses used to treat osteoporosis, plain sodium fluoride can cause pain in the legs and incomplete stress fractures when the doses are too high. Slow-release and enteric-coated versions of sodium fluoride do not have gastric side effects in any significant way, have milder and less frequent complications in the bones. In the lower doses used for water fluoridation, the only clear adverse effect is dental fluorosis, which can alter the appearance of children's teeth during tooth development. A chronic fluoride ingestion of 1 ppm of fluoride in drinking water can cause mottling of the teeth and an exposure of 1.7 ppm will produce mottling in 30–50 % of patients. As of 2014 there have been only three reported cases of fluoride toxicity associated with the ingestion of fluoride-containing toothpaste; as an example, one of these involved a 45 year old woman who came to her doctor complaining of unusual swelling and pain in her fingers. Tests showed elevated levels of fluoride in her blood.
When questioned about this, the woman admitted to the regular ingestion of large amounts of toothpaste, consuming a tube of it every two days and swallowing 68.5 mg of fluoride every day, because she "liked the taste". When asked to switch to a non-fluoride form of toothpaste, her fluoride levels dropped and her condition subsided. Sodium fluoride is an inorganic ionic compound, dissolving in water to give separated Na+ and F− ions. Like sodium chloride, it crystallizes in a cubic motif where both Na+ and F− occupy octahedral coordination sites.
Hygroscopy is the phenomenon of attracting and holding water molecules from the surrounding environment, at normal or room temperature. This is achieved through either absorption or adsorption with the adsorbing substance becoming physically changed somewhat; this could be an increase in volume, boiling point, viscosity, or other physical characteristic or property of the substance, as water molecules can become suspended between the substance's molecules in the process. The word hygroscopy uses combining forms of hygro- and -scopy. Unlike any other -scopy word, it no longer refers to a viewing or imaging mode, it did begin that way, with the word hygroscope referring in the 1790s to measuring devices for humidity level. These hygroscopes used materials, such as certain animal hairs, that appreciably changed shape and size when they became damp; such materials were said to be hygroscopic because they were suitable for making a hygroscope. Though, the word hygroscope ceased to be used for any such instrument in modern usage.
But the word hygroscopic lived on, thus hygroscopy. Nowadays an instrument for measuring humidity is called a hygrometer. Hygroscopic substances include cellulose fibers, caramel, glycerol, wood, sulfuric acid, many fertilizer chemicals, many salts, a wide variety of other substances. If a compound absorbs enough moisture so that it dissolves it is classed as hydrophilic. Zinc chloride and calcium chloride, as well as potassium hydroxide and sodium hydroxide, are so hygroscopic that they dissolve in the water they absorb: this property is called deliquescence. Not only is sulfuric acid hygroscopic in concentrated form but its solutions are hygroscopic down to concentrations of 10% v/v or below. A hygroscopic material will tend to become cakey when exposed to moist air; because of their affinity for atmospheric moisture, hygroscopic materials might require storage in sealed containers. When added to foods or other materials for the express purpose of maintaining moisture content, such substances are known as humectants.
Materials and compounds exhibit different hygroscopic properties, this difference can lead to detrimental effects, such as stress concentration in composite materials. The volume of a particular material or compound is affected by ambient moisture and may be considered its coefficient of hygroscopic expansion or coefficient of hygroscopic contraction —the difference between the two terms being a difference in sign convention. Differences in hygroscopy can be observed in plastic-laminated paperback book covers—often, in a moist environment, the book cover will curl away from the rest of the book; the unlaminated side of the cover absorbs more moisture than the laminated side and increases in area, causing a stress that curls the cover toward the laminated side. This is similar to the function of a thermostat's bi-metallic strip. Inexpensive dial-type hygrometers make use of this principle using a coiled strip. Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution.
Deliquescence occurs when the vapour pressure of the solution, formed is less than the partial pressure of water vapour in the air. While some similar forces are at work here, it is different from capillary attraction, a process where glass or other solid substances attract water, but are not changed in the process; the amount of moisture held by hygroscopic materials is proportional to the relative humidity. Tables containing this information can be found in many engineering handbooks and is available from suppliers of various materials and chemicals. Hygroscopy plays an important role in the engineering of plastic materials; some plastics are hygroscopic. The seeds of some grasses have hygroscopic extensions that bend with changes in humidity, enabling them to disperse over the ground. An example is Needle-and-Thread, Hesperostipa comata; each seed has an awn. Increased moisture causes it to untwist, upon drying, to twist again, thereby drilling the seed into the ground. Thorny dragons collect moisture in the dry desert via nighttime condensation of dew that forms on their skin and is channeled to their mouths in hygroscopic grooves between the spines of their skin.
Water collects in these grooves when it rains. Capillary action allows the lizard to suck in water from all over its body. Deliquescence, like hygroscopy, is characterized by a strong affinity for water and tendency to absorb moisture from the atmosphere if exposed to it. Unlike hygroscopy, deliquescence involves absorbing sufficient water to form an aqueous solution. Most deliquescent materials are salts, including calcium chloride, magnesium chloride, zinc chloride, ferric chloride, potassium carbonate, potassium phosphate, ferric ammonium citrate, ammonium nitrate, potassium hydroxide, sodium hydroxide. Owing to their high affinity for water, these substances are used as desiccants an application for concentrated sulfuric and phosphoric acids; these compounds are used in the chemical industry to remove the water produced by chemical reactions. Many engineering polymers are hygroscopic, including nylon, ABS, polycarbonate and poly. Other polyme
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.