Both strontium and strontianite are named after Strontian, a village in Scotland near which the mineral was discovered in 1790 by Adair Crawford and William Cruickshank; it was identified as a new element the next year from its crimson-red flame test color. Strontium was first isolated as a metal in 1808 by Humphry Davy using the then newly discovered process of electrolysis. During the 19th century, strontium was mostly used in the production of sugar from sugar beets (see strontian process). At the peak of production of television cathode-ray tubes, as much as 75% of strontium consumption in the United States was used for the faceplate glass.[7] With the replacement of cathode-ray tubes with other display methods, consumption of strontium has dramatically declined.[7]

While natural strontium (which is mostly the isotope strontium-88) is stable, the synthetic strontium-90 is radioactive and is one of the most dangerous components of nuclear fallout, as strontium is absorbed by the body in a similar manner to calcium. Natural stable strontium, on the other hand, is not hazardous to health.

Characteristics

Oxidized dendritic strontium

Strontium is a divalent silvery metal with a pale yellow tint whose properties are mostly intermediate between and similar to those of its group neighbors calcium and barium.[8] It is softer than calcium and harder than barium. Its melting (777 °C) and boiling (1377 °C) points are lower than those of calcium (842 °C and 1484 °C respectively); barium continues this downward trend in the melting point (727 °C), but not in the boiling point (1900 °C). The density of strontium (2.64 g/cm3) is similarly intermediate between those of calcium (1.54 g/cm3) and barium (3.594 g/cm3).[9] Three allotropes of metallic strontium exist, with transition points at 235 and 540 °C.[10]

The standard electrode potential for the Sr2+/Sr couple is −2.89 V, approximately midway between those of the Ca2+/Ca (−2.84 V) and Ba2+/Ba (−2.92 V) couples, and close to those of the neighboring alkali metals.[11] Strontium is intermediate between calcium and barium in its reactivity toward water, with which it reacts on contact to produce strontium hydroxide and hydrogen gas. Strontium metal burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380 °C, at room temperature it forms only the oxide spontaneously.[9] Besides the simple oxide SrO, the peroxide SrO2 can be made by direct oxidation of strontium metal under a high pressure of oxygen, and there is some evidence for a yellow superoxide Sr(O2)2.[12] Strontium hydroxide, Sr(OH)2, is a strong base, though it is not as strong as the hydroxides of barium or the alkali metals.[13] All four dihalides of strontium are known.[14]

Due to the large size of the heavy s-block elements, including strontium, a vast range of coordination numbers is known, from 2, 3, or 4 all the way to 22 or 24 in SrCd11 and SrZn13. The Sr2+ ion is quite large, so that high coordination numbers are the rule.[15] The large size of strontium and barium plays a significant part in stabilising strontium complexes with polydentate macrocyclic ligands such as crown ethers: for example, while 18-crown-6 forms relatively weak complexes with calcium and the alkali metals, its strontium and barium complexes are much stronger.[16]

Organostrontium compounds contain one or more strontium–carbon bonds. They have been reported as intermediates in Barbier-type reactions.[17][18][19] Although strontium is in the same group as magnesium, and organomagnesium compounds are very commonly used throughout chemistry, organostrontium compounds are not similarly widespread because they are more difficult to make and more reactive. Organostrontium compounds tend to be more similar to organoeuropium or organosamarium compounds due to the similar ionic radii of these elements (Sr2+ 118 pm; Eu2+ 117 pm; Sm2+ 122 pm). Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favor stability. For example, strontium dicyclopentadienyl, Sr(C5H5)2, must be made by directly reacting strontium metal with mercurocene or cyclopentadiene itself; replacing the C5H5 ligand with the bulkier C5(CH3)5 ligand on the other hand increases the compound's solubility, volatility, and kinetic stability.[20]

Because of its extreme reactivity with oxygen and water, strontium occurs naturally only in compounds with other elements, such as in the minerals strontianite and celestine. It is kept under a liquid hydrocarbon such as mineral oil or kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal is pyrophoric, meaning that it will ignite spontaneously in air at room temperature. Volatile strontium salts impart a bright red color to flames, and these salts are used in pyrotechnics and in the production of flares.[9] Like calcium and barium, as well as the alkali metals and the divalent lanthanides europium and ytterbium, strontium metal dissolves directly in liquid ammonia to give a dark blue solution of solvated electrons.[8]

Isotopes

Main article: Isotopes of strontium

Natural strontium is a mixture of four stable isotopes: 84Sr, 86Sr, 87Sr, and 88Sr.[9] On these isotopes, 88Sr is the most abundant, makes up about 82.6% of all natural strontium, though the abundance varies due to the production of radiogenic 87Sr as the daughter of long-lived beta-decaying 87Rb.[21] This is the basis of rubidium–strontium dating. Of the unstable isotopes, the primary decay mode of the isotopes lighter than 85Sr is electron capture or positron emission to isotopes of rubidium, and that of the isotopes heavier than 88Sr is electron emission to isotopes of yttrium. Of special note are 89Sr and 90Sr. The former has a half-life of 50.6 days and is used to treat bone cancer due to strontium's chemical similarity and hence ability to replace calcium.[22][23] While 90Sr (half-life 28.90 years) has been used similarly, it is also an isotope of concern in fallout from nuclear weapons and nuclear accidents due to its production as a fission product. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia.[24] The 1986 Chernobyl nuclear accident contaminated about 30,000 km2 with greater than 10 kBq/m2 with 90Sr, which accounts for about 5% of the 90Sr which was in the reactor core.[25]

History

Flame test for strontium

Strontium is named after the Scottish village of Strontian (Gaelic Sròn an t-Sìthein), where it was discovered in the ores of the lead mines.[26]

In 1790, Adair Crawford, a physician engaged in the preparation of barium, and his colleague William Cruickshank, recognised that the Strontian ores exhibited properties that differed from those in other "heavy spars" sources.[27] This allowed Crawford to conclude on page 355 "... it is probable indeed, that the scotch mineral is a new species of earth which has not hitherto been sufficiently examined." The physician and mineral collector Friedrich Gabriel Sulzer analysed together with Johann Friedrich Blumenbach the mineral from Strontian and named it strontianite. He also came to the conclusion that it was distinct from the witherite and contained a new earth (neue Grunderde).[28] In 1793 Thomas Charles Hope, a professor of chemistry at the University of Glasgow studied the mineral[29][30] and proposed the name strontites.[31][32][33] He confirmed the earlier work of Crawford and recounted: "... Considering it a peculiar earth I thought it necessary to give it an name. I have called it Strontites, from the place it was found; a mode of derivation in my opinion, fully as proper as any quality it may possess, which is the present fashion." The element was eventually isolated by Sir Humphry Davy in 1808 by the electrolysis of a mixture containing strontium chloride and mercuric oxide, and announced by him in a lecture to the Royal Society on 30 June 1808.[34] In keeping with the naming of the other alkaline earths, he changed the name to strontium.[35][36][37][38][39]

The first large-scale application of strontium was in the production of sugar from sugar beet. Although a crystallisation process using strontium hydroxide was patented by Augustin-Pierre Dubrunfaut in 1849[40] the large scale introduction came with the improvement of the process in the early 1870s. The German sugar industry used the process well into the 20th century. Before World War I the beet sugar industry used 100,000 to 150,000 tons of strontium hydroxide for this process per year.[41] The strontium hydroxide was recycled in the process, but the demand to substitute losses during production was high enough to create a significant demand initiating mining of strontianite in the Münsterland. The mining of strontianite in Germany ended when mining of the celestine deposits in Gloucestershire started.[42] These mines supplied most of the world strontium supply from 1884 to 1941. Although the celestine deposits in the Granada basin were known for some time the large scale mining did not start before the 1950s.[43]

During atmospheric nuclear weapons testing, it was observed that strontium-90 is one of the nuclear fission products with a relatively high yield. The similarity to calcium and the chance that the strontium-90 might become enriched in bones made research on the metabolism of strontium an important topic.[44][45]

Since they are metals, they are lustrous and have good electrical and thermal conductivity. Most (with the exception of group 11 and group 12) are hard and strong, and have high melting and boiling temperatures. They form compounds in any of two or more different oxidation states and bind to a variety of ligands to form coordination complexes that are often coloured. They form many useful alloys and are often employed as catalysts in elemental form or in compounds such as coordination complexes and oxides. Most are strongly paramagnetic because of their unpaired d electrons, as are many of their compounds. All of the elements that are ferromagnetic near room temperature are transition metals (iron, cobalt and nickel) or inner transition metals (gadolinium).

English chemist Charles Rugeley Bury (1890–1968) first used the word transition in this context in 1921, when he referred to a transition series of elements during the change of an inner layer of electrons (for example n = 3 in the 4th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32.[1][2][3] These elements are now known as the d-block.

The first row of transition metals in order. Definition and classification The 2011 IUPAC Principles of Chemical Nomenclature describe a "transition metal" as any element in groups 3 to 12 on the periodic table.[4] This corresponds exactly to the d-block elements, and many scientists use this definition.[5][6] In actual practice, the f-block lanthanide and actinide series are called "inner transition metals". The 2005 Red Book allows for the group 12 elements to be excluded, but not the 2011 Principles.[7]

The IUPAC Gold Book[8] defines a transition metal as "an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell", but this definition is taken from an old edition of the Red Book and is no longer present in the current edition.[7]

In the d-block, the atoms of the elements have between zero and ten d electrons.

Transition metals in the d-block Group 3 4 5 6 7 8 9 10 11 12 Period 4 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 5 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 6 71Lu 72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt 79Au 80Hg 7 103Lr 104Rf 105Db 106Sg 107Bh 108Hs 109Mt 110Ds 111Rg 112Cn Published texts and periodic tables show variation regarding the heavier members of group 3.[9] The common placement of lanthanum and actinium in these positions is not supported by physical, chemical, and electronic evidence,[10][11][12] which overwhelmingly favour putting lutetium and lawrencium in those places.[13][14] Some authors prefer to leave the spaces below yttrium blank as a third option, but there is confusion on whether this format implies that group 3 contains only scandium and yttrium, or if it also contains all the lanthanides and actinides;[15][16][17][18][19] additionally, it creates a 15-element-wide f-block, when quantum mechanics dictates that the f-block should only be 14 elements wide.[15] The form with lutetium and lawrencium in group 3 is supported by a 1988 IUPAC report on physical, chemical, and electronic grounds,[20] and again by a 2021 IUPAC preliminary report as it is the only form that allows simultaneous (1) preservation of the sequence of increasing atomic numbers, (2) a 14-element-wide f-block, and (3) avoidance of the split in the d-block.[15] Argumentation can still be found in the contemporary literature purporting to defend the form with lanthanum and actinium in group 3, but many authors consider it to be logically inconsistent (a particular point of contention being the differing treatment of actinium and thorium, which both can use 5f as a valence orbital but have no 5f occupancy as single atoms);[14][21][22] the majority of investigators considering the problem agree with the updated form with lutetium and lawrencium.[14]

The group 12 elements zinc, cadmium, and mercury are sometimes excluded from the transition metals.[1] This is because they have the electronic configuration [ ]d10s2, where the d shell is complete,[23] and they still have a complete d shell in all their known oxidation states. The group 12 elements Zn, Cd and Hg may therefore, under certain criteria, be classed as post-transition metals in this case. However, it is often convenient to include these elements in a discussion of the transition elements. For example, when discussing the crystal field stabilization energy of first-row transition elements, it is convenient to also include the elements calcium and zinc, as both Ca2+ and Zn2+ have a value of zero, against which the value for other transition metal ions may be compared. Another example occurs in the Irving–Williams series of stability constants of complexes. Moreover, Zn, Cd, and Hg can use their d orbitals for bonding even though they are not known in oxidation states that would formally require breaking open the d-subshell, which sets them apart from the p-block elements.[24][25][26]

The recent (though disputed and so far not reproduced independently) synthesis of mercury(IV) fluoride (HgF 4) has been taken by some to reinforce the view that the group 12 elements should be considered transition metals,[27] but some authors still consider this compound to be exceptional.[28] Copernicium is expected to be able to use its d electrons for chemistry as its 6d subshell is destabilised by strong relativistic effects due to its very high atomic number, and as such is expected to have transition-metal-like behaviour when it shows higher oxidation states than +2 (which are not definitely known for the lighter group 12 elements).

Although meitnerium, darmstadtium, and roentgenium are within the d-block and are expected to behave as transition metals analogous to their lighter congeners iridium, platinum, and gold, this has not yet been experimentally confirmed. Whether copernicium behaves more like mercury or has properties more similar to those of the noble gas radon is not clear.

Early transition metals are on the left side of the periodic table from group 3 to group 7. Late transition metals are on the right side of the d-block, from group 8 to 11 (and 12 if it is counted as transition metals).

The heavy group 2 elements calcium, strontium, and barium do not have filled d-orbitals as single atoms, but are known to have d-orbital bonding participation in some compounds, and for that reason have been called "honorary" transition metals.[29]

Conventions

The placeholder R is often used as a generic placeholder for alkyl (saturated hydrocarbon) group side chains in chemical structure diagrams. To indicate other non-carbon groups in structure diagrams, X, Y, or Z are often used.

History

The R symbol was introduced by 19th-century French chemist Charles Frédéric Gerhardt, who advocated its adoption on the grounds that it would be widely recognizable and intelligible given its correspondence in multiple European languages to the initial letter of "root" or "residue": French racine ("root") and résidu ("residue"), these terms' respective English translations along with radical (itself derived from Latin radix below), Latin radix ("root") and residuum ("residue"), and German Rest ("remnant" and, in the context of chemistry, both "residue" and "radical").[3]

Usage

Organic chemistry

In polymer science, the side chain of an oligomeric or polymeric offshoot extends from the backbone chain of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its crystallinity and density. An oligomeric branch may be termed a short-chain branch, and a polymeric branch may be termed a long-chain branch. Side groups are different from side chains; they are neither oligomeric nor polymeric.[4]

Biochemistry

In proteins, which are composed of amino acid residues, the side chains are attached to the alpha-carbon atoms of the amide backbone. The side chain connected to the alpha-carbon is specific for each amino acid and is responsible for determining charge and polarity of the amino acid. The amino acid side chains are also responsible for many of the interactions that lead to proper protein folding and function.[5] Amino acids with similar polarity are usually attracted to each other, while nonpolar and polar side chains usually repel each other. Nonpolar/polar interactions can still play an important part in stabilizing the secondary structure due to the relatively large amount of them occurring throughout the protein.[6] Spatial positions of side-chain atoms can be predicted based on protein backbone geometry using computational tools for side-chain reconstruction.[7]

Table of amino acids

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See also

• Alkyl

• Backbone chain

• Branching (polymer chemistry)

• Functional group

• Pendant group

• residue

• Substituent

• Backbone-dependent rotamer library

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A functional group is a group of atoms in a molecule with distinctive chemical properties, regardless of the other atoms in the molecule. The atoms in a functional group are linked to each other and to the rest of the molecule by covalent bonds. For repeating units of polymers, functional groups attach to their nonpolar core of carbon atoms and thus add chemical character to carbon chains. Functional groups can also be charged, e.g. in carboxylate salts (–COO−), which turns the molecule into a polyatomic ion or a complex ion. Functional groups binding to a central atom in a coordination complex are called ligands. Complexation and solvation are also caused by specific interactions of functional groups. In the common rule of thumb "like dissolves like", it is the shared or mutually well-interacting functional groups which give rise to solubility. For example, sugar dissolves in water because both share the hydroxyl functional group (–OH) and hydroxyls interact strongly with each other. Plus, when functional groups are more electronegative than atoms they attach to, the functional groups will become polar, and the otherwise nonpolar molecules containing these functional groups become polar and so become soluble in some aqueous environment.

Combining the names of functional groups with the names of the parent alkanes generates what is termed a systematic nomenclature for naming organic compounds. In traditional nomenclature, the first carbon atom after the carbon that attaches to the functional group is called the alpha carbon; the second, beta carbon, the third, gamma carbon, etc. If there is another functional group at a carbon, it may be named with the Greek letter, e.g., the gamma-amine in gamma-aminobutyric acid is on the third carbon of the carbon chain attached to the carboxylic acid group. IUPAC conventions call for numeric labeling of the position, e.g. 4-aminobutanoic acid. In traditional names various qualifiers are used to label isomers, for example, isopropanol (IUPAC name: propan-2-ol) is an isomer of n-propanol (propan-1-ol). The term moiety has some overlap with the term "functional group". However, a moiety is an entire "half" of a molecule, which can be not only a single functional group, but also a larger unit consisting of multiple functional groups. For example, an "aryl moiety" may be any group containing an aromatic ring, regardless of how many functional groups the said aryl has.

Table of common functional groups

The following is a list of common functional groups.[3] In the formulas, the symbols R and R' usually denote an attached hydrogen, or a hydrocarbon side chain of any length, but may sometimes refer to any group of atoms.

Hydrocarbons

Hydrocarbons are a class of molecule that is defined by functional groups called hydrocarbyls that contain only carbon and hydrogen, but vary in the number and order of double bonds. Each one differs in type (and scope) of reactivity.

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History Earlier symbols for chemical elements stem from classical Latin and Greek vocabulary. For some elements, this is because the material was known in ancient times, while for others, the name is a more recent invention. For example, Pb is the symbol for lead (plumbum in Latin); Hg is the symbol for mercury (hydrargyrum in Greek); and He is the symbol for helium (a Neo-Latin name) because helium was not known in ancient Roman times. Some symbols come from other sources, like W for tungsten (Wolfram in German) which was not known in Roman times.

A three-letter temporary symbol may be assigned to a newly synthesized (or not yet synthesized) element. For example, "Uno" was the temporary symbol for hassium (element 108) which had the temporary name of unniloctium, based on the digits of its atomic number. There are also some historical symbols that are no longer officially used.

Extension of the symbol

Annotated example of an atomic symbol In addition to the letters for the element itself, additional details may be added to the symbol as superscripts or subscripts a particular isotope, ionization, or oxidation state, or other atomic detail.[1] A few isotopes have their own specific symbols rather than just an isotopic detail added to their element symbol.

Attached subscripts or superscripts specifying a nuclide or molecule have the following meanings and positions:

The nucleon number (mass number) is shown in the left superscript position (e.g., 14N). This number defines the specific isotope. Various letters, such as "m" and "f" may also be used here to indicate a nuclear isomer (e.g., 99mTc). Alternately, the number here can represent a specific spin state (e.g., 1O2). These details can be omitted if not relevant in a certain context. The proton number (atomic number) may be indicated in the left subscript position (e.g., 64Gd). The atomic number is redundant to the chemical element, but is sometimes used to emphasize the change of numbers of nucleons in a nuclear reaction. If necessary, a state of ionization or an excited state may be indicated in the right superscript position (e.g., state of ionization Ca2+). The number of atoms of an element in a molecule or chemical compound is shown in the right subscript position (e.g., N2 or Fe2O3). If this number is one, it is normally omitted - the number one is implicitly understood if unspecified. A radical is indicated by a dot on the right side (e.g., Cl• for a neutral chlorine atom). This is often omitted unless relevant to a certain context because it is already deducible from the charge and atomic number, as generally true for nonbonded valence electrons in skeletal structures. Many functional groups also have their own chemical symbol, e.g. Ph for the phenyl group, and Me for the methyl group.

A list of current, dated, as well as proposed and historical signs and symbols is included here with its signification. Also given is each element's atomic number, atomic weight, or the atomic mass of the most stable isotope, group and period numbers on the periodic table, and etymology of the symbol.

Symbols for chemical elements List of chemical elements Z Symbol Name Origin of name[2][3] 1 H Hydrogen Greek elements hydro- and -gen, meaning 'water-forming' 2 He Helium Greek hḗlios, 'sun' 3 Li Lithium Greek líthos, 'stone' 4 Be Beryllium beryl, a mineral (ultimately from the name of Belur in southern India) 5 B Boron borax, a mineral (from Arabic bawraq) 6 C Carbon Latin carbo, 'coal' 7 N Nitrogen Greek nítron and -gen, meaning 'niter-forming' 8 O Oxygen Greek oxy- and -gen, meaning 'acid-forming' 9 F Fluorine Latin fluere, 'to flow' 10 Ne Neon Greek néon, 'new' 11 Na Sodium English soda (the symbol Na is derived from Neo-Latin natrium, coined from German Natron, 'natron') 12 Mg Magnesium Magnesia, a district of Eastern Thessaly in Greece 13 Al Aluminium alumina, from Latin alumen (gen. alumni), 'bitter salt, alum' 14 Si Silicon Latin silex, 'flint' (originally silicium) 15 P Phosphorus Greek phōsphóros, 'light-bearing' 16 S Sulfur Latin sulphur, 'brimstone' 17 Cl Chlorine Greek chlōrós, 'greenish yellow' 18 Ar Argon Greek argós, 'idle' (because of its inertness) 19 K Potassium Neo-Latin potassa, 'potash' (the symbol K is derived from Latin kalium) 20 Ca Calcium Latin calx, 'lime' 21 Sc Scandium Latin Scandia, 'Scandinavia' 22 Ti Titanium Titans, the sons of the Earth goddess of Greek mythology 23 V Vanadium Vanadis, an Old Norse name for the Scandinavian goddess Freyja 24 Cr Chromium Greek chróma, 'colour' 25 Mn Manganese corrupted from magnesia negra; see Magnesium 26 Fe Iron English word (the symbol Fe is derived from Latin ferrum) 27 Co Cobalt German Kobold, 'goblin' 28 Ni Nickel Nickel, a mischievous sprite of German miner mythology 29 Cu Copper English word, from Latin cuprum, from Ancient Greek Kýpros 'Cyprus' 30 Zn Zinc Most likely from German Zinke, 'prong' or 'tooth', though some suggest Persian sang, 'stone' 31 Ga Gallium Latin Gallia, 'France' 32 Ge Germanium Latin Germania, 'Germany' 33 As Arsenic French arsenic, from Greek arsenikón 'yellow arsenic' (influenced by arsenikós, 'masculine' or 'virile'), from a West Asian wanderword ultimately from Old Iranian *zarniya-ka, 'golden' 34 Se Selenium Greek selḗnē, 'moon' 35 Br Bromine Greek brômos, 'stench' 36 Kr Krypton Greek kryptós, 'hidden' 37 Rb Rubidium Latin rubidus, 'deep red'