Silver is a chemical element; it has symbol Ag () and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity and reflectivity of any metal. Silver is found in the Earth's crust in the pure, free elemental form ("native silver"), as an alloy with gold (electrum) and other metals, and in minerals such as acanthite, argentite and chlorargyrite. Most silver is recovered as a byproduct of lead–zinc, copper and gold mining and refining.
Silver has long been valued as a precious metal and coinage metal. It is one of the seven metals of antiquity, and has had an enduring role in monetary systems, jewellery, tableware and decorative arts. Its purity is commonly measured on a per-mille basis; a 94%-pure alloy is described as "0.940 fine". Because pure silver is relatively soft, silver used in jewellery, coinage and household objects is often alloyed with copper or other metals.
Beyond currency and investment uses such as coins and bullion, silver is an important industrial material. It is used in photovoltaics, electrical contacts and conductors, printed electronics, brazing alloys, catalysis, specialised mirrors and window coatings, and antimicrobial materials. Silver halides are important in photographic film, photographic paper and photochromic glass, although demand for photographic silver has declined with the spread of digital imaging.
Characteristics
Silver is similar in its physical and chemical properties to its two vertical neighbours in group 11 of the periodic table: copper, and gold. Its 47 electrons are arranged in the configuration [Kr]4d105s1, similarly to copper ([Ar]3d104s1) and gold ([Xe]4f145d106s1); group 11 is one of the few groups in the d-block which has a completely consistent set of electron configurations. This distinctive electron configuration, with a single electron in the highest occupied s subshell over a filled d subshell, accounts for many of the singular properties of metallic silver.
Silver is a relatively soft and extremely ductile and malleable transition metal, though it is slightly less malleable than gold. Silver crystallises in a face-centred cubic lattice with bulk coordination number 12, where only the single 5s electron is delocalised, similarly to copper and gold.Greenwood and Earnshaw, p. 1178 Unlike metals with incomplete d-shells, metallic bonds in silver are lacking a covalent character and are relatively weak. This observation explains the low hardness and high ductility of single crystals of silver.
Silver has a brilliant, white, metallic lustre that can take a high polish, and which is so characteristic that the name of the metal itself has become a colour name.Greenwood and Earnshaw, p. 1177 Protected silver has greater optical reflectivity than aluminium at all wavelengths longer than ~450 nm. At wavelengths shorter than 450 nm, silver's reflectivity is inferior to that of aluminium and drops to zero near 310 nm.
Very high electrical and thermal conductivity are common to the elements in group 11, because their single s electron is free and does not interact with the filled d subshell, as such interactions (which occur in the preceding transition metals) lower electron mobility. The thermal conductivity of silver is among the highest of all materials, although the thermal conductivity of carbon (in the diamond allotrope) and superfluid helium-4 are higher. The electrical conductivity of silver is the highest of all metals, greater even than copper. Silver also has the lowest contact resistance of any metal. Silver is rarely used for its electrical conductivity, due to its high cost, although an exception is in radio-frequency engineering, particularly at VHF and higher frequencies where silver plating improves electrical conductivity because those currents tend to flow on the surface of conductors rather than through the interior. During World War II in the US, tons of silver were used for the electromagnets in calutrons for enriching uranium, mainly because of the wartime shortage of copper.
Silver readily forms alloys with copper, gold, and zinc. Zinc-silver alloys with low zinc concentration may be considered as face-centred cubic solid solutions of zinc in silver, as the structure of the silver is largely unchanged while the electron concentration rises as more zinc is added. Increasing the electron concentration further leads to body-centred cubic (electron concentration 1.5), complex cubic (1.615), and hexagonal close-packed phases (1.75).
Isotopes
Naturally occurring silver is composed of two stable isotopes, Ag and Ag, with Ag being slightly more abundant (51.839% natural abundance). This almost equal abundance is rare in the periodic table. The atomic weight is ; this value is very important because of the importance of silver compounds, particularly halides, in gravimetric analysis. Both isotopes of silver are produced in stars via the s-process (slow neutron capture), as well as in supernovas via the r-process (rapid neutron capture).
Twenty-eight radioisotopes have been characterised, the most stable being Ag with a half-life of 41.29 days, Ag with a half-life of 7.45 days, and Ag with a half-life of 3.13 hours. Silver has numerous nuclear isomers, the most stable being Ag (t1/2 = 418 years), Ag (t1/2 = 249.79 days) and Ag (t1/2 = 8.28 days). All of the remaining radioactive isotopes have half-lives of less than an hour, and the majority of these have half-lives of less than three minutes.
Isotopes of silver range in atomic mass from 92.950 Da (Ag) to 129.950 Da (Ag); the primary decay mode before the most abundant stable isotope, Ag, is electron capture and the primary mode after is beta decay. The primary decay products before Ag are palladium (element 46) isotopes, and the primary products after are cadmium (element 48) isotopes.
The palladium isotope Pd decays by beta emission to Ag with a half-life of 6.5 million years. Iron meteorites are the only objects with a high-enough palladium-to-silver ratio to yield measurable variations in Ag abundance. Radiogenic Ag was first discovered in the Santa Clara meteorite in 1978. Pd–Ag correlations observed in bodies that have clearly been melted since the accretion of the Solar System must reflect the presence of unstable nuclides in the early Solar System.
Chemistry + Oxidation states and molecular geometries of silver Oxidation state Coordination number Moleculargeometry Representativecompound 0 (d10s1) 3 Trigonal–planar 1 (d10) 2 Linear 3 Trigonal–planar 4 Tetrahedral 6 Octahedral AgF, AgCl, AgBr 2 (d9) 4 Square–planar 3 (d8) 4 Square–planar 6 Octahedral
Silver is a rather unreactive metal. This is because its filled 4d shell is not very effective in shielding the electrostatic forces of attraction from the nucleus to the outermost 5s electron, and hence silver is near the bottom of the electrochemical series (E0(/Ag) = +0.799 V). In group 11, silver has the lowest first ionisation energy (showing the instability of the 5s orbital), but has higher second and third ionisation energies than copper and gold (showing the stability of the 4d orbitals), so that the chemistry of silver is predominantly that of the +1 oxidation state, reflecting the increasingly limited range of oxidation states along the transition series as the d-orbitals fill and stabilise.Greenwood and Earnshaw, p. 1180 Unlike copper, for which the larger hydration energy of as compared to is the reason why the former is the more stable in aqueous solution and solids despite lacking the stable filled d-subshell of the latter, with silver this effect is swamped by its larger second ionisation energy. Hence, is the stable species in aqueous solution and solids, with being much less stable as it oxidises water.
Most silver compounds have significant covalent character due to the small size and high first ionisation energy (730.8 kJ/mol) of silver. Furthermore, silver's Pauling electronegativity of 1.93 is higher than that of lead (1.87), and its electron affinity of 125.6 kJ/mol is much higher than that of hydrogen (72.8 kJ/mol) and not much less than that of oxygen (141.0 kJ/mol).Greenwood and Earnshaw, p. 1176 Due to its full d-subshell, silver in its main +1 oxidation state exhibits relatively few properties of the transition metals proper from groups 4 to 10, forming rather unstable organometallic compounds, forming linear complexes showing very low coordination numbers like 2, and forming an amphoteric oxideLidin RA 1996, Inorganic substances handbook, Begell House, New York, . p. 5 as well as Zintl phases like the post-transition metals.Goodwin F, Guruswamy S, Kainer KU, Kammer C, Knabl W, Koethe A, Leichtfreid G, Schlamp G, Stickler R & Warlimont H 2005, 'Noble metals and noble metal alloys', in Springer Handbook of Condensed Matter and Materials Data, W Martienssen & H Warlimont (eds), Springer, Berlin, pp. 329–406, . p. 341 Unlike the preceding transition metals, the +1 oxidation state of silver is stable even in the absence of π-acceptor ligands.
Silver does not react with air, even at red heat, and thus was considered by alchemists to be a noble metal. Its reactivity is intermediate between that of copper (which forms copper(I) oxide when heated in air to red heat) and gold. Like copper, silver reacts with sulfur and its compounds; in their presence, silver tarnishes in air to form the black silver sulfide (copper forms the green sulfate instead, while gold does not react). While silver is not attacked by non-oxidising acids, the metal dissolves readily in hot concentrated sulfuric acid, as well as dilute or concentrated nitric acid. In the presence of air, and especially in the presence of hydrogen peroxide, silver dissolves readily in aqueous solutions of cyanide.Greenwood and Earnshaw, p. 1179
The three main forms of deterioration in historical silver artifacts are tarnishing, formation of silver chloride due to long-term immersion in salt water, as well as reaction with nitrate ions or oxygen. Fresh silver chloride is pale yellow, becoming purplish on exposure to light; it projects slightly from the surface of the artifact or coin. The precipitation of copper in ancient silver can be used to date artifacts, as copper is nearly always a constituent of silver alloys."Silver Artifacts" in Corrosion – Artifacts. NACE Resource Center
Silver metal is attacked by strong oxidant such as potassium permanganate () and potassium dichromate (), and in the presence of potassium bromide (KBr). These compounds are used in photography to bleach silver images, converting them to silver bromide that can either be fixed with thiosulfate or redeveloped to intensify the original image. Silver forms cyanide complexes (silver cyanide) that are soluble in water in the presence of an excess of cyanide ions. Silver cyanide solutions are used in electroplating of silver.
The common oxidation states of silver are (in order of commonness): +1 (the most stable state; for example, silver nitrate, ); +2 (highly oxidising; for example, silver(II) fluoride, ); and even very rarely +3 (extreme oxidising; for example, potassium tetrafluoroargentate(III), ). The +3 state requires very strong oxidising agents to attain, such as fluorine or peroxodisulfate, and some silver(III) compounds react with atmospheric moisture and attack glass.Greenwood and Earnshaw, p. 1188 Indeed, silver(III) fluoride is usually obtained by reacting silver or silver monofluoride with the strongest known oxidising agent, krypton difluoride.Greenwood and Earnshaw, p. 903
Compounds Oxides and chalcogenides