The silver sulfide is an inorganic compound whose chemical formula is AgtwoS. It consists of a greyish-black solid formed by cations Ag+ and S anionstwo- in a 2: 1 ratio. The Stwo- is very akin to Ag+, because both are soft ions and manage to stabilize with each other.
Silver ornaments tend to darken, losing their characteristic luster. The color change is not a product of the oxidation of silver, but of its reaction with hydrogen sulfide present in the environment at low concentrations; this can come from the rotting or degradation of plants, animals or foods rich in sulfur.
The HtwoS, whose molecule carries a sulfur atom, reacts with silver according to the following chemical equation: 2Ag (s) + HtwoS (g) => AgtwoS (s) + Htwo(g)
Therefore, the AgtwoS is responsible for the black layers formed on silver. However, in nature this sulfide can also be found in the minerals Acantite and Argentite. The two minerals are distinguished from many others by their shiny black crystals, like the one in the solid in the image above..
The AGtwoS has polymorphic structures, attractive electronic and optoelectronic properties, is semiconductor and promises to be a material for the manufacture of photovoltaic devices, such as solar cells.
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The upper image illustrates the crystal structure of silver sulfide. The blue spheres correspond to the Ag cations+, while the yellow ones to the S anionstwo-. The AGtwoS is polymorphic, which means that it can adopt various crystal systems under certain temperature conditions..
How? Through a phase transition. The ions are rearranged in such a way that the increase in temperature and the vibrations of the solid do not disturb the electrostatic balance of attraction-repulsion. When this happens it is said that there is a phase transition, and the solid therefore exhibits new physical properties (such as luster and color)..
The AGtwoS at normal temperatures (below 179ºC), it has a monoclinic crystalline structure (α- AgtwoS). In addition to this solid phase there are two others: the bcc (cubic centered on the body) between 179 to 586ºC, and the fcc (cubic centered on the faces) at very high temperatures (δ- AgtwoS).
The mineral argentite consists of the fcc phase, also known as β-AgtwoS. Once cooled and transformed into acanthite, its structural features prevail combined. Therefore, both crystalline structures coexist: the monoclinic and the bcc. Hence, black solids with bright and interesting overtones arise..
247.80 g / mol
Grayish black crystals
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836 ° C. This value agrees with the fact that AgtwoS is a compound with little ionic character and, therefore, melts at temperatures below 1000ºC.
In water only 6.21 ∙ 10-fifteen g / L at 25 ° C. That is, the amount of the black solid that is solubilized is negligible. This, again, is due to the low polar character of the Ag-S bond, where there is no significant difference in electronegativity between the two atoms..
Also, the AgtwoS is insoluble in all solvents. No molecule can efficiently separate its crystalline layers into Ag ions+ and Stwo- solvated.
In the image of the structure you can also see four layers of S-Ag-S bonds, which move over each other when the solid is subjected to compression. This behavior means that, despite being a semiconductor, it is ductile like many metals at room temperature..
S-Ag-S layers fit properly due to their angular geometries which are seen as a zigzag. As there is a compression force, they move on a displacement axis, thus causing new non-covalent interactions between silver and sulfur atoms..
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The AGtwoS is an amphoteric semiconductor, that is, it behaves as if it were of the type n and of the type p. In addition, it is not brittle, so it has been studied for its application in electronic devices..
The AGtwoS can be reduced to metallic silver by bathing the black pieces with hot water, NaOH, aluminum and salt. The following reaction takes place:
3AgtwoS (s) + 2Al (s) + 3HtwoO (l) => 6Ag (s) + 3HtwoS (ac) + AltwoOR3(s)
Silver, whose electron configuration is [Kr] 4d105s1, it can lose only one electron: that of its outermost orbital 5s. Thus, the cation Ag+ is left with an electron configuration [Kr] 4d10. Therefore, it has a unique valence of +1, which determines what its compounds should be called..
Sulfur, on the other hand, has electron configuration [Ne] 3stwo3p4, and it takes two electrons to complete its valence octet. When it gains these two electrons (from silver), it transforms into the sulfide anion, Stwo-, with [Ar] setting. That is, it is isoelectronic to the noble gas argon.
So the AgtwoS must be called according to the following nomenclatures:
Monkeysulfide gavesilver. Here the number of atoms of each element is considered and they are marked with the prefixes of Greek numerators.
Silver sulfide. As it has a unique valence of +1, it is not specified with Roman numerals in parentheses: silver (I) sulfide; which is wrong.
Silver sulfideico. Since silver "works" with a valence of +1, the suffix -ico is added to its name. argentum In latin.
Some of the Novel Uses for AgtwoS are as follows:
-The colloidal solutions of its nanoparticles (with different sizes), have antibacterial activity, are not toxic, and therefore can be used in the fields of medicine and biology..
-Its nanoparticles can form what is known as quantum dots. They absorb and emit radiation with greater intensity than many fluorescent organic molecules, so they can supplant the latter as biological markers.
-The structures of α-AgtwoS cause it to exhibit striking electronic properties to be used as solar cells. Likewise, it represents a starting point for the synthesis of new thermoelectric materials and sensors..
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