Zinc

Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny blue whitish appearance when surface oxidation is removed. It is the first element in group 12 of the periodic table. Zinc is the 24th most abundant element in Earth's crust, with an average concentration of 70 grams per ton. Zinc also has five stable isotopes; the most abundant of which, Zn-64, comprises nearly half of zinc's total abundance. In some respects, zinc is chemically similar to magnesium: both elements exhibit only one normal oxidation state, and the Zn²⁺ and Mg²⁺ ions are of similar size. The most common zinc ore is sphalerite, a zinc sulfide mineral. The largest concentration of economically feasible lodes in descending order are located in China, Peru, and Australia, among others. Zinc is refined industrially by froth flotation of the ore, roasting, and final extraction using electricity.
Zinc is an essential trace element for humans, animals, plants and for microorganisms and is necessary for both prenatal and postnatal development. It is the second most abundant trace metal in humans after iron, an important cofactor for many enzymes, and the only metal which appears in all enzyme classes. Zinc is also an essential nutrient element for coral growth.
Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans. Deficiency of zinc intake affects about two billion people in the developing world and is associated with many diseases. In children, deficiency causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhea. However, consumption of excess zinc may cause ataxia, lethargy, and copper deficiency. In marine biomes, notably within polar regions, a deficit of zinc can compromise the vitality of primary algal communities, potentially destabilizing the intricate marine trophic structures and consequently impacting biodiversity.
Brass, an alloy of copper and zinc in various proportions, was used as early as the third millennium BC in the Aegean area and the region which currently includes Iraq, the United Arab Emirates, Kalmykia, Turkmenistan and Georgia. In the second millennium BC it was used in the regions currently including West India, Uzbekistan, Iran, Syria, Iraq, and Israel. Zinc metal was not produced on a large scale until the 12th century in India, though it was known to the ancient Romans and Greeks. The mines of Rajasthan have given definite archeological evidence of zinc production harking back to the 6th century BC. The oldest man-made pure zinc comes from Zawar, Rajasthan, as early as the 9th century AD, when a distillation process was utilized to make virtually pure zinc. Alchemists would burn zinc metal in air to form what they called "philosopher's wool" or "white snow".
The element was probably named by the alchemist Paracelsus after the German word Zinke. German chemist Andreas Sigismund Marggraf is credited with discovering pure metallic zinc in 1746. By 1800, work done by Luigi Galvani and Alessandro Volta had uncovered the electrochemical properties of zinc.
Plating of corrosion-resistant zinc on iron, achieved by a process named hot-dip galvanization, is the major application for zinc. Other applications include electrical batteries, small non-structural casts, and alloys such as brass. A variety of zinc compounds are commonly used, such as zinc carbonate, zinc gluconate, zinc chloride, zinc pyrithione, and zinc sulfide. In addition, dimethylzinc and diethylzinc are used for the chemical syntheses of organic compounds.

Characteristics

Physical properties

Zinc is a bluish-white, lustrous, diamagnetic metal, though most common commercial grades of the metal have a dull finish. It is somewhat less dense than iron and has a hexagonal crystal structure, with a distorted form of hexagonal close packing, in which each atom has six nearest neighbors in its own plane and six others at a greater distance of 290.6 pm. The metal is hard and brittle at most temperatures but becomes malleable between 100 and 150 °C. Above 210 °C, the metal becomes brittle again and can be pulverized by beating. Zinc is a fair conductor of electricity. For a metal, zinc has relatively low melting and boiling point. This melting point is the lowest of all the d-block metals aside from mercury and cadmium. For this reason among others, zinc, cadmium, and mercury are often not considered to be transition metals like the rest of the d-block metals.
Many alloys contain zinc, the most famous example being brass. Other metals long known to form binary alloys with zinc are aluminium, antimony, bismuth, gold, iron, lead, mercury, silver, tin, magnesium, cobalt, nickel, tellurium, and sodium. Although neither zinc nor zirconium is ferromagnetic, their alloy,, exhibits ferromagnetism below 35 K.

Occurrence

Zinc makes up about 70 ppm of Earth's crust in mass, making it the 24th-most abundant crustal element. It also makes up 312 ppm of the Solar System, where it is the 22nd most abundant element. Typical background concentrations of zinc do not exceed: 1 μg/m³ in atmosphere, 300 mg/kg in soil, 100 mg/kg in vegetation, 20 μg/L in freshwater, and 5 μg/L in seawater. The element is normally found in association with other base metals such as copper and lead in the form of ores. Zinc is a chalcophile, meaning the element is more likely to be found associated with sulfur and other heavy chalcogens, rather than with the light chalcogen oxygen or with non-chalcogen electronegative elements such as the halogens. Sulfides formed as the crust solidified under the highly reducing conditions of the young Earth's atmosphere. Sphalerite, a crystalline form of zinc sulfide, is the most heavily mined zinc-containing ore as it contains 60–62% zinc by mass.
Other source minerals for zinc include smithsonite, hemimorphite, wurtzite, and sometimes hydrozincite. With the exception of wurtzite, all the other minerals were formed by weathering of the primordial zinc sulfides.
Currently identified zinc resources across the globe total 1.9–2.8 billion tonnes. Large deposits are in Australia, China, Canada, and the United States, with the largest potential reserves in Iran. The most recent estimate of reserve base for zinc was made in 2009 and was calculated to be roughly 480 Mt. Zinc reserves, on the other hand, are geologically identified ore bodies whose suitability for recovery is economically practical at the time of determination. Since exploration and mine development is an ongoing process, the amount and number of zinc reserves is not a fixed number, and sustainability of zinc ore supplies cannot be judged by simply extrapolating the combined mine life of today's zinc mines. This concept is well supported by data from the United States Geological Survey, which illustrates that although refined zinc production increased 80% between 1990 and 2010, the reserve lifetime for zinc has remained unchanged, through the discovery of additional zinc deposits. About 346 million tonnes have been extracted throughout history up to 2002, and scholars have estimated that about 109–305 million tonnes are presently in use.

Isotopes

Five stable isotopes of zinc occur in nature, with ⁶⁴Zn being the most abundant isotope. The other isotopes found in nature are , , , and .
Several dozen radioisotopes have been characterized., which has a half-life of 243.66 days, is the least active radioisotope, followed by with a half-life of 46.5 hours. Zinc has 10 nuclear isomers, of which ^69mZn has the longest half-life, at 13.75 hours. The superscript m indicates a metastable isotope, whose nucleus is in an excited state and which will eventually return to its ground state, through the emission of excess energy in the form of one or more photons, with the nucleus decaying to the ground state by the end of the process.
The most common decay mode of a radioisotope of zinc with a mass number lower than 66 is electron capture. The resulting decay product will be an isotope of copper.
The most common decay mode of a radioisotope of zinc with a mass number higher than 66 is beta decay, producing an isotope of gallium.

Compounds and chemistry

Reactivity

Zinc has an electron configuration of 4s²3d¹⁰ and is a member of the group 12 of the periodic table. It is a moderately reactive metal and strong reducing agent; in the reactivity series it is comparable to manganese. The surface of the pure metal tarnishes quickly, eventually forming a protective passivating layer of the basic zinc carbonate,, by reaction with atmospheric carbon dioxide.
Zinc burns in air with a bright bluish-green flame, giving off fumes of zinc oxide. Zinc reacts readily with acids, alkalis and other non-metals. Extremely pure zinc reacts only slowly at room temperature with acids. Strong acids, such as hydrochloric or sulfuric acid, can remove the passivating layer and the subsequent reaction with the acid releases hydrogen gas.
Zinc chemistry resembles that of the late first-row transition metals, nickel and copper, as well as certain main group elements. Almost all zinc compounds have the element in the +2 oxidation state. When Zn²⁺ compounds form, the outer shell s electrons are lost, yielding a bare zinc ion with the electronic configuration 3d¹⁰. The filled interior d shell generally does not participate in bonding, producing diamagnetic and mostly colorless compounds. In aqueous solution an octahedral complex, is the predominant species.
The ionic radii of zinc and magnesium happen to be nearly identical. Consequently, some of the equivalent salts have the same crystal structure, and in other circumstances where ionic radius is a determining factor, the chemistry of zinc has much in common with that of magnesium. Compared to the transition metals, zinc tends to form bonds with a greater degree of covalency. Complexes with N- and S- donors are much more stable. Complexes of zinc are mostly 4- or 6- coordinate, although 5-coordinate complexes are known.
Other oxidation states require unusual physical conditions, and the only positive oxidation states demonstrated are +1 or +2. The volatilization of zinc in combination with zinc chloride at temperatures above 285 °C indicates the formation of, a zinc compound with a +1 oxidation state. Calculations indicate that a zinc compound with the oxidation state of +4 is unlikely to exist. Zn is predicted to exist in the presence of strongly electronegative trianions; however, there exists some doubt around this possibility.