The United Kingdom the United Kingdom of Great Britain and Northern Ireland, sometimes referred to as Britain, is a sovereign country located off the north-western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland, many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state, the Republic of Ireland. Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world; the Irish Sea lies between Great Ireland. With an area of 242,500 square kilometres, the United Kingdom is the 78th-largest sovereign state in the world, it is the 22nd-most populous country, with an estimated 66.0 million inhabitants in 2017. The UK is constitutional monarchy; the current monarch is Queen Elizabeth II, who has reigned since 1952, making her the longest-serving current head of state.
The United Kingdom's capital and largest city is London, a global city and financial centre with an urban area population of 10.3 million. Other major urban areas in the UK include Greater Manchester, the West Midlands and West Yorkshire conurbations, Greater Glasgow and the Liverpool Built-up Area; the United Kingdom consists of four constituent countries: England, Scotland and Northern Ireland. Their capitals are London, Edinburgh and Belfast, respectively. Apart from England, the countries have their own devolved governments, each with varying powers, but such power is delegated by the Parliament of the United Kingdom, which may enact laws unilaterally altering or abolishing devolution; the nearby Isle of Man, Bailiwick of Guernsey and Bailiwick of Jersey are not part of the UK, being Crown dependencies with the British Government responsible for defence and international representation. The medieval conquest and subsequent annexation of Wales by the Kingdom of England, followed by the union between England and Scotland in 1707 to form the Kingdom of Great Britain, the union in 1801 of Great Britain with the Kingdom of Ireland created the United Kingdom of Great Britain and Ireland.
Five-sixths of Ireland seceded from the UK in 1922, leaving the present formulation of the United Kingdom of Great Britain and Northern Ireland. There are fourteen British Overseas Territories, the remnants of the British Empire which, at its height in the 1920s, encompassed a quarter of the world's land mass and was the largest empire in history. British influence can be observed in the language and political systems of many of its former colonies; the United Kingdom is a developed country and has the world's fifth-largest economy by nominal GDP and ninth-largest economy by purchasing power parity. It has a high-income economy and has a high Human Development Index rating, ranking 14th in the world, it was the world's first industrialised country and the world's foremost power during the 19th and early 20th centuries. The UK remains a great power, with considerable economic, military and political influence internationally, it is sixth in military expenditure in the world. It has been a permanent member of the United Nations Security Council since its first session in 1946.
It has been a leading member state of the European Union and its predecessor, the European Economic Community, since 1973. The United Kingdom is a member of the Commonwealth of Nations, the Council of Europe, the G7, the G20, NATO, the Organisation for Economic Co-operation and Development and the World Trade Organization; the 1707 Acts of Union declared that the kingdoms of England and Scotland were "United into One Kingdom by the Name of Great Britain". The term "United Kingdom" has been used as a description for the former kingdom of Great Britain, although its official name from 1707 to 1800 was "Great Britain"; the Acts of Union 1800 united the kingdom of Great Britain and the kingdom of Ireland in 1801, forming the United Kingdom of Great Britain and Ireland. Following the partition of Ireland and the independence of the Irish Free State in 1922, which left Northern Ireland as the only part of the island of Ireland within the United Kingdom, the name was changed to the "United Kingdom of Great Britain and Northern Ireland".
Although the United Kingdom is a sovereign country, Scotland and Northern Ireland are widely referred to as countries. The UK Prime Minister's website has used the phrase "countries within a country" to describe the United Kingdom; some statistical summaries, such as those for the twelve NUTS 1 regions of the United Kingdom refer to Scotland and Northern Ireland as "regions". Northern Ireland is referred to as a "province". With regard to Northern Ireland, the descriptive name used "can be controversial, with the choice revealing one's political preferences"; the term "Great Britain" conventionally refers to the island of Great Britain, or politically to England and Wales in combination. However, it is sometimes used as a loose synonym for the United Kingdom as a whole; the term "Britain" is used both as a synonym for Great Britain, as a synonym for the United Kingdom. Usage is mixed, with the BBC preferring to use Britain as shorthand only for Great Britain and the UK Government, while accepting that both terms refer to the United K
A mineral is, broadly speaking, a solid chemical compound that occurs in pure form. A rock may consist of a single mineral, or may be an aggregate of two or more different minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are excluded, but some minerals are biogenic and/or are organic compounds in the sense of chemistry. Moreover, living beings synthesize inorganic minerals that occur in rocks. In geology and mineralogy, the term "mineral" is reserved for mineral species: crystalline compounds with a well-defined chemical composition and a specific crystal structure. Minerals without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound may occur with different crystal structures, each structure is considered different mineral species. Thus, for example and stishovite are two different minerals consisting of the same compound, silicon dioxide; the International Mineralogical Association is the world's premier standard body for the definition and nomenclature of mineral species.
As of November 2018, the IMA recognizes 5,413 official mineral species. Out of more than 5,500 proposed or traditional ones; the chemical composition of a named mineral species may vary somewhat by the inclusion of small amounts of impurities. Specific varieties of a species sometimes have official names of their own. For example, amethyst is a purple variety of the mineral species quartz; some mineral species can have variable proportions of two or more chemical elements that occupy equivalent positions in the mineral's structure. Sometimes a mineral with variable composition is split into separate species, more or less arbitrarily, forming a mineral group. Besides the essential chemical composition and crystal structure, the description of a mineral species includes its common physical properties such as habit, lustre, colour, tenacity, fracture, specific gravity, fluorescence, radioactivity, as well as its taste or smell and its reaction to acid. Minerals are classified by key chemical constituents.
Silicate minerals comprise 90% of the Earth's crust. Other important mineral groups include the native elements, oxides, carbonates and phosphates. One definition of a mineral encompasses the following criteria: Formed by a natural process. Stable or metastable at room temperature. In the simplest sense, this means. Classical examples of exceptions to this rule include native mercury, which crystallizes at −39 °C, water ice, solid only below 0 °C. Modern advances have included extensive study of liquid crystals, which extensively involve mineralogy. Represented by a chemical formula. Minerals are chemical compounds, as such they can be described by fixed or a variable formula. Many mineral groups and species are composed of a solid solution. For example, the olivine group is described by the variable formula 2SiO4, a solid solution of two end-member species, magnesium-rich forsterite and iron-rich fayalite, which are described by a fixed chemical formula. Mineral species themselves could have a variable composition, such as the sulfide mackinawite, 9S8, a ferrous sulfide, but has a significant nickel impurity, reflected in its formula.
Ordered atomic arrangement. This means crystalline. An ordered atomic arrangement gives rise to a variety of macroscopic physical properties, such as crystal form and cleavage. There have been several recent proposals to classify amorphous substances as minerals; the formal definition of a mineral approved by the IMA in 1995: "A mineral is an element or chemical compound, crystalline and, formed as a result of geological processes." Abiogenic. Biogenic substances are explicitly excluded by the IMA: "Biogenic substances are chemical compounds produced by biological processes without a geological component and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound the product can be accepted as a mineral."The first three general characteristics are less debated than the last two. Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. Recent changes have included the addition of an organic class, in both the new Dana and the Strunz classification schemes.
The organic class includes a rare group of minerals with hydrocarbons. The IMA Commission on New Minerals and Mineral Names adopted in 2009 a hierarchical scheme for the naming and classification of mineral groups and group names and established seven commissions and four working groups to review and classify minerals into an official listing of their published names. According to these new r
Molybdenum is a chemical element with symbol Mo and atomic number 42. The name is from Neo-Latin molybdaenum, from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered in 1778 by Carl Wilhelm Scheele; the metal was first isolated in 1781 by Peter Jacob Hjelm. Molybdenum does not occur as a free metal on Earth; the free element, a silvery metal with a gray cast, has the sixth-highest melting point of any element. It forms hard, stable carbides in alloys, for this reason most of world production of the element is used in steel alloys, including high-strength alloys and superalloys. Most molybdenum compounds have low solubility in water, but when molybdenum-bearing minerals contact oxygen and water, the resulting molybdate ion MoO2−4 is quite soluble. Industrially, molybdenum compounds are used in high-pressure and high-temperature applications as pigments and catalysts. Molybdenum-bearing enzymes are by far the most common bacterial catalysts for breaking the chemical bond in atmospheric molecular nitrogen in the process of biological nitrogen fixation.
At least 50 molybdenum enzymes are now known in bacteria and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation. These nitrogenases contain molybdenum in a form different from other molybdenum enzymes, which all contain oxidized molybdenum in a molybdenum cofactor; these various molybdenum cofactor enzymes are vital to the organisms, molybdenum is an essential element for life in all higher eukaryote organisms, though not in all bacteria. In its pure form, molybdenum is a silvery-grey metal with a Mohs hardness of 5.5, a standard atomic weight of 95.95 g/mol. It has a melting point of 2,623 °C, it has one of the lowest coefficients of thermal expansion among commercially used metals. The tensile strength of molybdenum wires increases about 3 times, from about 10 to 30 GPa, when their diameter decreases from ~50–100 nm to 10 nm. Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale. It does not visibly react with water at room temperature.
Weak oxidation of molybdenum starts at 300 °C. Like many heavier transition metals, molybdenum shows little inclination to form a cation in aqueous solution, although the Mo3+ cation is known under controlled conditions. There are 35 known isotopes of molybdenum, ranging in atomic mass from 83 to 117, as well as four metastable nuclear isomers. Seven isotopes occur with atomic masses of 92, 94, 95, 96, 97, 98, 100. Of these occurring isotopes, only molybdenum-100 is unstable. Molybdenum-98 is the most abundant isotope, comprising 24.14% of all molybdenum. Molybdenum-100 has a half-life of about 1019 y and undergoes double beta decay into ruthenium-100. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of 150 ns. All unstable isotopes of molybdenum decay into isotopes of niobium and ruthenium; as noted below, the most common isotopic molybdenum application involves molybdenum-99, a fission product. It is a parent radioisotope to the short-lived gamma-emitting daughter radioisotope technetium-99m, a nuclear isomer used in various imaging applications in medicine.
In 2008, the Delft University of Technology applied for a patent on the molybdenum-98-based production of molybdenum-99. Molybdenum forms chemical compounds in oxidation states from -II to +VI. Higher oxidation states are more relevant to its terrestrial occurrence and its biological roles, mid-level oxidation states are associated with metal clusters, low oxidation states are associated with organomolybdenum compounds. Mo and W chemistry shows strong similarities; the relative rarity of molybdenum, for example, contrasts with the pervasiveness of the chromium compounds. The highest oxidation state is seen in molybdenum oxide, whereas the normal sulfur compound is molybdenum disulfide MoS2. From the perspective of commerce, the most important compounds are molybdenum disulfide and molybdenum trioxide; the black disulfide is the main mineral. It is roasted in air to give the trioxide: 2 MoS2 + 7 O2 → 2 MoO3 + 4 SO2The trioxide, volatile at high temperatures, is the precursor to all other Mo compounds as well as alloys.
Molybdenum has several oxidation states, the most stable being +4 and +6. Molybdenum oxide is soluble in strong alkaline water, forming molybdates. Molybdates are weaker oxidants than chromates, they tend to form structurally complex oxyanions by condensation at lower pH values, such as 6− and 4−. Polymolybdates can incorporate other ions; the dark-blue phosphorus-containing heteropolymolybdate P3− is used for the spectroscopic detection of phosphorus. The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides: Molybdenum chloride MoCl2, which exists as the hexamer Mo6Cl12 and the related dianion 2-. Molybdenum chloride MoCl3, a dark red solid, which converts to the anion trianionic complex 3-. Molybdenum chloride MoCl4, a black solid, which adopts a polymeric structure. Molybdenum chloride MoCl5 dark green solid that
In economics, a country's current account is one of the two components of its balance of payments, the other being the capital account. The current account consists of the balance of trade, net primary income or factor income and net cash transfers, that have taken place over a given period of time; the current account balance is one of two major measures of a country's foreign trade. A current account surplus indicates that the value of a country's net foreign assets grew over the period in question, a current account deficit indicates that it shrank. Both government and private payments are included in the calculation, it is called the current account because goods and services are consumed in the current period. The current account is an important indicator of an economy's health, it is defined as the sum of the balance of trade, net income from abroad, net current transfers. A positive current account balance indicates the nation is a net lender to the rest of the world, while a negative current account balance indicates that it is a net borrower from the rest of the world.
A current account surplus increases a nation's net foreign assets by the amount of the surplus, a current account deficit decreases it by that amount. A country's balance of trade is the net or difference between the country's exports of goods and services and its imports of goods and services, excluding all financial transfers and other components, over a given period of time. A country is said to have a trade surplus if its exports exceed its imports, a trade deficit if its imports exceed its exports. Positive net sales abroad contribute to a current account surplus; because exports generate positive net sales, because the trade balance is the largest component of the current account, a current account surplus is associated with positive net exports. In the net factor income or income account, income payments are outflows, income receipts are inflows. Income are receipts from investments made abroad and money sent by individuals working abroad, known as remittances, to their families back home.
If the income account is negative, the country is paying more than it is taking in interest, etc. The various subcategories in the income account are linked to specific respective subcategories in the capital account, as income is composed of factor payments from the ownership of capital or the negative capital abroad. From the capital account and central banks determine implied rates of return on the different types of capital; the United States, for example, gleans a larger rate of return from foreign capital than foreigners do from owning United States capital. In the traditional accounting of balance of payments, the current account equals the change in net foreign assets. A current account deficit implies a reduction of net foreign assets: Current account = change in net foreign assets. If an economy is running a current account deficit, it is absorbing more than; this can only happen if some other economies are lending their savings to it or the economy is running down its foreign assets such as official foreign currency reserve.
On the other hand, if an economy is running a current account surplus it is absorbing less than that it is producing. This means; as the economy is open, this saving is being invested abroad and thus foreign assets are being created. The current account is calculated by adding up the 4 components of current account: goods, services and current transfers. Goods Being movable and physical in nature, goods are traded by countries all over the world; when a transaction of certain good's ownership from a local country to a foreign country takes place, this is called an "export". The other way around, when a good's owner changes to a local inhabitant from a foreigner, is defined to be an "import". In calculating current account, exports are marked as credit and imports as debit. Services When an intangible service is used by a foreigner in a local land and the local resident receives the money from a foreigner, this is counted as an export, thus a credit. Income A credit of income happens when an individual or a company of domestic nationality receives money from a company or individual with foreign identity.
A foreign company's investment upon a domestic company or a local government is considered as a debit. Current transfers Current transfers take place when a certain foreign country provides currency to another country with nothing received as a return; such transfers are done in the form of donations, aids, or official assistance. A country's current account can be calculated by the following formula: C A = + N Y + N C T Where CA is the current account, X and M are the export and import of goods and services, NY the net income from abroad, NCT the net current transfers. A nation's current account balance is influenced by numerous factors – its trade policies, exchange rate, competit
The tögrög or tugrik is the official currency of Mongolia. It was subdivided into 100 möngö; the lowest denomination in regular use is the 10-tögrög note and the highest is the 20,000-tögrög note. In unicode, the currency sign is U+20AE ₮ TUGRIK SIGN. In 2010, the tögrög rose 15% against the dollar, due to the growth of the mining industry in Mongolia. However, its exchange rate eroded by 24% from early 2013 to June 2014 due to falling foreign investment and mining revenue; the word tögrög refers to "circle", or a "circular object". Today, it is used outside of referring to the currency, with the exception of the phrase tögrög sar, meaning "full moon"; the tögrög was introduced on December 9, 1925 at a value equal to one Soviet ruble, where one ruble or tögrög was equal to 18 grams of silver. It replaced the Mongolian dollar and other currencies and became the sole legal currency on April 1, 1928. Möngö coins are no longer in circulation as currency, owing to their negligible value. Today, they are to tourists as collectibles.
During socialism, the tögrög coin denominations were 1, 2, 5, 10, 15, 20, 50 möngö, 1 tögrög. After the Mongolian People's Republic came to an end in 1992 and inflation surged, möngö coins were abandoned and larger tögrög values introduced; the asterisk: this coin is non-circulating Like coins, the tögrög banknotes were similar to the Soviet ruble during the Mongolian People's Republic era. The similarities included color theme, overall design, the lineup of the denominations, which were 1, 3, 5, 10, 25, 50, 100 tögrög unless stated otherwise; the color for each value is 1 tögrög: brown 3 tögrög: green 5 tögrög: blue 10 tögrög: green 20 tögrög: red 25 tögrög: lilac 50 tögrög: green 100 tögrög: brownFormerly, all banknotes were printed in the Soviet Union. Modern tögrög banknotes are printed in Great Britain. Images shown are the earliest variations of each value Issued dates are listed for up to 2003, it is known that there is a 2005 edition of 10 tögrög, but it is yet unclear whether or not it was the only value for the 2005 edition.
Lower value notes issued in 2000 and after have line-patterned color underprint on the entire note, where the previous edition had near-white solid color. But one exception to the rule is the 2000 edition of 500 tögrög. High value notes issued in 2002 and after have a patch on the lower right hand side of obverse as an improved anti-counterfeit device, which used to be printed only on the two highest values; the new patch is more sophisticated than the ones in the 1990s. The Soyombo symbol was upgraded to a hologram on the two highest values. In China, there is a homonymic currency called tügürig, the Mongol name for the Renminbi, divided into 100 mönggü. 5 tögrög: No longer in common usage 10 tögrög: Used for change. 100 tögrög: A small ice cream 200 tögrög: A lollipop 500 tögrög: Average price for public transport in Ulaanbaatar.
United States dollar
The United States dollar is the official currency of the United States and its territories per the United States Constitution since 1792. In practice, the dollar is divided into 100 smaller cent units, but is divided into 1000 mills for accounting; the circulating paper money consists of Federal Reserve Notes that are denominated in United States dollars. Since the suspension in 1971 of convertibility of paper U. S. currency into any precious metal, the U. S. dollar is, de facto, fiat money. As it is the most used in international transactions, the U. S. dollar is the world's primary reserve currency. Several countries use it as their official currency, in many others it is the de facto currency. Besides the United States, it is used as the sole currency in two British Overseas Territories in the Caribbean: the British Virgin Islands and Turks and Caicos Islands. A few countries use the Federal Reserve Notes for paper money, while still minting their own coins, or accept U. S. dollar coins. As of June 27, 2018, there are $1.67 trillion in circulation, of which $1.62 trillion is in Federal Reserve notes.
Article I, Section 8 of the U. S. Constitution provides that the Congress has the power "To coin money". Laws implementing this power are codified at 31 U. S. C. § 5112. Section 5112 prescribes the forms; these coins are both designated in Section 5112 as "legal tender" in payment of debts. The Sacagawea dollar is one example of the copper alloy dollar; the pure silver dollar is known as the American Silver Eagle. Section 5112 provides for the minting and issuance of other coins, which have values ranging from one cent to 100 dollars; these other coins are more described in Coins of the United States dollar. The Constitution provides that "a regular Statement and Account of the Receipts and Expenditures of all public Money shall be published from time to time"; that provision of the Constitution is made specific by Section 331 of Title 31 of the United States Code. The sums of money reported in the "Statements" are being expressed in U. S. dollars. The U. S. dollar may therefore be described as the unit of account of the United States.
The word "dollar" is one of the words in the first paragraph of Section 9 of Article I of the Constitution. There, "dollars" is a reference to the Spanish milled dollar, a coin that had a monetary value of 8 Spanish units of currency, or reales. In 1792 the U. S. Congress passed a Coinage Act. Section 9 of that act authorized the production of various coins, including "DOLLARS OR UNITS—each to be of the value of a Spanish milled dollar as the same is now current, to contain three hundred and seventy-one grains and four sixteenth parts of a grain of pure, or four hundred and sixteen grains of standard silver". Section 20 of the act provided, "That the money of account of the United States shall be expressed in dollars, or units... and that all accounts in the public offices and all proceedings in the courts of the United States shall be kept and had in conformity to this regulation". In other words, this act designated the United States dollar as the unit of currency of the United States. Unlike the Spanish milled dollar, the U.
S. dollar is based upon a decimal system of values. In addition to the dollar the coinage act established monetary units of mill or one-thousandth of a dollar, cent or one-hundredth of a dollar, dime or one-tenth of a dollar, eagle or ten dollars, with prescribed weights and composition of gold, silver, or copper for each, it was proposed in the mid-1800s that one hundred dollars be known as a union, but no union coins were struck and only patterns for the $50 half union exist. However, only cents are in everyday use as divisions of the dollar. XX9 per gallon, e.g. $3.599, more written as $3.599⁄10. When issued in circulating form, denominations equal to or less than a dollar are emitted as U. S. coins while denominations equal to or greater than a dollar are emitted as Federal Reserve notes. Both one-dollar coins and notes are produced today, although the note form is more common. In the past, "paper money" was issued in denominations less than a dollar and gold coins were issued for circulation up to the value of $20.
The term eagle was used in the Coinage Act of 1792 for the denomination of ten dollars, subsequently was used in naming gold coins. Paper currency less than one dollar in denomination, known as "fractional currency", was sometimes pejoratively referred to as "shinplasters". In 1854, James Guthrie Secretary of the Treasury, proposed creating $100, $50 and $25 gold coins, which were referred to as a "Union", "Half Union", "Quarter Union", thus implying a denomination of 1 Union = $100. Today, USD notes are made from cotton fiber paper, unlike most common paper, made of wood fiber. U. S. coins are produced by the United States Mint. U. S. dollar banknotes are printed by the Bureau of Engraving and Printing and, since 1914, have been issued by t
Copper is a chemical element with symbol Cu and atomic number 29. It is a soft and ductile metal with high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, constantan used in strain gauges and thermocouples for temperature measurement. Copper is one of the few metals; this led to early human use in several regions, from c. 8000 BC. Thousands of years it was the first metal to be smelted from sulfide ores, c. 5000 BC, the first metal to be cast into a shape in a mold, c. 4000 BC and the first metal to be purposefully alloyed with another metal, tin, to create bronze, c. 3500 BC. In the Roman era, copper was principally mined on Cyprus, the origin of the name of the metal, from aes сyprium corrupted to сuprum, from which the words derived and copper, first used around 1530.
The encountered compounds are copper salts, which impart blue or green colors to such minerals as azurite and turquoise, have been used and as pigments. Copper used in buildings for roofing, oxidizes to form a green verdigris. Copper is sometimes used in decorative art, both in its elemental metal form and in compounds as pigments. Copper compounds are used as bacteriostatic agents and wood preservatives. Copper is essential to all living organisms as a trace dietary mineral because it is a key constituent of the respiratory enzyme complex cytochrome c oxidase. In molluscs and crustaceans, copper is a constituent of the blood pigment hemocyanin, replaced by the iron-complexed hemoglobin in fish and other vertebrates. In humans, copper is found in the liver and bone; the adult body contains between 2.1 mg of copper per kilogram of body weight. Copper and gold are in group 11 of the periodic table; the filled d-shells in these elements contribute little to interatomic interactions, which are dominated by the s-electrons through metallic bonds.
Unlike metals with incomplete d-shells, metallic bonds in copper are lacking a covalent character and are weak. This observation explains the low high ductility of single crystals of copper. At the macroscopic scale, introduction of extended defects to the crystal lattice, such as grain boundaries, hinders flow of the material under applied stress, thereby increasing its hardness. For this reason, copper is supplied in a fine-grained polycrystalline form, which has greater strength than monocrystalline forms; the softness of copper explains its high electrical conductivity and high thermal conductivity, second highest among pure metals at room temperature. This is because the resistivity to electron transport in metals at room temperature originates from scattering of electrons on thermal vibrations of the lattice, which are weak in a soft metal; the maximum permissible current density of copper in open air is 3.1×106 A/m2 of cross-sectional area, above which it begins to heat excessively. Copper is one of a few metallic elements with a natural color other than silver.
Pure copper acquires a reddish tarnish when exposed to air. The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells corresponds to orange light; as with other metals, if copper is put in contact with another metal, galvanic corrosion will occur. Copper does not react with water, but it does react with atmospheric oxygen to form a layer of brown-black copper oxide which, unlike the rust that forms on iron in moist air, protects the underlying metal from further corrosion. A green layer of verdigris can be seen on old copper structures, such as the roofing of many older buildings and the Statue of Liberty. Copper tarnishes when exposed to some sulfur compounds, with which it reacts to form various copper sulfides. There are 29 isotopes of copper. 63Cu and 65Cu are stable, with 63Cu comprising 69% of occurring copper. The other isotopes are radioactive, with the most stable being 67Cu with a half-life of 61.83 hours.
Seven metastable isotopes have been characterized. Isotopes with a mass number above 64 decay by β−, whereas those with a mass number below 64 decay by β+. 64Cu, which has a half-life of 12.7 hours, decays both ways.62Cu and 64Cu have significant applications. 62Cu is used in 62Cu-PTSM as a radioactive tracer for positron emission tomography. Copper is produced in massive stars and is present in the Earth's crust in a proportion of about 50 parts per million. In nature, copper occurs in a variety of minerals, including native copper, copper sulfides such as chalcopyrite, digenite and chalcocite, copper sulfosalts such as tetrahedite-tennantite, enargite, copper carbonates such as azurite and malachite, as copper or copper oxides such as cuprite and tenorite, respectively; the largest mass of elemental copper discovered weighed 420 tonnes and was found in 1857 on the Keweenaw Peninsula in Michigan, US. Native copper is a polycrystal