Silicon is a metalloid with an atomic number of 14. It has a melting point of 1410°C and boiling point of 2355°C. Silicon is the second most abundant element on Earth and is used to make computer chips, solar panels, and other electronics.
Silicates are minerals consisting of silica combined with metal oxides, forming a major component of the earth's crust. They have a basic tetrahedral structure and comprise 40% of common minerals. The main types are ortho, pyro, sheet, ring, and chain silicates. Silicates have many important uses including in microchips, quartz crystals, glass, ceramics, and as allied products such as sodium silicate, aluminum silicate, and potassium silicate. China is the world's largest producer and consumer of silicates.
Soda lime glass is the most common and inexpensive type of glass, containing 70% silica, 15% soda, and 9% lime. It has a chemical formula of Na2O.CaO.6SiO2 and makes up about 90% of all manufactured glass. Soda lime glass is produced through heating silica, soda, lime, and other minor ingredients to high temperatures, which allows chemical reactions to form a fused material. It has properties such as a refractive index of 1.46, 70-80% transmittance, and a melting temperature of 1000°C that make it suitable for many applications including windows, bottles, light bulbs, and laboratory equipment.
The chemical properties of a substance can be determined by performing experiments that use specific materials or processes with known characteristics. If a material affects the substance in a given way, the substance has a particular property. If a process changes the substance, more properties can be deduced.
1. Ion beam techniques were used to analyze Roman imperial vessel and window glass samples from Mt. Kosmaj in Serbia. Three distinct compositional groups were identified.
2. Group K1 consisted of window glass with unusually high manganese levels of around 1%, likely due to impurities and inexperienced local glassworkers. Group K2 included vessel glass and colorless/colored glass with manganese around 0.3% and antimony around 0.2% as decolorants. Group K3 was a small amount of pale green glass with natural manganese impurity levels of around 0.2%.
3. The glass was determined to be of the common Roman natron-lime-silica variety
The document discusses the process of mining and extracting gold. It describes how gold was originally mined during the 1800s gold rush and is now located using soil sampling. It explains two mining methods - lode deposits where gold is visible in rock and panning where gold accumulates in sediment. The extraction process is outlined, involving crushing ore, grinding into a slurry with steel balls, adding cyanide to dissolve the gold, and separating gold from carbon. Refining removes impurities and gold is often alloyed for hardness. Gold has various modern uses including currency, jewelry, dentistry, and industrial applications.
Eksitonit is a unique synthetic Yttrium Aluminium Garnet (YAG) gemstone patented by Tais Diamonds and Gems Co. Ltd. It was developed by scientist Boris Mikhaylovich Sinelnikov and has properties that make it suitable for use in laser equipment, jewelry, and luxury lighting such as high hardness (8.5 on Mohs scale), high light refraction, and durability. Tais Diamonds and Gems can produce Eksitonit in large sizes up to 60kg and in a variety of colors for customized luxury lighting designs.
Preparation of cremation Diamonds (DNA 2 DIAMONDS)Ankit Basoya
The student thanks their teacher for the opportunity to do a project on creating cremation diamonds from ashes. Cremation diamonds are real diamonds created from the cremated remains of a loved one through a process of converting the carbon in the ashes to pure carbon, creating a core, and placing it under high temperature and pressure to form a diamond. The process involves 5 steps - preparing the ashes into a powder, converting it to pure carbon, making a carbon core, placing the core under heat and pressure to form a diamond, and finally cutting and polishing the rough diamond. Cremation diamonds provide a meaningful way to memorialize and keep loved ones close.
Sodium chloride is used to make chlorine, sodium hydroxide, and hydrogen through industrial processes. It is an important raw material extracted from underground salt deposits through solution mining. Electrolysis of a sodium chloride solution produces chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide in the solution.
Silicates are minerals consisting of silica combined with metal oxides, forming a major component of the earth's crust. They have a basic tetrahedral structure and comprise 40% of common minerals. The main types are ortho, pyro, sheet, ring, and chain silicates. Silicates have many important uses including in microchips, quartz crystals, glass, ceramics, and as allied products such as sodium silicate, aluminum silicate, and potassium silicate. China is the world's largest producer and consumer of silicates.
Soda lime glass is the most common and inexpensive type of glass, containing 70% silica, 15% soda, and 9% lime. It has a chemical formula of Na2O.CaO.6SiO2 and makes up about 90% of all manufactured glass. Soda lime glass is produced through heating silica, soda, lime, and other minor ingredients to high temperatures, which allows chemical reactions to form a fused material. It has properties such as a refractive index of 1.46, 70-80% transmittance, and a melting temperature of 1000°C that make it suitable for many applications including windows, bottles, light bulbs, and laboratory equipment.
The chemical properties of a substance can be determined by performing experiments that use specific materials or processes with known characteristics. If a material affects the substance in a given way, the substance has a particular property. If a process changes the substance, more properties can be deduced.
1. Ion beam techniques were used to analyze Roman imperial vessel and window glass samples from Mt. Kosmaj in Serbia. Three distinct compositional groups were identified.
2. Group K1 consisted of window glass with unusually high manganese levels of around 1%, likely due to impurities and inexperienced local glassworkers. Group K2 included vessel glass and colorless/colored glass with manganese around 0.3% and antimony around 0.2% as decolorants. Group K3 was a small amount of pale green glass with natural manganese impurity levels of around 0.2%.
3. The glass was determined to be of the common Roman natron-lime-silica variety
The document discusses the process of mining and extracting gold. It describes how gold was originally mined during the 1800s gold rush and is now located using soil sampling. It explains two mining methods - lode deposits where gold is visible in rock and panning where gold accumulates in sediment. The extraction process is outlined, involving crushing ore, grinding into a slurry with steel balls, adding cyanide to dissolve the gold, and separating gold from carbon. Refining removes impurities and gold is often alloyed for hardness. Gold has various modern uses including currency, jewelry, dentistry, and industrial applications.
Eksitonit is a unique synthetic Yttrium Aluminium Garnet (YAG) gemstone patented by Tais Diamonds and Gems Co. Ltd. It was developed by scientist Boris Mikhaylovich Sinelnikov and has properties that make it suitable for use in laser equipment, jewelry, and luxury lighting such as high hardness (8.5 on Mohs scale), high light refraction, and durability. Tais Diamonds and Gems can produce Eksitonit in large sizes up to 60kg and in a variety of colors for customized luxury lighting designs.
Preparation of cremation Diamonds (DNA 2 DIAMONDS)Ankit Basoya
The student thanks their teacher for the opportunity to do a project on creating cremation diamonds from ashes. Cremation diamonds are real diamonds created from the cremated remains of a loved one through a process of converting the carbon in the ashes to pure carbon, creating a core, and placing it under high temperature and pressure to form a diamond. The process involves 5 steps - preparing the ashes into a powder, converting it to pure carbon, making a carbon core, placing the core under heat and pressure to form a diamond, and finally cutting and polishing the rough diamond. Cremation diamonds provide a meaningful way to memorialize and keep loved ones close.
Sodium chloride is used to make chlorine, sodium hydroxide, and hydrogen through industrial processes. It is an important raw material extracted from underground salt deposits through solution mining. Electrolysis of a sodium chloride solution produces chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide in the solution.
Silicon is the second most abundant element in the Earth's crust, found in rocks, sand, clays and soils. It is commonly found combined with oxygen as silicon dioxide or with oxygen and other elements as silicates. Pure silicon is a dark gray solid with the same crystalline structure as diamond, and it reacts with halogens and certain metals. When heated with carbon in an electric furnace, it produces silicon carbide.
Silicates are minerals composed of silicon and oxygen that make up approximately 90% of the Earth's crust. They exist as silicate minerals and aluminosilicate clays in the crust. Silicates can be classified based on their crystal structure as orthosilicates containing single SiO4 tetrahedra, pyrosilicates with linked pairs of tetrahedra, ring silicates containing silica rings, chain silicates with linked tetrahedral chains, sheet silicates with shared oxygen atoms between sheets, and framework silicates with a three-dimensional networked structure. Important aluminosilicates include micas, clays, zeolites, and many common rock-forming minerals.
The document discusses the properties and chemistry of the alkaline earth metals. Some key points:
1) The alkaline earth metals include beryllium, magnesium, calcium, strontium, and barium. They are silvery and less reactive than alkali metals but more reactive than transition metals.
2) Their properties include increasing density down the group. They form stable, colorless ionic compounds and react with nonmetal elements like chlorine and nitrogen.
3) Important compounds include oxides, hydroxides, and carbonates. Calcium compounds like calcium carbonate and calcium chloride have many applications. Magnesium is important in biology and its alloys.
- Metals have properties like being opaque, good conductors of heat and electricity, and high malleability. They have delocalized valence electrons that form energy bands.
- Aluminum is the most abundant metal in the Earth's crust, while noble metals like gold are only found in native form. Most metals are found in minerals.
- Metallurgy involves separating metals from minerals through physical or chemical processes like pyrometallurgy using heat, hydrometallurgy using aqueous solutions, and electrometallurgy using electrolysis.
- Alloys are mixtures of metals that can have substitutional or interstitial crystal structures that influence their properties. Common metal alloys include brass, bron
1). The document summarizes information about nickel, including its extraction from ores, properties, advantages, disadvantages, and classifications of nickel alloys. Nickel is extracted via pyrometallurgical or hydrometallurgical processes depending on the ore. 2). Important nickel alloys include nickel-copper alloys, nickel-chromium alloys, and nickel-base superalloys used in applications requiring high heat resistance. 3). Nickel has properties like corrosion resistance and is used to make alloys for applications across many industries.
The document provides information about cement, including its history, chemical composition, manufacturing process, hydration, types of cement and tests conducted on cement. It begins with describing how cement is made from raw materials such as limestone, clay and iron ore through grinding, heating and cooling processes. It then discusses the chemistry and reactions involved in cement hydration. The document also lists and describes common types of cement used in construction, such as ordinary Portland cement, rapid hardening cement, white cement, as well as tests to measure cement consistency, setting time and strength.
The document provides information on cement, including its history, chemical composition, manufacturing process, and hydration. It discusses how cement is made by heating limestone, clay, and other materials in a kiln to form clinker, which is then ground with gypsum. The manufacturing process involves quarrying limestone, grinding raw materials, sintering in a rotary kiln at high temperatures, cooling the clinker, and final grinding with gypsum. Hydration of cement occurs as its compounds (C3S, C2S, C3A, C4AF) react with water, releasing heat and forming hydrates that harden the concrete.
Glass is an amorphous, hard, brittle, transparent or translucent material made by fusing sand with soda, lime, and other ingredients at high temperatures. It has no definite melting point or structural formula. Glass is classified as soft or hard, with soft glass being more widely used for windows and containers due to its lower melting point and resistance to water and acids, while hard glass is used for chemical apparatus and heat-resistant items. The manufacturing process involves melting raw materials at high heat, forming the molten glass into shapes, annealing to slowly cool the glass to reduce breakage, and finishing steps like polishing.
Leaching of coppe anode slimes from copper electrowinningKenneyMbhetse
This document summarizes a student's project to recover precious metals from copper anode slime. The student tested different parameters for decopperization using sulfuric acid and then thiourea leaching to selectively recover gold and silver. Decopperization worked best at 600 g/L acid, 4 hours, and with an oxidant added. Thiourea leaching recovered up to 874 ppm gold and 27,972 ppm silver but did not selectively leach the metals. The student concluded more washing, oxidant addition, and longer leaching times were needed to fully remove copper and improve precious metal recovery.
The document summarizes key information about three materials: aluminum oxide, silicon carbide, and diamond. It discusses their crystal structures, properties like hardness and thermal conductivity, common processing methods, and applications. Aluminum oxide is used as an abrasive and refractory material. Silicon carbide is very hard and used for abrasives, armor, and power electronics. Diamond is the hardest known material and used in drilling, cutting, and jewelry due to its optical properties.
This document provides information about aluminum, copper, and nickel. It discusses their physical and chemical properties, including atomic number, density, and melting/boiling points. It also describes their common ores and uses in alloys. Aluminum is a reactive metal used in alloys for lightweight materials. Copper is highly conductive and used in wiring and alloys like brass. Nickel is corrosion resistant and commonly used in stainless steel and other alloys.
This document discusses the extractive metallurgy process for lead. It describes lead properties, history of lead use, health effects, lead ores and locations, ore dressing, size reduction, roasting, smelting, refining including the Parkes and Betts processes, and lead products and byproducts. The overall process involves extracting lead from its ore, reducing and refining it to remove impurities and produce a high purity lead product.
The document discusses using alkaline industrial wastes for mineral carbon sequestration. It describes mineral carbon sequestration techniques like direct and indirect carbonation that react carbon dioxide with minerals. Potential waste materials that could be used include steel slag, mining waste, ash, alkaline paper mill waste, and cement waste due to their alkaline properties and calcium content. Experiments were conducted carbonating these wastes using a batch reactor with carbon dioxide, and results found considerable amounts of carbon dioxide could be captured through mineral carbon sequestration.
Type, Processing and Transition of GlassesM Ali Mohsin
The document provides an overview of engineering ceramics and glasses, including:
1) The manufacturing process for glass involves melting raw materials at high temperatures, forming and shaping the molten glass, annealing to strengthen it, and finishing.
2) Different types of glasses are produced by varying the raw materials, including soda-lime glass, potash-lime glass, lead glass, borosilicate glass, and alumina silicate glass.
3) The glass transition is the reversible transition of an amorphous material from a hard, brittle state to a rubbery state as temperature increases, and is characterized by the glass transition temperature Tg.
Diamond and graphite are two allotropes of carbon that differ significantly in their physical properties. Diamonds form under high temperature and pressure deep in the Earth's mantle, while graphite forms through the metamorphism of carbonaceous sediments. Despite being made of the same element, diamond is transparent, extremely hard, and thermally conductive, while graphite is black, soft, and electrically conductive. Both have important industrial applications, with diamond used in cutting and optics and graphite used in pencils, lubricants, and batteries.
Nickel is a silvery-white metal that is commonly found in ores containing other elements. There are two important types of nickel ores - laterites, which are oxide or silicate ores predominantly found in tropical regions, and sulfides which contain about 1.5% nickel associated with other metals in more temperate areas. Nickel is extracted from these ores using pyrometallurgical or hydrometallurgical approaches depending on the specific mineral composition of the ore. Once extracted, nickel can be further refined to high purity using techniques like the Mond process or electrorefining.
The document outlines topics that will be covered on the Y10 End of Year Exam for different chapters.
For C1 Topics, Chapter 1 will cover fundamental ideas about atoms, elements, compounds and chemical bonding. Chapter 2 will discuss rocks and building materials like limestone and its uses. Chapter 3 will cover metals and their uses, including extracting metals like iron, aluminum and copper from ores.
For C2 Topics, Chapter 4 will focus on rates and energy.
For C3 Topics, Chapter 3 will cover energy calculations. Students can view a slideshow and click on topics to be directed to relevant sections of a PowerPoint presentation.
MANUFACTURING AND UNDERSTANDING ABOUT CEMENT ITS COMPOSITION, INTERNAL MECHANICS, VARIOUS METHODS OF MANUFACTURING, USES AND VARIOUS COMPOUNDS PRESENT IN CEMENT AND ITS IMPORTANCE
CHECKOUT MY YOUTUBE CHANNEL
http://www.youtube.com/c/beaCIVILEngineergovindsir_onlineclasses
Silicon is the second most abundant element in the Earth's crust, found in rocks, sand, clays and soils. It is commonly found combined with oxygen as silicon dioxide or with oxygen and other elements as silicates. Pure silicon is a dark gray solid with the same crystalline structure as diamond, and it reacts with halogens and certain metals. When heated with carbon in an electric furnace, it produces silicon carbide.
Silicates are minerals composed of silicon and oxygen that make up approximately 90% of the Earth's crust. They exist as silicate minerals and aluminosilicate clays in the crust. Silicates can be classified based on their crystal structure as orthosilicates containing single SiO4 tetrahedra, pyrosilicates with linked pairs of tetrahedra, ring silicates containing silica rings, chain silicates with linked tetrahedral chains, sheet silicates with shared oxygen atoms between sheets, and framework silicates with a three-dimensional networked structure. Important aluminosilicates include micas, clays, zeolites, and many common rock-forming minerals.
The document discusses the properties and chemistry of the alkaline earth metals. Some key points:
1) The alkaline earth metals include beryllium, magnesium, calcium, strontium, and barium. They are silvery and less reactive than alkali metals but more reactive than transition metals.
2) Their properties include increasing density down the group. They form stable, colorless ionic compounds and react with nonmetal elements like chlorine and nitrogen.
3) Important compounds include oxides, hydroxides, and carbonates. Calcium compounds like calcium carbonate and calcium chloride have many applications. Magnesium is important in biology and its alloys.
- Metals have properties like being opaque, good conductors of heat and electricity, and high malleability. They have delocalized valence electrons that form energy bands.
- Aluminum is the most abundant metal in the Earth's crust, while noble metals like gold are only found in native form. Most metals are found in minerals.
- Metallurgy involves separating metals from minerals through physical or chemical processes like pyrometallurgy using heat, hydrometallurgy using aqueous solutions, and electrometallurgy using electrolysis.
- Alloys are mixtures of metals that can have substitutional or interstitial crystal structures that influence their properties. Common metal alloys include brass, bron
1). The document summarizes information about nickel, including its extraction from ores, properties, advantages, disadvantages, and classifications of nickel alloys. Nickel is extracted via pyrometallurgical or hydrometallurgical processes depending on the ore. 2). Important nickel alloys include nickel-copper alloys, nickel-chromium alloys, and nickel-base superalloys used in applications requiring high heat resistance. 3). Nickel has properties like corrosion resistance and is used to make alloys for applications across many industries.
The document provides information about cement, including its history, chemical composition, manufacturing process, hydration, types of cement and tests conducted on cement. It begins with describing how cement is made from raw materials such as limestone, clay and iron ore through grinding, heating and cooling processes. It then discusses the chemistry and reactions involved in cement hydration. The document also lists and describes common types of cement used in construction, such as ordinary Portland cement, rapid hardening cement, white cement, as well as tests to measure cement consistency, setting time and strength.
The document provides information on cement, including its history, chemical composition, manufacturing process, and hydration. It discusses how cement is made by heating limestone, clay, and other materials in a kiln to form clinker, which is then ground with gypsum. The manufacturing process involves quarrying limestone, grinding raw materials, sintering in a rotary kiln at high temperatures, cooling the clinker, and final grinding with gypsum. Hydration of cement occurs as its compounds (C3S, C2S, C3A, C4AF) react with water, releasing heat and forming hydrates that harden the concrete.
Glass is an amorphous, hard, brittle, transparent or translucent material made by fusing sand with soda, lime, and other ingredients at high temperatures. It has no definite melting point or structural formula. Glass is classified as soft or hard, with soft glass being more widely used for windows and containers due to its lower melting point and resistance to water and acids, while hard glass is used for chemical apparatus and heat-resistant items. The manufacturing process involves melting raw materials at high heat, forming the molten glass into shapes, annealing to slowly cool the glass to reduce breakage, and finishing steps like polishing.
Leaching of coppe anode slimes from copper electrowinningKenneyMbhetse
This document summarizes a student's project to recover precious metals from copper anode slime. The student tested different parameters for decopperization using sulfuric acid and then thiourea leaching to selectively recover gold and silver. Decopperization worked best at 600 g/L acid, 4 hours, and with an oxidant added. Thiourea leaching recovered up to 874 ppm gold and 27,972 ppm silver but did not selectively leach the metals. The student concluded more washing, oxidant addition, and longer leaching times were needed to fully remove copper and improve precious metal recovery.
The document summarizes key information about three materials: aluminum oxide, silicon carbide, and diamond. It discusses their crystal structures, properties like hardness and thermal conductivity, common processing methods, and applications. Aluminum oxide is used as an abrasive and refractory material. Silicon carbide is very hard and used for abrasives, armor, and power electronics. Diamond is the hardest known material and used in drilling, cutting, and jewelry due to its optical properties.
This document provides information about aluminum, copper, and nickel. It discusses their physical and chemical properties, including atomic number, density, and melting/boiling points. It also describes their common ores and uses in alloys. Aluminum is a reactive metal used in alloys for lightweight materials. Copper is highly conductive and used in wiring and alloys like brass. Nickel is corrosion resistant and commonly used in stainless steel and other alloys.
This document discusses the extractive metallurgy process for lead. It describes lead properties, history of lead use, health effects, lead ores and locations, ore dressing, size reduction, roasting, smelting, refining including the Parkes and Betts processes, and lead products and byproducts. The overall process involves extracting lead from its ore, reducing and refining it to remove impurities and produce a high purity lead product.
The document discusses using alkaline industrial wastes for mineral carbon sequestration. It describes mineral carbon sequestration techniques like direct and indirect carbonation that react carbon dioxide with minerals. Potential waste materials that could be used include steel slag, mining waste, ash, alkaline paper mill waste, and cement waste due to their alkaline properties and calcium content. Experiments were conducted carbonating these wastes using a batch reactor with carbon dioxide, and results found considerable amounts of carbon dioxide could be captured through mineral carbon sequestration.
Type, Processing and Transition of GlassesM Ali Mohsin
The document provides an overview of engineering ceramics and glasses, including:
1) The manufacturing process for glass involves melting raw materials at high temperatures, forming and shaping the molten glass, annealing to strengthen it, and finishing.
2) Different types of glasses are produced by varying the raw materials, including soda-lime glass, potash-lime glass, lead glass, borosilicate glass, and alumina silicate glass.
3) The glass transition is the reversible transition of an amorphous material from a hard, brittle state to a rubbery state as temperature increases, and is characterized by the glass transition temperature Tg.
Diamond and graphite are two allotropes of carbon that differ significantly in their physical properties. Diamonds form under high temperature and pressure deep in the Earth's mantle, while graphite forms through the metamorphism of carbonaceous sediments. Despite being made of the same element, diamond is transparent, extremely hard, and thermally conductive, while graphite is black, soft, and electrically conductive. Both have important industrial applications, with diamond used in cutting and optics and graphite used in pencils, lubricants, and batteries.
Nickel is a silvery-white metal that is commonly found in ores containing other elements. There are two important types of nickel ores - laterites, which are oxide or silicate ores predominantly found in tropical regions, and sulfides which contain about 1.5% nickel associated with other metals in more temperate areas. Nickel is extracted from these ores using pyrometallurgical or hydrometallurgical approaches depending on the specific mineral composition of the ore. Once extracted, nickel can be further refined to high purity using techniques like the Mond process or electrorefining.
The document outlines topics that will be covered on the Y10 End of Year Exam for different chapters.
For C1 Topics, Chapter 1 will cover fundamental ideas about atoms, elements, compounds and chemical bonding. Chapter 2 will discuss rocks and building materials like limestone and its uses. Chapter 3 will cover metals and their uses, including extracting metals like iron, aluminum and copper from ores.
For C2 Topics, Chapter 4 will focus on rates and energy.
For C3 Topics, Chapter 3 will cover energy calculations. Students can view a slideshow and click on topics to be directed to relevant sections of a PowerPoint presentation.
MANUFACTURING AND UNDERSTANDING ABOUT CEMENT ITS COMPOSITION, INTERNAL MECHANICS, VARIOUS METHODS OF MANUFACTURING, USES AND VARIOUS COMPOUNDS PRESENT IN CEMENT AND ITS IMPORTANCE
CHECKOUT MY YOUTUBE CHANNEL
http://www.youtube.com/c/beaCIVILEngineergovindsir_onlineclasses
3. Silicon
• Group 14
• Carbon Family
• Metalloid
• Density of 2.33 g/cm3
• Silvery color
• Boiling point of 2355oC
• Combustible solid when in powder form
• Melting point of 1410oC
• Hardness of 6.5 Mohs
• Solid
7. Discovery Of Silicon
• Discoverer: Jons Berzelius
• Date of Discovery: 1823
• Discovered in Sweden
• Stable Life Span
• Name Origin: From the Latin word silex (flint)
• Uses: glass, semiconductors, computer chips
• Obtained From: clay, granite, quartz, sand
8. Impact
Environmental Social
• No negative environmental • Used many things people
effects have been reported. use everyday
• It is very sustainable • Used in technology
• Has a positive social impact
9.
10. Uses Cost
• Chips for computers and • Pure Silicon $5.4 per 100g
electronics • Bulk Silicon $0.14 per 100g
• In engines
• Enhances iron’s magnetic
abilities
• In concrete and bricks
• In watches and clocks
• Cookware
• Medical implants
• Adhesives
11. Interesting Facts
• The lowest purity electronic grade silicon can
be is 99.9999999% pure.
• Second most abundant element on our planet
is Silicon.
• 8th most abundant element in the universe is
Silicon
• Silicon is made in stars with a mass of eight or
more Earth sun