An introduction to Lead Chemistry

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Overview

Lead is the heaviest member of the carbon family. The carbon family consists of the five elements in Group 14 (IVA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Although a member of the carbon family, lead looks and behaves very differently from carbon.
Lead is one of only a few elements known to ancient peoples. One of the oldest examples of lead is a small statue found in Egypt. It was made during the First Dynasty, in about 3400 B.C. Mention of lead and lead objects can also be found in very old writing from India. And the Bible mentions lead in a number of passages.

SYMBOL 
Pb
ATOMIC NUMBER 
82
ATOMIC MASS 
207.2
FAMILY 
Group 14 (IVA)
Carbon
PRONUNCIATION 
LED

Throughout history, Lead has been used to make water and sewer pipes; roofing; cable coverings; type metal and other alloys; paints; wrappings for food, tobacco, and other products; and as an additive in gasoline. Since the 1960s, however, there has been a growing concern about the health effects of lead. For instance, scientists have found that lead can cause mental and physical problems in growing children. As a result, many common lead products are now being phased out.

Discovery and naming

Lead has been around for thousands of years. It is impossible to say when humans first discovered the element. It does not occur as an element in the earth very often. But one of its ores, lead sulfide (PbS), is fairly common. It is not difficult to obtain pure lead metal from lead sulfide. Humans probably discovered methods for doing so thousands of years ago.

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By Roman times, lead metal was widely used. The far-reaching system that brought water to Rome contained many lead pipes. Sheets of lead were used as writing tablets and some Roman coins were also made of lead. Perhaps of greatest interest was the use of lead in making pots and pans. Modern scientists believe many Romans may have become ill and died because of this practice. Cooking liquids in lead utensils tends to make the lead dissolve. It got into the food being cooked. People who ate those foods got more and more lead into their bodies. Eventually, the effects of lead poisoning must have begun to appear.
Of course, the Romans had little understanding of the connection between lead and disease. They probably never realized that they were poisoning themselves by using lead pots and pans.
No one is quite sure how lead got its name. The word has been traced to manuscripts that date to before the 12th century. Romans called the metal plumbum. It is from this name that the element's chemical symbol comes: Pb. Compounds of lead are sometimes called by this old name, such as plumbous chloride.

Physical properties

Lead is a heavy, soft, gray solid. It is both ductile and malleable. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. It has a shiny surface when first cut, but it slowly tarnishes (rusts) and becomes dull. Lead is easily worked. "Working" a metal means bending, cutting, shaping, pulling, and otherwise changing the shape of the metal.
The melting point of lead is 327.4°C (621.3°F), and its boiling point is 1,750 to 1,755°C (3,180 to 3,190°F). Its density is 11.34 grams per cubic centimeter. Lead does not conduct an electric current, sound, or vibrations very well.

Chemical properties

Lead is a moderately active metal. It dissolves slowly in water and in most cold acids. It reacts more rapidly with hot acids. It does not react with oxygen in the air readily and does not burn.

Occurrence in nature

The abundance of lead in the Earth's crust is estimated to be between 13 and 20 parts per million. It ranks in the upper third among the elements in terms of its abundance.
Lead rarely occurs as a pure element in the earth. Its most common ore is galena, or lead sulfide (PbS). Other ores of Lead are anglesite, or lead sulfate (PbSO ₄ ); cerussite, or lead carbonate (PbCO ₃ ); and mimetite (PbCL ₂ ○ Pb ₃ (AsO ₄ ) ₂ ).
The largest producers of lead ore in the world are Australia, China, the United States, Peru, Canada, Mexico, and Sweden. In the United States, more than 93 percent of all the lead produced comes from Missouri. Other lead-producing states are Montana, Colorado, Idaho, Illinois, New York, and Tennessee. In 1996, 426,000 metric tons of lead were produced in the United States.

Isotopes

Four naturally occurring isotopes of lead occur. They are lead-204, lead-206, lead-207, and lead-208. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
About sixteen radioactive isotopes of lead are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

Romans routinely ate food cooked in lead pots and pans. The connection between lead and disease was not known then, so many people became ill and died of lead poisoning.

One radioactive isotope of lead, lead-210, is sometimes used in medicine. This isotope gives off radiation that can kill cancer cells. It is also used to treat non-cancerous eye disorders.

Lead smelting.

Extraction

Lead is obtained from its ores by a method used with many metals. First, the ore is roasted (heated in air). Roasting, also called smelting, converts the ore to a compound of lead and oxygen, lead oxide (PbO ₂ ). Lead oxide is then heated with charcoal (pure carbon). The carbon takes oxygen away from the lead oxide. It leaves pure lead behind:
Lead obtained in this way is not very pure. It can be purified electrolytically. Electrolytic refining involves passing an electric current through a compound. Very pure lead is collected at one side of the container in which the reaction is carried out.

A major source of lead is recycled car batteries.

Lead is also recovered in recycling programs. Recycling is the process by which a material is retrieved from a product that is no longer used. For example, old car batteries were once just thrown away. Now they are sent to recycling plants where lead can be extracted and used over and over again. It is not necessary to get all the lead that industry needs from new sources, such as ores.

Uses

The lead industry is undergoing dramatic change. Many products once made with lead no longer use the element. The purpose of this change is to reduce the amount of lead that gets into the environment. Examples of such products include ammunition, such as shot and bullets; sheet lead used in building construction; solder; water and sewer pipes; ball bearings; radiation shielding; and gasoline. These changes are possible because manufacturers are finding safer elements to use in place of lead.

The price of a gallon of gas

F or many years, lead was regarded as a miracle chemical by the automotive industry. The power to run a car comes from the burning of gasoline in the engine. However, burning gasoline is not a simple process. Many things happen inside an engine when gasoline burns in the carburetor.
For example, an engine can "knock" if the gasoline does not burn properly. "Knocking" is a "bang-bang" sound from the engine. It occurs when low-grade gasoline is used.
One way to prevent knocking is to use high-grade gasoline. Another way is to add chemicals to the gasoline. The best gasoline additive discovered was a compound called tetraethyl lead (Pb(C ₂ H ₅ ) ₄ ). Tetraethyl lead was usually called "lead" by the automotive industry, the consumer, and advertisers. When someone bought "leaded" gasoline, it contained not lead metal, but tetraethyl lead.
Leaded gasoline was a great discovery. It could be made fairly cheaply and it prevented car engines from knocking. No wonder people thought it was a miracle chemical.
What people didn't realize was that tetraethyl lead breaks down in a car engine because of the high temperature at which engines operate. When tetraethyl lead breaks down, elemental lead (Pb) is formed:
The result—with millions of cars being driven every day—was more and more lead getting into the air. And more and more people inhaled that lead. Eventually, doctors began to see more people with leadrelated diseases.
The federal government finally decided that tetraethyl lead was too dangerous to use in gasoline. By 1990, the use of this compound had been banned by all governments in North America.

Other uses of lead have not declined. The best example is lead storage batteries. A lead storage battery is a device for converting chemical energy into electrical energy. Almost every car and truck has at least one lead storage battery. But no satisfactory substitute for it has been found. About 87 percent of all lead produced in the United States now goes to the manufacture of lead storage batteries. In addition to cars and trucks, these batteries are used for communication networks and emergency power supplies in hospitals, and in forklifts, airline ground equipment, and mining vehicles.

Compounds

A small percentage of lead is used to make lead compounds. Although the amount of lead is small, the variety of uses for these compounds is large. Some examples of important lead compounds are:

lead acetate (Pb(C ₂ H ₃ O ₂ ) ₂ ): insecticides; waterproofing; varnishes; dyeing of cloth; production of gold; hair dye
lead antimonate (Pb ₃ (SbO ₄ ) ₂ ): staining of glass, porcelain and other ceramics
lead azide (Pb(N ₃ ) ₂ ): used as a "primer" for high explosives
lead chromate ("chrome yellow"; PbCrO ₄ ): industrial paints (use restricted by law)
lead fluoride (PbF ₂ ): used to make lasers; specialized optical glasses
lead iodide (PbI ₂ ): photography; cloud seeding to produce rain
lead naphthenate (Pb(C ₇ H ₁₂ O ₂ )): wood preservative; insecticide; additive for lubricating oil; paint and varnish drier
lead phosphite (2PbO ○ PbHPO ₃ ): used to screen out ultraviolet radiation in plastics and paints
lead stearate (Pb(C ₁₈ H ₃₅ O ₂ ) ₂ ): used to make soaps, greases, waxes, and paints; lubricant; drier for paints and varnishes
lead telluride (PbTe): used to make semiconductors, photoconductors, and other electronic equipment

Health effects

The health effects of lead have become much better understood since the middle of the 20th century. At one time, the metal was regarded as quite safe to use for most applications. Now lead is known to cause both immediate and long-term health problems, especially with children. It is toxic when swallowed, eaten, or inhaled.
Young children are most at risk from lead poisoning. Some children have a condition known as pica. They have an abnormal desire to eat materials like dirt, paper, and chalk. Children with pica sometimes eat paint chips off walls. At one time, many interior house paints were made with lead compounds. Thus, crawling babies or children with pica ran the risk of eating large amounts of lead and being poisoned.
Some symptoms of lead poisoning include nausea, vomiting, extreme tiredness, high blood pressure, and convulsions (spasms). Over a long period of time, these children often suffer brain damage. They lose the ability to carry out normal mental functions.
Other forms of lead poisoning can also occur. For example, people who work in factories where lead is used can inhale lead fumes. The amount of fumes inhaled at any one time may be small. But over months or years, the lead in a person's body can build up. This kind of lead poisoning can lead to nerve damage and problems with the gastrointestinal system (stomach and intestines).

Lead causes both immediate and longterm health problems, especially with children. It is toxic when swallowed, eaten, or inhaled.

Today, there is an effort to reduce the use of lead in consumer products. For instance, older homes are often tested for lead paint before they are resold. Lead paint has also been removed from older school buildings.

Read more: http://www.chemistryexplained.com/elements/L-P/Lead.html#ixzz1YRlvM3k9

An Introduction to Sulfur

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Overview

Sulfur belongs to the chalcogen family. Other members of the family are oxygen, selenium, tellurium, and polonium. These elements make up Group 16 (VIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other.

The term chalcogen comes from two Greek words meaning "ore forming." An ore is a naturally occurring mineral used as a source for an element. Many ores are compounds of a metal and oxygen or a metal and sulfur. Compounds that contain two elements, one of which is sulfur, are called sulfides. For example, a beautiful gold-colored mineral is called pyrite, or "fool's gold," because it looks so much like real gold. Pyrite is iron sulfide (FeS ₂ ).
Sulfur was known to ancient peoples. Its physical and chemical properties are very distinctive. It often occurs as a brilliant yellow powder. When it burns, it produces a clear blue flame and a very strong odor.

SYMBOL 
S
ATOMIC NUMBER 
16
ATOMIC MASS 
32.064
FAMILY 
Group 16 (VIA)
Chalcogen
PRONUNCIATION 
SUL-fur

Sulfur, also spelled as sulphur, is a very important element in today's world. Its most important use is in the manufacture of sulfuric acid (H ₂ SO ₄ ). There is more sulfuric acid made than any other chemical in the world. It has an enormous number of important uses.

Discovery and naming

Sulfur must have been well known to ancient peoples. They sometimes referred to it as brimstone. Sulfur sometimes occurs in bright yellow layers on the top of the earth. It has a sharp, offensive odor. When it burns, it gives off a strong, suffocating smell. The odor is like that produced when a match is struck.

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The Bible mentions brimstone in a number of places. For example, Sodom and Gomorrah were two towns destroyed by God for the wicked ways of their citizens: "The Lord rained upon Sodom and upon Gomorrah brimstone and fire."
But ancient people certainly did not think about sulfur the way modern chemists do. In fact, they used the word "element" to talk about anything that was basic. Ancient Greek philosophers, for example, thought that everything consisted of four elements: earth, fire, water, and air. Other philosophers thought there were only two elements: sulfur and mercury.
But early thinkers were often confused as to what they meant by the word "sulfur." They often were talking about anything that burned and gave off large amounts of smoke. To them, "sulfur" was really a "burning substance." It took centuries for scientists to identify sulfur as an element.

Physical properties

Sulfur exists in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The two forms of sulfur are known as α-form and β-form (the Greek letters alpha and beta, respectively). Both allotropes are yellow, with the α-form a brighter yellow and the β-form a paler, whitish-yellow. The α-form changes to the β-form at about 94.5°C (202°F). The α-form can be melted at 112.8°C (235.0°F) if it is heated quickly. The β-form has a melting point of 119°C (246°F). The boiling point of the α-form is 444.6°C (832.3°F).
The two allotropes have densities of 2.06 grams per cubic centimeter (α-form) and 1.96 grams per cubic centimeter (β-form). Neither allotrope will dissolve in water. Both are soluble

Solid sulfur.

in other Liquids, such as benzene (C ₆ H ₆ ), carbon tetrachloride (CCl ₄ ), and carbon disulfide (CS ₂ ).
Another allotrope of sulfur is formed when the element is melted. This allotrope has no crystalline shape. It looks like a dark brown, thick, melted plastic.

Chemical properties

Sulfur's most prominent chemical property is that it burns. When it does so, it gives off a pale blue flame and sulfur dioxide (SO ₂ ) gas. Sulfur dioxide has a very obvious strong, choking odor.

Sulfur sometimes occurs in bright yellow layers on the top of the earth. It has a sharp, offensive odor.

Sulfur also combines with most other elements. Sometimes it combines with them easily at room temperature. In other cases, it must be heated. The reaction between magnesium and sulfur is typical. When the two elements are heated, they combine to form magnesium sulfide (MgS):

A chemical reaction involving sulfur.

Sulfur also combines with hydrogen gas:
The compound formed in this reaction is hydrogen sulfide (H ₂ S). Hydrogen sulfide has one of the best known odors of all compounds. It smells like rotten eggs. Hydrogen sulfide is added to natural gas (methane) used in homes for cooking and heating. Methane is odorless. So the unique smell of hydrogen sulfide makes it easy to know when there is a methane leak.

Occurrence in nature

At one time, sulfur occurred in layers along the Earth's surface. They were easy for humans to find and take. Deposits like these are more difficult to find today. One place they still occur is in the vicinity of volcanoes. Sulfur is released from volcanoes as a gas. When it reaches the cold air, it changes back to a solid. It forms beautiful yellow deposits along the edge of a volcano.
Large supplies of sulfur still occur underground. They are removed by the Frasch process (see accompanying sidebar).
Sulfur also occurs in a number of important minerals. Some examples are barite, or barium sulfate (BaSO ₄ ); celestite, or strontium sulfate (SrSO ₄ ); cinnabar, or mercury sulfide (HgS); galena, or lead sulfide (PbS); pyrites, or iron sulfide (FeS ₂ ); sphalerite, or zinc sulfide (ZnS); and stibnite, or antimony sulfide (Sb ₂ S ₃ ).
The abundance of sulfur in the Earth's crust is thought to be about 0.05 percent. It ranks about number 16 among the elements in terms of their abundance in the earth. It is more abundant than carbon, but less abundant than barium or strontium.
The largest producers of sulfur in the world are the United States, Canada, China, Russia, Mexico, and Japan. In 1996, the United States produced about 11,800,000 metric tons of sulfur. It is mined in 30 states, Puerto Rico, and the U.S. Virgin Islands.

Isotopes

There are four naturally occurring isotopes of sulfur: sulfur-32, sulfur-33, sulfur-34, and sulfur-36. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Sulfur occurs in the vicinity of volcanoes.

Six radioactive isotopes of sulfur are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
One radioactive isotope of sulfur, sulfur-35, is used commercially. In medicine, the isotope is used to study the way fluids occur inside the body. It also has applications in research as a tracer. A tracer is a radioactive isotope whose presence in a system can easily be detected. The isotope is injected into the system at some point. Inside the system, the isotope gives off radiation. That radiation can be followed by means of detectors placed around the system.
As an example, a company that makes rubber tires might want to know what happens to the sulfur added to tires. Sulfur-35 is added to rubber along with non-radioactive sulfur. Researchers follow the radioactive isotope in the tires to see what happens to the sulfur when the tires are used.

The Frasch method of removing sulfur

T he Frasch method is one of the most famous mining systems ever invented. It was developed by German-American chemist Herman Frasch (1851-1914) in 1887.
The Frasch method is based on the low melting point of sulfur. The element melts at a temperature slightly higher than that of boiling water (100°C). Here is how the method works:
A set of three nested pipes (one inside each other) is sunk into the ground. The innermost pipe has a diameter of about an inch. The middle pipe has a diameter of about four inches. And the outer pipe has a diameter of about eight inches.
A stream of superheated water is injected into the outer pipe. Superheated water is water that is hotter than its boiling point, but that has not started to boil. Superheated water can be made by raising the pressure on the water. Its temperature can reach 160°C (320°F).
The superheated water passes down the outer pipe into the underground sulfur, causing it to melt. The molten (melted) sulfur forms a lake at the bottom of the pipe.
At the same time, a stream of hot air under pressure is forced down the innermost (one-inch) pipe. The hot air stirs up the molten sulfur and hot water at the bottom of the pipe. A foamy, soupy mixture of sulfur and water is formed. The mixture is forced upward through the middle pipe. When it reaches the surface, it is collected. The sulfur cools and separates from the water.

Similar applications of sulfur-35 involve studying sulfur in steel when it is made, seeing how sulfur affects the way engines operate, following what happens when proteins (which contain sulfur) are digested, and learning how drugs that contain sulfur are processed in the body.

Extraction

Like coal, sulfur sometimes occurs in thick layers under ground. One way to remove sulfur would be to mine it the way coal is mined. But a much easier method for removing sulfur from the ground is the Frasch method (see accompanying sidebar).

Uses

Sulfur has relatively few uses as an element. One of the most important of those uses is in vulcanization. Vulcanization is the process of adding sulfur to rubber to make it stiff and hard. It keeps the rubber from melting as it gets warmer. The discovery of vulcanization by Charles Goodyear (1800-60) in 1839 is one of the greatest industrial accomplishments of modern times.
Some powdered sulfur is also used as an insecticide. It can be spread on plants to kill or drive away insects that feed on the plants. By far the majority of sulfur is used, however, to make sulfur compounds. The most important of these is sulfuric acid (H ₂ SO ₄ ).

Compounds

Nearly 90 percent of all sulfur produced goes into sulfuric acid. Sulfuric acid is the number one chemical in the world in terms of the amount produced. Each year, almost twice as much sulfuric acid is made as the next highest chemical, nitrogen. In 1996, more than 45 million tons of sulfuric acid were produced in the United States alone.
The greatest portion, nearly 75 percent, of sulfuric acid is used to make fertilizers. The next most important use, 10 percent, is in the petroleum industry. Other important uses of sulfuric acid are in the treatment of copper ores; the production of paper and paper products; the manufacture of other agricultural chemicals; and the production of plastics, synthetic rubber, and other synthetic materials.

Vulcanization is the process of adding sulfur to rubber to make it stiff and hard.

Sulfuric acid is also used in smaller amounts to make explosives, water treatment chemicals, storage batteries, pesticides,

Reaction of sulfuric acid and sugar.

drugs, synthetic fibers, and many other chemicals used in everyday life.

Health effects

The cleansing power of sulfur has been known for many centuries. At one time, ancient physicians burned sulfur in a house to cleanse it of impurities. Creams made with sulfur were used to treat infections and diseases. In fact, sulfur is still used to treat certain medical problems. Sulfur is prepared in one of three forms. Precipitated sulfur (milk of sulfur) is made by boiling sulfur with lime. Sublimed sulfur (flowers of sulfur) is pure sulfur powder. And washed sulfur is sulfur treated with ammonia water. Washed sulfur is used to kill parasites (organisms that live on other organisms) such as fleas and ticks. It is also used as a laxative, a substance that helps loosen the bowels.
Sulfur is a macronutrient for both plants and animals. A macronutrient is an element needed in relatively large amounts to insure the good health of an organism. Sulfur is used to make proteins and nucleic acids, such as DNA. It also occurs in many essential enzymes. Enzymes are chemicals that make chemical reactions occur more quickly in cells. Humans usually have no problem getting enough sulfur in their diets. Eggs and meats are especially rich in sulfur.
A person who does not get enough sulfur in his or her diet develops certain health problems. These include itchy and flaking skin and improper development of hair and nails. Under very unusual conditions, a lack of sulfur can lead to death. Such conditions would be very rare, however.

The cleansing power of sulfur has been known for many centuries.

Plants require sulfur for normal growth and development. When plants do not get enough sulfur from the soil, their young leaves start to turn yellow. Eventually, this yellowing extends to the whole plant. The plant may develop other diseases as a result.

Read more: http://www.chemistryexplained.com/elements/P-T/Sulfur.html#ixzz1YRZDWLry

An Introduction Silver

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Overview

Chemists classify silver as a transition metal. The transition metals are elements between Groups 2 and 13 in the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. More than 40 elements, all metals, fall within the transition metal range.
Silver is also classified as a precious metal. Precious metals are not very abundant in the Earth's crust. They are attractive and not very chemically active. These properties make the metal desirable in jewelry, coins, and art. About a half dozen metals near silver in the periodic table are also precious metals. These include gold, platinum, palladium, rhodium, and indium.
Silver has been used by humans for thousands of years. It often occurs as a free element in nature. It can also be extracted from its ores fairly easy. These properties made it easy for early humans to learn about silver.

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SYMBOL 
Ag
ATOMIC NUMBER
47
ATOMIC MASS
107.868
FAMILY 
Group 11 (IB)
Transition metal
PRONUNCIATION 
SIL-ver

Today, the most important use of silver is in photography. Three silver compounds used in photography are silver chloride (AgCl), silver bromide (AgBr), and silver iodide (AgI). Silver is also used to make electrical equipment, mirrors, medical and dental equipment, and jewelry. It is often used to make alloys with gold for some of these applications. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals.

Discovery and naming

Silver was probably first discovered after gold and copper. Gold and copper often occur as free elements in nature. They have very distinctive colors, which made it easy for early humans to find these metals.
Silver also occurs as a free metal, but much less often than gold or copper. At some point, humans learned to extract silver from its ores. But that discovery must have occurred very early on in human history. Archaeologists (scientists who study ancient civilizations) have found silver objects dating to about 3400 B.C. in Egypt. Drawings on some of the oldest pyramids show men working with metal, probably extracting silver from its ores.
Other early cultures also used silver. Written records from India describe the metal as far back as about 900 B.C. Silver was in common use in the Americas when Europeans first arrived.
The Bible contains many references to silver. The metal was used as a way of paying for objects. It also decorated temples, palaces, and other important buildings. The Bible also contains sections that describe the manufacture of silver.
The word silver goes back to at least the 12th century, A.D. It seems to have come from an old English word used to describe the metal, seolfor. The symbol for silver (Ag), however, comes from its Latin name, argentum. The name may have originated from the Greek word argos,meaning "shiny" or "white."

Physical properties

Silver is a soft, white metal with a shiny surface. It is the most ductile and most malleable metal. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. Silver has two other unique properties. It conducts heat and electricity better than any other element. It also reflects light very well.

Hot, glowing silver.

Silver's melting point is 961.5°C (1,762°F) and its boiling point is about 2,000 to 2,200°C (3,600 to 4,000°F). Its density is 10.49 grams per cubic centimeter.

Drawings on some of the oldest pyramids show men working with metal, probably extracting silver from its ores.

Chemical properties

Silver is a very inactive metal. It does not react with oxygen in the air under normal circumstances. It does react slowly with sulfur compounds in the air, however. The product of this reaction is silver sulfide (Ag ₂ S), a black compound. The tarnish that develops over time on silverware and other silver-plated objects is silver sulfide.
Silver does not react readily with water, acids, or many other compounds. It does not burn except as silver powder.

Occurrence in nature

Silver is a fairly rare element in the Earth's crust. Its abundance is estimated to be about 0.1 parts per million. It is also found in seawater. Its abundance there is thought to be about 0.01 parts per million.
Silver usually occurs in association with other metal ores, especially those of lead . The most common silver ores are argentite (Ag ₂ S); cerargyrite, or "horn silver" (AgCl); proustite (3Ag ₂ S ○ As ₂ S ₃ ); and pyrargyrite (Ag ₂ S ○ Sb ₂ S ₃ ).
The largest producers of silver in the world are Mexico, Peru, the United States, Canada, Poland, Chile, and Australia. In the United States, silver is produced at about 76 mines in 16 states. The largest state producers are Nevada, Idaho, and Arizona. These three states account for about two-thirds of all the silver mined in the United States.

Isotopes

Two naturally occurring isotopes of silver exist: silver-107 and silver-109. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
About 16 radioactive isotopes of silver are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
None of the radioactive isotopes of silver has any commercial use.

The tarnish that develops over time on silverware and other silver-plated objects is silver sulfide.

Extraction

Ores rich in silver disappeared long ago due to mining. Today, silver usually comes from ores that contain very small amounts of the metal. These amounts can range from about a few thousandths

A small percent of silver produced in the United States is used for coins. The old "Peace" silver dollar, shown here, was minted from 1921 to 1935.

of an ounce per ton of ore to 100 ounces per ton. The metal is most commonly produced as a by-product of mining for other metals. After the primary metal has been removed, the waste often contains small amounts of silver. These wastes are treated with chemicals that react with the silver. The silver can then be extracted by electrolysis. Electrolysis is a process by which a compound is broken down by passing an electric current through it.

Uses and compounds

About 10 percent of silver produced in the United States is used in coins, jewelry, and artwork. One way silver is used is in alloys with gold. Gold is highly desired for coins and jewelry. But it is much too soft to use in its pure form. Adding silver to gold, however, makes an alloy that is much stronger and longer lasting. Most "gold" objects today are actually alloys, often alloys of silver and gold.
Other objects use much more of the silver metal, however. About half of the silver produced in the United States goes into photographic film. Pure silver is first converted to a compound: silver chloride, silver bromide, or silver iodide. The compound is then used to make photographic film (see accompanying sidebar).
The second most important use of silver is in electrical and electronic equipment. About 20 percent of all silver produced is used for this purpose. Silver is actually the most desirable of all metals for electrical equipment. Electricity flows through silver more easily than it does through any other metal. In most cases, however, metals such as copper or aluminum are used because they are less expensive.

Silver's important role in film

T aking a photograph depends on a simple chemical idea: Light can cause electrons to move around. Here is what that means:
Silver metal will combine with chlorine, bromine, or iodine to form compounds. As an example:
In this reaction, each silver atom loses one electron to a chlorine atom. The silver atom becomes "one electron short" of what it usually has. The one-electron-short silver atom is called a silver ion.
Photographic film is coated with a thin layer of silver chloride, silver bromide, or silver iodide. That means the film is covered with many silver ions. Silver ions are colorless, so photographic film has no color to it.
What happens when photographic film is exposed to light? Light gives energy to electrons in the photographic film. Some of these electrons find their way back to silver ions, transforming them back to atoms:
But silver atoms are not colorless. They are black. So, a photographic film exposed to light turns black at every point where light strikes a silver ion.
In taking a picture, of course, not all of the film gets equal amounts of light. A picture of a person, for example, will have areas that get much more light than others. So some places on the film become very dark, and other places become less dark.
Additional steps are necessary to "develop" photographic film or to produce a picture from it. But the first step in taking a photograph is changing silver ions back to silver atoms with light.

But sometimes, an electrical device is so important that cost is not a consideration. For example, electrical devices on spacecraft, satellites, and aircraft must work reliably and efficiently. The cost of using silver is not as important as it would be in a home appliance. Thus, silver is used for electrical wiring and connections in these devices.
In some cases, silver plating solves a practical problem where the more expensive silver would work best. Silver plating is the process by which a very thin layer of silver metal is laid down on top of another metal. Silver is so malleable that it can be hammered into sheets thinner than a sheet of paper. Silver this thin can be applied to another metal. Then the other metal takes on some of the properties of the silver coating. For example, it may work very well as a reflector because silver is such a good reflector. It does not matter if the second metal is a good reflector or not. The silver coating serves as the reflecting surface in the combination.
About a fifth of all silver produced is used in a variety of other products. For example, it is often used in dental amalgams. An amalgam is an alloy in which mercury is one of the metals used. Silver amalgams work well for filling decayed teeth. They are non-toxic and do not break down or react with other materials very readily. Silver is also used in specialized batteries, including silver- zinc and silver- cadmium batteries.

Electricity flows through silver more easily than it does through any other metal.

Health effects

Silver is a mildly toxic element. When the metal or its compounds get on the skin, they can cause a bluish appearance known as argyria or argyrosis. Breathing in silver dust can have serious long-term health effects also. The highest recommended exposure for silver dust is 0.1 milligrams per cubic meter of air.

Read more: http://www.chemistryexplained.com/elements/P-T/Silver.html#ixzz1YRQPFoJa

An Introduction Manganese

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Overview

Manganese is a transition metal. The transition metals are the large block of elements in the middle of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. The transition metals make up Rows 4 through 7 in Groups 3 through 12 of the periodic table. Many of the best known and most widely used metals are in this group of elements.
It took chemists some time to discover the difference between manganese and iron. The two metals have very similar properties and often occur together in the Earth's crust. The first person to clearly identify the differences between the two elements was Swedish mineralogist Johann Gottlieb Gahn (1745-1818) in 1774.

SYMBOL 
Mn
ATOMIC NUMBER 
25
ATOMIC MASS 
54.9380
FAMILY 
Group 7 (VIIB)
Transition metal
PRONUNCIATION 
MANG-guh-neez

Manganese plays an interesting role in the U.S. economy. It is absolutely essential in the production of iron and steel. No element has been found that can replace manganese is such applications. The United States has essentially no manganese supplies of its own, so it depends on imports from other nations.

Discovery and naming

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One of the main ores of manganese is pyrolusite. Pyrolusite is made up primarily of the compound manganese dioxide (MnO ₂ ). Early artists were familiar with pyrolusite. They used the mineral to give glass a beautiful purple color. They also used the mineral to remove color from a glass. When glass is made, it often contains impurities that give the glass an unwanted color. The presence of iron, for example, can give glass a yellowish tint. Adding pyrolusite to yellowish glass removes the color. The purple tint of pyrolusite balances out the yellow color of the glass. The glass ends up being clear and colorless.
By the mid-1700s, chemists began to suspect that pyrolusite might contain a new element. Some authorities credit German chemist Ignatius Gottfried Kaim with isolating the element in 1770. However, Kaim's report was not read by many chemists and was quickly lost.
During this period, some of the most famous chemists in Europe were trying to analyze pyrolusite, but none of them was successful. The problem was solved in 1774 when Gahn developed a method for removing the new element from pyrolusite. He heated pyrolusite with charcoal (pure carbon ). The carbon tookoxygen away from manganese dioxide, leaving behind pure manganese:
The origin of manganese's name is a bit confusing. Early chemists associated the new element with a mineral called magnesia. That mineral got its name because it is magnetic. Magnesia does not contain manganese, but the name stuck.

Physical properties

Manganese is a steel-gray, hard, shiny, brittle metal. It is so brittle, in fact, that it cannot be machined in its pure form. Machining refers to the bending, cutting, and shaping of a metal by mechanical means. The melting point of manganese is 1,245°C (2,273°F) and its boiling point is about 2,100°C (3,800°F). Its density is 7.47 grams per cubic centimeter.
Manganese exists in four allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The element changes from one form to another as the temperature rises. The form that exists from room temperature up to about 700°C (1,300°F) is the most common form.

Chemical properties

Manganese is a moderately active metal. It combines slowly with oxygen in the air to form manganese dioxide (MnO ₂ ). At higher temperatures, it reacts more rapidly. It may even burn, giving off a bright white light. Manganese reacts slowly with cold water, but more rapidly with hot water or steam. It dissolves in most acids with the release of hydrogen gas. It also combines with fluorine and chloride to make manganese difluoride (MnF ₂ ) and manganese dichloride (MnCl ₂ ).

Occurrence in nature

Manganese never occurs as a pure element in nature. It always combines with oxygen or other elements. The most common ores of manganese are pyrolusite, manganite, psilomelane, and rhodochrosite. Manganese is also found mixed with iron ores. The largest producers of manganese ore in the world are China, South Africa, the Ukraine, Brazil, Australia, Gabon, and Kazakstan.
Manganese also occurs abundantly on the ocean floor in the form of nodules. These nodules are fairly large lumps of metallic ores. They usually contain cobalt, nickel, copper, and iron, as well as manganese. Scientists estimate that up to 1.5 trillion metric tons of manganese nodules may lie on the floors of the world's oceans and large lakes. Currently, there is no profitable method for removing these ores.
Manganese is the 12th most abundant element in the Earth's crust. Its abundance is estimated to be 0.085 to 0.10 percent. That makes it about as abundant as fluorine or phosphorus.

Up to 1.5 trillion metric tons of manganese nodules (large lumps of metallic ores) may lie on ocean floors.

Isotopes

Only one naturally occurring isotope of manganese exists, manganese-22. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Men and machine lay railroad tracks. A common alloy of manganese, ferromanganese, is contained in the steel used to produce railroad tracks.

Nine radioactive isotopes of manganese are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
None of the radioactive isotopes of manganese has any important commercial uses.

Extraction

The usual method for producing pure manganese is to heat manganese dioxide (MnO ₂ ) with carbon or aluminum. These elements remove the oxygen and leave pure metal: 

Uses

Up to 90 percent of all manganese produced is made into steel alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. The addition of manganese to steel makes the final product hard, as well as resistant to corrosion (rusting) and mechanical shock.

The heavy steel found in bank vaults contains ferromanganese, a manganese alloy.

The most common alloy of manganese is ferromanganese. This alloy contains about 48 percent manganese combined with iron and carbon. Ferromanganese is the starting material for making a very large variety of steel products, including tools, heavy-duty machinery, railroad tracks, bank vaults, construction components, and automotive parts. About 60 percent of the manganese used in the United States in 1996 went to the manufacture of ferromanganese.
Another common alloy of manganese is silicomanganese. It contains manganese, silicon, and carbon in addition to iron. It is used for structural components and in springs. The production of silicomanganese accounted for about 33 percent of the manganese used in the United States in 1996.
Manganese is also used to make alloys with metals other than iron or steel. For example, the alloy known as manganin is 84 percent copper, 12 percent manganese, and 4 percent nickel. Manganin is used in electrical instruments.

Compounds

Less than 10 percent of all the manganese used in the United States goes to the production of manganese compounds. Perhaps the most important commercial use of these compounds is manganese dioxide (MnO ₂ ). Manganese dioxide is used to make dry-cell batteries. These batteries are used in electronic equipment, flashlights, and pagers. Dry cell batteries hold a black pasty substance containing manganese dioxide. The use of manganese dioxide in a dry cell prevents hydrogen gas from collecting in the battery as electricity is produced.
Another manganese compound, manganous chloride (MnCl ₂ ), is an additive in animal food for cows, horses, goats, and other domestic animals. Fertilizers also contain manganous chloride so that plants get all the manganese they need.
Finally, small amounts of manganese compounds are used as coloring agents in bricks, textiles, paints, inks, glass, and ceramics. Manganese compounds can be found in shades of pink, rose, red, yellow, green, purple, and brown.

Health effects

Manganese is one of the chemical elements that has both positive and negative effects on living organisms. A very small amount of the element is needed to maintain good health in plants and animals. The manganese is used by enzymes in an organism. An enzyme is a molecule that makes chemical reactions occur more quickly in cells. Enzymes are necessary to keep any cell operating properly. If manganese is missing from the diet, enzymes do not operate efficiently. Cells begin to die, and the organism becomes ill.
Fortunately, the amount of manganese needed by organisms is very small. It is not necessary to take extra manganese to meet the needs of cells.

Manganous chloride (MnCl ₂ ) is an additive in animal food.

In fact, an excess of manganese can create health problems. These problems include weakness, sleepiness, tiredness, emotional disturbances, and even paralysis. The only way to receive such a large dose is in a factory or mine. Workers may inhale manganese dust in the air.

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