Silver (revised)

SILVER (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

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.

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.

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 (Ag₂S), 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 (Ag₂S); cerargyrite, or "horn silver" (AgCl); proustite (3Ag₂S ○ As₂S₃); and pyrargyrite (Ag₂S ○ Sb₂S₃).

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 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.