January 2005 

Favorite photographs are the keepsakes of a lifetime. Pictures of family and friends, summer vacations and graduations--all sorts of meaningful images--are preserved for us in photos. Photography is just one of the amazing gifts of science and technology that add enjoyment to our lives. Chlorine chemistry plays a role in traditional film photography in the form of the light-sensitive compound, silver chloride, AgCl.

What's in a name?
The chemical symbol for silver is Ag, short for the Latin word for this element--argentum. The compound AgCl is found naturally in small amounts in silver deposits and is known as the mineral chlorargyrite (chlor-ARE-jur-ite). This name reflects its chemical composition ("chlor" for chlorine and "argyr" for argentum). The "ite" ending indicates a mineral name.

Silver chloride, silver bromide (AgBr) and silver iodide (AgI) are the three silver "halide" compounds used in photography. The halide elements include fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). Groups of elements with similar chemical properties, such as the halides, are arranged in vertical columns on the Periodic Table of the Elements. Can you locate the halides using the link provided?

Photochemistry

The light sensitivity of the silver halides is key to the photographic process. Tiny crystals of all three of these compounds are used in making photographic film. When exposed to light, a chemical reaction darkens the film to produce an image.

AgCl, for example, consists of crystals of tightly packed ions of silver and chlorine, denoted Ag+ and Cl-. The "+" and "-" symbols tell us that Ag ion (Ag+) is missing one negatively charged electron and that Cl ion (Cl-) has an extra electron. When film containing Ag+ and Cl- is exposed to light energy, the chlorine ion's extra electron is ejected and then captured by a silver ion.

Electron ejected from chlorine (Oxidation):

Electron captured by silver (Reduction):

When silver metal forms as a result of the electron capture, it forms a dark image on film. Chemically, we say that Ag+ has been "reduced" to Ag (metal). At the same time, Cl- is said to be "oxidized."

Portrait of Carl Wilhelm
Scheele by J.
Falander (Edgar Fahs
Smith Collection,
University of
Pennsylvania Library)

The Science Center Salutes One of the Giants of Chemistry: Carl Wilhelm Scheele (1742-1786)

The light sensitivity of natural silver salts, such as AgCl, has been known by scientists for centuries. The great Swedish chemist Carl Wilhelm Scheele, who discovered seven of the natural elements--chlorine, nitrogen, oxygen, manganese, molybdenum, barium and tungsten--and many organic compounds, was one of the early scientists who experimented with silver chloride. In 1777 Scheele correctly identified the chemical reaction that produces metallic silver when AgCl is exposed to light.

One of eleven children, Scheele became an apprentice to a local pharmacist (called an apothecary) at age 14. With access to chemical compounds, laboratory equipment and chemistry books, Scheele developed a deep interest in the science of matter. He studied and conducted experiments whenever he could. Later in life, Scheele was recognized and honored for his many scientific discoveries. When offered university positions, however, he refused, preferring to work in a small laboratory in his home.

Follow-up Questions

  1. Halide atoms naturally have seven electrons in their outermost electron shells. This is an unstable arrangement--an outer shell of eight electrons is much more stable, so halides try to "capture" electrons wherever they can. Does that make them good chemical oxidizers or reducers? Explain.

  2. Make a list of naturally occurring minerals containing chlorine. List their names and their chemical formulas.

  3. Research the contributions of the following people to the development of photography: Daguerre, Talbot, Herschel and Eastman.

For a list of previous "Chlorine Compound of the Month" features, click here.

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