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Chemistry of Arsenic (Z=33)

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  • Figure 1: Sample of solid Arsenic. Image used with permission from Wikipedia.

    Isotopes of Arsenic

    The atomic mass of Arsenic listed on the periodic table is 74.92 u. Arsenic has over 33 different isotopes, ranging from 60 to 92 in atomic mass. However, only one of these isotopes is stable, which is 75As. Therefore, Arsenic is a monoisotpoic element. Like the rest of the nitrogen group, Arsenic has five valence electrons. In group five, Arsenic is most closely related to antimony, both being metalloids. Since nitrogen and phosphorus are non-metal, they share very little commonalities with arsenic and antimony. As with bismuth being a P-block metal, they too share very little similarities.

    Old Usage

    Some arsenic compounds are used in the manufacture of everything from wallpaper to ceramics. Arsenic in the 1800s was commonly used in the form of copper arsenite, a green compound that was used as a pigment to color wallpaper and paint. This caused a lot of health problems. As mold and bacteria fed on the wallpaper and paint, they released trimethylarsine, C3H9As, which became airborne and inhaled. This would lead to arsenic poisoning and in some cases, death. It was substituted by copper carbonate by the end of the 1800s as the primary green pigment.

    Of course, arsenic is most often thought of in terms of its toxicity and it is used as a weed killer and rat poison. Intentional arsenic poisoning is now pretty much the stuff of old murder mysteries. Originally arsenious oxide was administered in small doses over a period of time, precipitating death accompanied by symptoms reminiscent of pneumonia. Today the arsenic remaining in the body can be detected during autopsy so this is not (happily) a very effective way to dispatch people undetected.

    Since Arsenic is toxic to most organisms, it is praised as an herbicide, and pesticide. For example, copper arsenate, \(Cu_3(AsO_4)_2\), mixed with chromium is a pesticide used to treat wood. As of 2004, CCA, chromium copper arsenate, has been put into restricted use by the EPA. In the 1900s copper acetoarsenite, or Paris Green, was used as an insecticide and fungicide in many agricultural farmland. Agent blue is one of the herbicides used during the Vietnam War by the US to destroy Vietnam rice farm. It is a blend of cacodylic acid, \((CH_3)_2AsO_2H\), and sodium cacodylic, an arsenic salt.

    Modern Usage

    Tiny amounts of arsenic are used today in the semiconductor industry to create light emitting diodes (LEDs). Since arsenic is a semi-metal, arsenic in the form of gallium arsenide is now used as a semiconductor for transistors and other electronic devices. It is used to make LED, light emitting diodes, and in some cases, it can substitute silicon in integrated circuits.


    1. What is Gallium Arsenide now used for in the electronic industry?
    2. Complete and balance: 2As + 5F2 --> ?
    3. True or false: Arsenic is more like antimony than phosphorus.
    4. True or False: Arsenic trioxide is a gas.
    5. Complete and balance: As(s) + H2O(l) --> ?


    Highlight to see: (Easy-Medium-Hard)

    1. Gallium arsenide is now used for transistors. (H)
    2. 2As+5F2->2AsF5 (E)
    3. True, both arsenic and antimony are semi-metals. (M)
    4. False, arsenic trioxide is a solid. It can, however, be disperse in the air as dust. (M)
    5. No reaction. Solid arsenic does not react well with water. (M)


    1. Jones, Harry. Principles of inorganic chemistry. New York: The MacMillan Company, 1903. Print.
    2. Mellor, Joseph. Modern inorganic chemistry. New York: Longmans, Green, and Company, 1912. Print.
    3. Young, Jay A. “Arsenic (III) Chloride” J. Chem. Educ., 2006, 83 (2), p 207


    • Freddie Chak, Jonathan Molina, Tiffany Lui (University of California, Davis)

    Stephen R. Marsden (ChemTopics)