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Formation and uses of negative ions

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Iodine can be found in seaweed

Credit: Courtesy of Shane Anderson/NOAA
Source: http://commons.wikimedia.org/wiki/File:Kelp_300.jpg
License: CC BY-NC 3.0

What does the amount of salt in seaweed tell us?

Before iodized salt was developed, some people experienced a number of developmental difficulties, including problems with thyroid gland function and mental retardation.  In the 1920s, we learned that these conditions could usually be treated easily with the addition of iodide anion to the diet.  One easy way to increase iodide intake was to add the anion to table salt.  This simple step greatly enhanced health and development. Large amounts of iodide ion are also found in seaweed such as kelp (see picture above) and saltwater fish.

When a metal loses an electron, energy is needed to remove that electron.  The other part of this process involves the addition of the electron to another element.  The electron adds to the outer shell of the new element.  Just as the loss of the electron from the metal produces a full shell, when the electron or electrons are added to the new element, it also results in a full shell.


Anions are negative ions that are formed when a nonmetal atom gains one or more electrons. Anions are so named because they are attracted to the anode (positive field) in an electric field.  Atoms typically gain electrons so that they will have the electron configuration of a noble gas. All the elements in Group 17 have seven valence electrons due to the outer ns2np5 configuration. Therefore, each of these elements would gain one electron and become an anion with a 1− charge. Likewise, Group 16 elements form ions with a 2− charge, and the Group 15 nonmetals form ions with a 3− charge.

Pauling electronegativity values of the periodic table

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

Naming anions is slightly different than naming cations. The ending of the element’s name is dropped and replaced with the –ide suffix. For example, F- is the fluoride ion, while O2- is the oxide ion.  As is the case with cations, the charge on the anion is indicated by a superscript following the symbol. Common anions are listed in Table below:

Anion Name

Symbol and Charge















Uses for Anions

Fluoride ion is widely used in water supplies to help prevent tooth decay. Chloride is an important component in ion balance in blood. Iodide ion is needed by the thyroid gland to make the hormone thyroxine.


  • Anions are formed by the addition of one or more electrons to the outer shell of an atom.
  • Group 17 elements add one electron to the outer shell, group 16 elements add two electrons, and group 15 elements add three electrons.
  • Anions are named by dropping the ending of the element’s name and adding -ide.


  1. What is an anion?
  2. How are anions formed?
  3. Why do anions form?
  4. How are anions named?
  5. List three examples of anions with names, charges, and chemical symbols.
  6. List three ways anions are used.

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    Image Attributions

    1. [1]^ Credit: Courtesy of Shane Anderson/NOAA; Source: http://commons.wikimedia.org/wiki/File:Kelp_300.jpg; License: CC BY-NC 3.0
    2. [2]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0

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