4: Polyatomic Ions
Some ions consist of more than one atom. The atoms within a polyatomic ion are held together by covalent bonds and the entity of these covalently bonded atoms has an overall electric charge. You will learn more about covalent bonds in the next unit ("4 Covalent Bonds").
Several polyatomic ions play an important role in environmental sciences (table 1).
Table 1: Examples of polyatomic ions of environmental significance
Examples of polyatomic ions in ionic compounds
Example 1 - Calcium carbonate (CaCO3)
The carbonate ion (CO32-) consists of one carbon atom and three oxygen atoms. Together, they comprise a single ion with an overall charge of -2.
Carbonate in an ionic bond with calcium ions (Ca2+) is called calcium carbonate (CaCO3). CaCO3 is the chief constituent of limestone, which is a common rock of the earth’s crust (fig. 1). Limestone is used for example as building material or as raw material for the production of lime.
Figure 1: Limestone rock [CC0 Public Domain]
Example 2 - Calcium hydroxide Ca(OH)2
Calcium hydroxide Ca(OH)2 has several applications in environmental technologies, for example in drinking water and waste water treatment or in flue gas cleaning of waste incineration.
Calcium hydroxide is comprised of a calcium cation (Ca2+) and two hydroxide anions (OH-), resulting in the chemical formula Ca(OH)2. The hydroxide ion consists of one oxygen atom and one hydrogen atom and carries a net charge of -1.
The application of Ca(OH)2 in environmental technologies is often based on the ability of the OH- ion to reduce acidity, which you will learn more about in the units “8 Acids and Bases” and “9 pH Scale”.
Example 3 - Ammonium nitrate (NH4NO3)
Nitrogen (N) is an essential plant nutrient (fig. 2). A common nitrogen fertilizer is ammonium nitrate (NH4NO3). It is an ionic bond made of two polyatomic ions, an ammonium cation (NH4+) and a nitrate anion (NO3-). Note, that even though the NH4+ ion contains only non-metal atoms its net charge is positive, which makes it a cation.
Figure 2: A young cabbage plant exhibiting nitrogen deficiency. [Creative Commons Attribution-Share Alike 3.0 Unported license, https://commons.wikimedia.org/wiki/File:Spitskool_stikstofgebrek_(nitrogen_deficiency)_Brassica_oleracea_convar._capitata_var._alba.jpg], licensed under the
- CK-12 Foundation (S. Bewick et al.), 2016. Map: Introductory Chemistry (CK-12), ch. 7.9: Polyatomic Ions. Libre Texts/UC Davis. http://chem.libretexts.org/Textbook_Maps/Introductory_Chemistry_Textbook_Maps/Map%3A_Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.09%3A_Polyatomic_Ions
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- J.C. Crittenden et al., 2012. MWH's Water Treatment: Principles and Design, p. 234 and p. 567. John Wiley & Sons, Inc., Hoboken (NJ).
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