6.4 Naming Compounds Containing Polyatomic Ions

Learning Objectives

By the end of this section, you will be able to:

  • Generate a proper name for an ionic compound containing polyatomic ions

Indigenous Perspective: Soapstone

Three stones against a black backdrop are pictured.
Figure 6.4a: Soapstone (Speckstein) (credit: work by Ra’ike, CC BY-SA 3.0)

Explore two ionic compounds used by Inuit. These include stone such as soapstone (also known as talc carbonate – Figure 6.4a) and composite materials such as bone which contains 70% hydroxyapatite, Ca5(PO4)3 (OH).The information can be found within the article Making Chemistry Relevant to Indigenous People [New Tab] from the University of Waterloo.


Source: Rayner-Canham, G., Taylor, R., & Lee, Y.R. (2016, February). Making chemistry relevant to Indigenous peoples. Chem 13 News Magazine. https://uwaterloo.ca/chem13-news-magazine/february-2016/feature/making-chemistry-relevant-indigenous-peoples

Naming Compounds with Polyatomic ions

There also exists a group of ions that contain more than one atom. These are called polyatomic ions. Table 6.4a lists the formulas, charges, and names of some common polyatomic ions. Only one of them, the ammonium ion, is a cation; the rest are anions. Most of them also contain oxygen atoms, so sometimes they are referred to as oxyanions. Some of them, such as nitrate and nitrite, and sulfate and sulfite, have very similar formulas and names, so care must be taken to get the formulas and names correct. Note that the -ite polyatomic ion has one less oxygen atom in its formula than the -ate ion but with the same ionic charge. For an accessible version of Table 6.4a, refer to Appendix E.

Table 6.4a: Common Polyatomic Ions
Symbol Name Symbol Name Symbol Name
CrO42- Chromate BO33- Borate SO42- Sulfate
CrO72- Dichromate AsO43- Arsenate SO32- Sulfite
CN Cyanide BrO Hypobromite HSO4 Hydrogen sulfate (bisulfate)
SCN Thiocyanide BrO3 Bromate HSO3 Hydrogen sulfite (bisulfite)
NO3 Nitrate CIO Hypochlorite PO43- Phosphate
NO2 Nitrite CIO2 Chlorite PO33- Phosphite
MnO4 Permanganate CIO3 Chlorate HPO42- Hydrogen phosphate
OH Hydroxide CIO4 Perchlorate H2PO42- Dihydrogen phosphate
O22- Peroxide IO4 Periodate CO32- Carbonate
NH2 Amide IO3 Iodate HCO3 Hydrogen carbonate
C2H3O2 Acetate IO Hypoiodite HC2O4 Hydrogen oxalate
C2O42- Oxalate NH4+ Ammonium

The naming of ionic compounds that contain polyatomic ions follows the same rules as the naming for other ionic compounds: simply combine the name of the cation and the name of the anion. Do not use numerical prefixes in the name if there is more than one polyatomic ion; the only exception to this is if the name of the ion itself contains a numerical prefix, such as dichromate or triiodide.

Writing the formulas of ionic compounds has one important difference. If more than one polyatomic ion is needed to balance the overall charge in the formula, enclose the formula of the polyatomic ion in parentheses and write the proper numerical subscript to the right and outside the parentheses. Thus, the formula between calcium ions, Ca2+, and nitrate ions, NO3, is properly written Ca(NO3)2, not CaNO32 or CaN2O6. Use parentheses where required. The name of this ionic compound is simply calcium nitrate.

Example 6.4a


Write the proper formula and give the proper name for each ionic compound formed between the two listed ions.

  1. NH4+ and S2−
  2. Al3+ and PO43−
  3. Fe2+ and PO43−


  1. Because the ammonium ion has a 1+ charge and the sulfide ion has a 2− charge, we need two ammonium ions to balance the charge on a single sulfide ion. Enclosing the formula for the ammonium ion in parentheses, we have (NH4)2S. The compound’s name is ammonium sulfide.
  2. Because the ions have the same magnitude of charge, we need only one of each to balance the charges. The formula is AlPO4, and the name of the compound is aluminum phosphate.
  3. Neither charge is an exact multiple of the other, so we have to go to the least common multiple of 6. To get 6+, we need three iron(II) ions, and to get 6−, we need two phosphate ions. The proper formula is Fe3(PO4)2, and the compound’s name is iron(II) phosphate.

Exercise 6.4a

Write the proper formula and give the proper name for each ionic compound formed between the two listed ions.

  1. NH4+ and PO43−
  2. Co3+ and NO2

Check Your Answer[1]

Food and Drink App: Sodium in Your Food

The element sodium, at least in its ionic form as Na+, is a necessary nutrient for humans to live. In fact, the human body is approximately 0.15% sodium, with the average person having one-twentieth to one-tenth of a kilogram in their body at any given time, mostly in fluids outside cells and in other bodily fluids.

Sodium is also present in our diet. The common table salt we use on our foods is an ionic sodium compound. Many processed foods also contain significant amounts of sodium added to them as a variety of ionic compounds. Why are sodium compounds used so much? Usually sodium compounds are inexpensive, but, more importantly, most ionic sodium compounds dissolve easily. This allows processed food manufacturers to add sodium-containing substances to food mixtures and know that the compound will dissolve and distribute evenly throughout the food. Simple ionic compounds such as sodium nitrite (NaNO2) are added to cured meats, such as bacon and deli-style meats, while a compound called sodium benzoate is added to many packaged foods as a preservative. Table 6.4b is a partial list of some sodium additives used in food. Some of them you may recognize after reading this chapter. Others you may not recognize, but they are all ionic sodium compounds with some negatively charged ion also present.

Table 6.4b: Some Sodium Compounds Added to Food
Sodium Compound Use in Food
Sodium acetate preservative, acidity regulator
Sodium adipate food acid
Sodium alginate thickener, vegetable gum, stabilizer, gelling agent, emulsifier
Sodium aluminum phosphate acidity regulator, emulsifier
Sodium aluminosilicate anticaking agent
Sodium ascorbate antioxidant
Sodium benzoate preservative
Sodium bicarbonate mineral salt
Sodium bisulfite preservative, antioxidant
Sodium carbonate mineral salt
Sodium carboxymethylcellulose emulsifier
Sodium citrates food acid
Sodium dehydroacetate preservative
Sodium erythorbate antioxidant
Sodium erythorbin antioxidant
Sodium ethyl para-hydroxybenzoate preservative
Sodium ferrocyanide anticaking agent
Sodium formate preservative
Sodium fumarate food acid
Sodium gluconate stabilizer
Sodium hydrogen acetate preservative, acidity regulator
Sodium hydroxide mineral salt
Sodium lactate food acid
Sodium malate food acid
Sodium metabisulfite preservative, antioxidant, bleaching agent
Sodium methyl para-hydroxybenzoate preservative
Sodium nitrate preservative, color fixative
Sodium nitrite preservative, color fixative
Sodium orthophenyl phenol preservative
Sodium propionate preservative
Sodium propyl para-hydroxybenzoate preservative
Sodium sorbate preservative
Sodium stearoyl lactylate emulsifier
Sodium succinates acidity regulator, flavour enhancer
Sodium salts of fatty acids emulsifier, stabilizer, anticaking agent
Sodium sulfite mineral salt, preservative, antioxidant
Sodium sulfite preservative, antioxidant
Sodium tartrate food acid
Sodium tetraborate preservative

The use of so many sodium compounds in prepared and processed foods has alarmed some physicians and nutritionists. They argue that the average person consumes too much sodium from their diet. The average person needs only about 500 mg of sodium every day; most people consume more than this—up to 10 times as much. Some studies have implicated increased sodium intake with high blood pressure; newer studies suggest that the link is questionable. However, there has been a push to reduce the amount of sodium most people ingest every day: avoid processed and manufactured foods, read labels on packaged foods (which include an indication of the sodium content – see Figure 6.4b), don’t oversalt foods, and use other herbs and spices besides salt in cooking.

Nutrition facts. There are 75 mg of sodium in each portion of this product.
Figure 6.4b: “Nutrition Facts.” Food labels include the amount of sodium per serving. This particular label shows that there are 75 mg of sodium in one serving of this particular food item (credit: Introductory Chemistry: 1st Canadian Edition by David W. Ball and Jessica A. Key, CC BY-NC-SA 4.0).

Links to Interactive Learning Tools

Attribution & References

Except where otherwise noted, this page is adapted by Adrienne Richards from “Chapter 3: Ions and Ionic Compounds & Molecules and Chemical Nomenclature” In Introductory Chemistry: 1st Canadian Edition by David W. Ball and Jessica A. Key, licensed under CC BY-NC-SA 4.0.

    1. (NH4)3PO4, ammonium phosphate
    2. Co(NO2)3, cobalt(III) nitrite


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6.4 Naming Compounds Containing Polyatomic Ions Copyright © 2023 by Gregory Anderson; Caryn Fahey; Jackie MacDonald; Adrienne Richards; Samantha Sullivan Sauer; J.R. van Haarlem; and David Wegman is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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