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5.3 Naming Compounds that contain Polyatomic Ions

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    There also exists a group of ions that contain more than one atom. These are called polyatomic ions. Table \(\PageIndex{2}\) 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.

    Table \(\PageIndex{2}\): Common Polyatomic Ions
    Name Formula and Charge   Name Formula and Charge
    ammonium NH4+   hydroxide OH
    acetate C2H3O2, or CH3COO nitrate NO3
    bicarbonate (hydrogen carbonate) HCO3 nitrite NO2
    bisulfate (hydrogen sulfate) HSO4 peroxide O22−
    carbonate CO32− perchlorate ClO4
    chlorate ClO3 phosphate PO43−
    chromate CrO42− sulfate SO42−
    cyanide CN sulfite SO32−
    dichromate Cr2O72− triiodide I3

    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 \(\PageIndex{4}\):

    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 \(\PageIndex{4}\)

    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


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

    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 \(\PageIndex{3}\) 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 \(\PageIndex{3}\): 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, flavor 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 his or her 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), don’t oversalt foods, and use other herbs and spices besides salt in cooking.


    Figure \(\PageIndex{2}\): 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.

    Key Takeaways

    • Ions form when atoms lose or gain electrons.
    • Ionic compounds have positive ions and negative ions.
    • Ionic formulas balance the total positive and negative charges.
    • Ionic compounds have a simple system of naming.
    • Groups of atoms can have an overall charge and make ionic compounds.

    5.3 Naming Compounds that contain Polyatomic Ions is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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