M2Q6: Polyatomic Ions


This section explores polyatomic ions, covalent compounds, and naming compounds in greater detail. The section below provides more detailed description of these topics, worked examples, practice problems and a glossary of important terms.

Learning Objectives for Polyatomic Ions and Covalent Compounds

  • Know the names and symbols of certain elements, as well as common molecules and ions. Write formulas for ionic and common covalent compounds.
    | Polyatomic Ions | Covalent Compounds |

| Key Concepts and SummaryGlossary | End of Section Exercises |

Polyatomic Ions

The ions that we have discussed so far are called monatomic ions, that is, they are ions formed from only one atom. We also find many polyatomic ions. These ions, which act as discrete units, are electrically charged molecules (a group of bonded atoms with an overall charge). Oxyanions are polyatomic ions that contain one or more oxygen atoms. At this point in your study of chemistry, you should memorize the names, formulas, and charges of the most common polyatomic ions (Table 1). Because you will use them repeatedly, they will all (Table 1 and Table 2) soon become familiar.

Polyatomic Ion Name Chemical Formula
ammonium NH4+
hydronium H3O+
hydroxide OH
acetate CH3COO
nitrate NO3
permanganate MnO4
hydrogen carbonate HCO3
carbonate CO32-
hydrogen sulfate HSO4
sulfate SO42-
phosphate PO43-
Table 1. Common Polyatomic Ions to Be Memorized
Name Formula
oxide O2-
peroxide O22-
cyanide CN
azide N3
nitrite NO2
hydrogen sulfite HSO3
sulfite SO32-
hydrogen phosphate HPO42-
dihydrogen phosphate H2PO4
perchlorate ClO4
chlorate ClO3
chlorite ClO2
hypochlorite ClO
chromate CrO42-
dichromate Cr2O72-
Table 2. Common Polyatomic Ions You May Encounter

Note that there is a system for naming some polyatomic ions; -ate and -ite are suffixes designating polyatomic ions containing more or fewer oxygen atoms. Per- (short for “hyper”) and hypo- (meaning “under”) are prefixes meaning more oxygen atoms than -ate and fewer oxygen atoms than -ite, respectively. For example, perchlorate is ClO4, chlorate is ClO3, chlorite is ClO2 and hypochlorite is ClO. Unfortunately, the number of oxygen atoms corresponding to a given suffix or prefix is not consistent; for example, nitrate is NO3 while sulfate is SO42−.

The nature of the attractive forces that hold atoms or ions together within a compound is the basis for classifying chemical bonding. When electrons are transferred and ions form, ionic bonds result. Ionic bonds are electrostatic forces of attraction, that is, the attractive forces experienced between objects of opposite electrical charge (in this case, cations and anions). When electrons are “shared” and molecules form, covalent bonds result. Covalent bonds are the attractive forces between the positively charged nuclei of the bonded atoms and one or more pairs of electrons that are located between the atoms. Compounds are classified as ionic or covalent (molecular) on the basis of the bonds present in them.

Compounds Containing Polyatomic Ions

Compounds containing polyatomic ions are named similarly to those containing only monatomic ions, except there is no need to change to an –ide ending, since the suffix is already present in the name of the anion. Examples are shown in Table 3.

KC2H3O2, potassium acetate NH4Cl, ammonium chloride
NaHCO3, sodium bicarbonate CaSO4, calcium sulfate
Al2(CO3)3, aluminum carbonate Mg3(PO4)2, magnesium phosphate
Table 3. Names of Some Polyatomic Ionic Compounds

Ionic Compounds in Your Cabinets

Every day you encounter and use a large number of ionic compounds. Some of these compounds, where they are found, and what they are used for are listed in Table 4. Look at the label or ingredients list on the various products that you use during the next few days, and see if you run into any of those in this table, or find other ionic compounds that you could now name or write as a formula.

Ionic Compound Use
NaCl, sodium chloride ordinary table salt
KI, potassium iodide added to “iodized” salt for thyroid health
NaF, sodium fluoride ingredient in toothpaste
NaHCO3, sodium bicarbonate baking soda; used in cooking (and as antacid)
Na2CO3, sodium carbonate washing soda; used in cleaning agents
NaOCl, sodium hypochlorite active ingredient in household bleach
CaCO3 calcium carbonate ingredient in antacids
Mg(OH)2, magnesium hydroxide ingredient in antacids
Al(OH)3, aluminum hydroxide ingredient in antacids
NaOH, sodium hydroxide lye; used as drain cleaner
K3PO4, potassium phosphate food additive (many purposes)
MgSO4, magnesium sulfate added to purified water
Na2HPO4, sodium hydrogen phosphate anti-caking agent; used in powdered products
Na2SO3, sodium sulfite preservative
Table 4. Everyday Ionic Compounds

Many ionic compounds contain polyatomic ions (Table 1 and Table 2) as the cation, the anion, or both. As with simple ionic compounds, these compounds must also be electrically neutral, so their formulas can be predicted by treating the polyatomic ions as discrete units. We use parentheses in a formula to indicate a group of atoms that behave as a unit. For example, the formula for calcium phosphate, one of the minerals in our bones, is Ca3(PO4)2. This formula indicates that there are three calcium ions (Ca2+) for every two phosphate (PO43−) groups. The PO43− groups are discrete units, each consisting of one phosphorus atom and four oxygen atoms, and having an overall charge of 3−. The compound is electrically neutral, and its formula shows a total count of three Ca, two P, and eight O atoms.

Example 1

Predicting the Formula of a Compound with a Polyatomic Anion
Baking powder contains calcium dihydrogen phosphate, an ionic compound composed of the ions Ca2+ and H2PO4. What is the formula of this compound?

The positive and negative charges must balance, and this ionic compound must be electrically neutral. Thus, we must have two negative charges to balance the 2+ charge of the calcium ion. This requires a ratio of one Ca2+ ion to two H2PO4 ions. We designate this by enclosing the formula for the dihydrogen phosphate ion in parentheses and adding a subscript 2. The formula is Ca(H2PO4)2.

Check Your Learning
Predict the formula of the ionic compound formed between the lithium ion and the peroxide ion, O22− (Hint: Use the periodic table to predict the sign and the charge on the lithium ion.)



Because an ionic compound is not made up of single, discrete molecules, it may not be properly symbolized using a molecular formula. Instead, ionic compounds must be symbolized by a formula indicating the relative numbers of its constituent ions. For compounds containing only monatomic ions (such as NaCl) and for many compounds containing polyatomic ions (such as CaSO4), these formulas are just the empirical formulas introduced earlier in this chapter. However, the formulas for some ionic compounds containing polyatomic ions are not empirical formulas. For example, the ionic compound sodium oxalate is comprised of Na+ and C2O42− ions combined in a 2:1 ratio, and its formula is written as Na2C2O4. The subscripts in this formula are not the smallest-possible whole numbers, as each can be divided by 2 to yield the empirical formula, NaCO2. This is not the accepted formula for sodium oxalate, however, as it does not accurately represent the compound’s polyatomic anion, C2O42−. We’ll revisit crystal structures of salts in Module 11.

Covalent (Molecular) Compounds

The bonding characteristics of inorganic covalent compounds are different from ionic compounds, and they are named using a different system as well. The charges of cations and anions dictate their ratios in ionic compounds, so specifying the names of the ions provides sufficient information to determine chemical formulas. However, because covalent bonding allows for significant variation in the combination ratios of the atoms in a molecule, the names for molecular compounds must explicitly identify these ratios.

Compounds Composed of Two Elements

When two nonmetallic elements form a covalent compound, several combination ratios are often possible. For example, carbon and oxygen can form the compounds CO and CO2. Since these are different substances with different properties, they cannot both have the same name (they cannot both be called carbon oxide). To deal with this situation, we use a naming method that is somewhat similar to that used for ionic compounds, but with added prefixes to specify the numbers of atoms of each element. The name of the more metallic element (the one farther to the left and/or bottom of the periodic table) is first, followed by the name of the more nonmetallic element (the one farther to the right and/or top) with its ending changed to the suffix –ide. The numbers of atoms of each element are designated by the Greek prefixes shown in Table 5.

Number Prefix Number Prefix
1 (sometimes omitted) mono- 6 hexa-
2 di- 7 hepta-
3 tri- 8 octa-
4 tetra- 9 nona-
5 penta- 10 deca-
Table 5. Nomenclature Prefixes

When only one atom of the first element is present, the prefix mono– is usually omitted from that part. Thus, CO is named carbon monoxide, and CO2 is called carbon dioxide. When two vowels are adjacent, the a in the Greek prefix is usually dropped. Some other examples are shown in Table 6.

Compound Name Compound Name
SO2 sulfur dioxide BCl3 boron trichloride
SO3 sulfur trioxide SF6 sulfur hexafluoride
NO2 nitrogen dioxide PF5 phosphorus pentafluoride
N2O4 dinitrogen tetroxide P4O10 tetraphosphorus decaoxide
N2O5 dinitrogen pentoxide IF7 iodine heptafluoride
Table 6. Names of Some Molecular Compounds Composed of Two Elements

There are a few common names that you will encounter as you continue your study of chemistry. For example, although NO is often called nitric oxide, its proper name is nitrogen monoxide. Similarly, N2O is known as nitrous oxide even though our rules would specify the name dinitrogen monoxide. (And H2O is usually called water, not dihydrogen monoxide.) You should commit to memory the common names of compounds as you encounter them.

Example 2

Naming Covalent Compounds
Name the following covalent compounds:

  1. SF6
  2. N2O3
  3. Cl2O7
  4. P4O6

Because these compounds consist solely of nonmetals, we use prefixes to designate the number of atoms of each element:

  1. sulfur hexafluoride
  2. dinitrogen trioxide
  3. dichlorine heptoxide
  4. tetraphosphorus hexoxide

Check Your Learning
Write the formulas for the following compounds:

  1. phosphorus pentachloride
  2. dinitrogen monoxide
  3. iodine heptafluoride
  4. carbon tetrachloride


(a) PCl5; (b) N2O; (c) IF7; (d) CCl4


The following website provides practice with naming chemical compounds and writing chemical formulas. You can choose binary, polyatomic, and variable charge ionic compounds, as well as molecular compounds.

Key Concepts and Summary

Chemists use nomenclature rules to clearly name compounds. Ionic and molecular compounds are named using somewhat different methods. Some compounds contain polyatomic ions; the names of common polyatomic ions should be memorized. Molecular compounds can form with different ratios of their elements, so prefixes are used to specify the numbers of atoms of each element in a molecule of the compound. Examples include SF6, sulfur hexafluoride, and N2O4, dinitrogen tetroxide.


polyatomic ion
ion composed of more than one atom
monoatomic ion
ion composed of one atom
polyatomic ion that contains one or more oxygen atoms
ionic bond
electrons are transferred to form an ionic compound held by electrostatic forces
covalent bond
electrons are shared to form a covalent compound held by attractive forces

Chemistry End of Section Exercises

  1. For each of the following pairs of ions, write the symbol for the formula of the compound they will form:
    1. NH4+, SO42−
    2. Na+, HPO42−
    3. Mg2+, PO43−
    4. NH4+, PO43−
    5. Na+, CO32−
    6. Ba2+, PO43−
  2. Write the formulas of the following compounds:
    1. chlorine dioxide
    2. dinitrogen tetraoxide
    3. potassium phosphide
    4. silver(I) sulfide
    5. aluminum nitride
    6. silicon dioxide
  3. Write the formulas of the following compounds:
    1. lithium carbonate
    2. sodium perchlorate
    3. barium hydroxide
    4. ammonium carbonate
    5. sulfuric acid
    6. calcium acetate
    7. magnesium phosphate
    8. sodium sulfite

Answers to Chemistry End of Section Exercises

  1. (a) (NH4)2SO4; (b) Na2HPO4; (c) Mg3(PO4)2; (d) (NH4)3PO4; (e) Na2CO3; (f) Ba3(PO4)2
  2. (a) ClO2; (b) N2O4; (c) K3P; (d) Ag2S; (e) AlN; (f) SiO2
  3. (a) Li2CO3; (b) NaClO4; (c) Ba(OH)2; (d) (NH4)2CO3; (e) H2SO4; (f) Ca(C2H3O2)2; (g) Mg3(PO4)2; (h) Na2SO3
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