5.4 Chemical Nomenclature

Learning Objectives

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

  • Name and write formulas of ionic compounds, binary molecular compounds, and acids using IUPAC rules

Nomenclature, a collection of rules for naming things, is important in science and in many other situations. This module describes an approach that is used to name simple ionic and molecular compounds, such as NaCl, CaCO3, and N2O4. The simplest of these are binary compounds, those containing only two elements, but we will also consider how to name ionic compounds containing polyatomic ions, and one specific, very important class of compounds known as acids (subsequent chapters in this text will focus on these compounds). We will limit our attention here to inorganic compounds, compounds that are composed principally of elements other than carbon, and will follow the nomenclature guidelines proposed by IUPAC.

Ionic Compounds

To name an inorganic compound, we need to consider the answers to several questions. First, is the compound ionic or molecular? If the compound is ionic, does the metal form ions of only one type (fixed charge) or more than one type (variable charge)? Are the ions monatomic (simple) or polyatomic? If the compound is molecular, does it contain hydrogen? If so, does it also contain oxygen? From the answers we derive, we place the compound in an appropriate category and then name it accordingly.

Compounds Containing Fixed Charged Metals and Simple Anions

The name of a binary compound containing fixed charged metals consists of the name of the cation (the name of the metal) followed by the name of the anion (the name of the nonmetallic element with its ending replaced by the suffix –ide). Some examples are given in Table 1.

Table 1. Names of Some Ionic Compounds
NaCl, sodium chloride Na2O, sodium oxide
KBr, potassium bromide CdS, cadmium sulfide
CaI2, calcium iodide Mg3N2, magnesium nitride
CsF, cesium fluoride Ca3P2, calcium phosphide
LiCl, lithium chloride Al4C3, aluminum carbide

Compounds Containing a Metal Ion with a Variable Charge

Most of the transition metals can form two or more cations with different charges (Note: tin and lead are outside the transition metal area and have variable charges, both can be either 2+ or 4+). Compounds of these metals with nonmetals are named with the same method as compounds in the first category, except the charge of the metal ion is specified by a Roman numeral in parentheses after the name of the metal. The charge of the metal ion is determined from the formula of the compound and the charge of the anion. For example, consider binary ionic compounds of iron and chlorine. Iron typically exhibits a charge of either 2+ or 3+, and the two corresponding compound formulas are FeCl2 and FeCl3. The simplest name, “iron chloride,” will, in this case, be ambiguous, as it does not distinguish between these two compounds. In cases like this, the charge of the metal ion is included as a Roman numeral in parentheses immediately following the metal name. These two compounds are then unambiguously named iron(II) chloride and iron(III) chloride, respectively. Other examples are provided in Table 2.

Table 2. Names of Some Transition Metal Ionic Compounds
Transition Metal Ionic Compound Name
FeCl3 iron(II) chloride
Hg2O mercury(I) oxide
HgO mercury(II) oxide
Cu3P2 copper(II) phosphide

Out-of-date nomenclature used the suffixes –ic and –ous to designate metals with higher and lower charges, respectively: Iron(III) chloride, FeCl3, was previously called ferric chloride, and iron(II) chloride, FeCl2, was known as ferrous chloride. Though this naming convention has been largely abandoned by the scientific community, it remains in use by some segments of industry. For example, you may see the words stannous fluoride on a tube of toothpaste. This represents the formula SnF2, which is more properly named tin(II) fluoride. The other fluoride of tin is SnF4, which was previously called stannic fluoride but is now named tin(IV) fluoride.

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. Most polyatomic ions end in –ate and –ite. Examples of ionic compounds containing polyatomic ions are shown in Table 3.

Table 3. Names of Some Polyatomic Ionic Compounds
KC2H3O2, potassium acetate Cu2NO3, copper(I) nitrate
Al2(CO3)3, aluminum carbonate Sn(OH)2, tin(II) hydroxide
NaHCO3, sodium bicarbonate Mn(SO4)2, manganese(IV) sulfite
Mg3(PO4)2, magnesium phosphate Ni(CN)2, nickel(II) cyanide
CaSO4, calcium sulfate AuSCN, gold(I) thiocyanate
(NH4)Cl, ammonium chloride Pb(ClO4)2, lead(II) perchlorate
Table 4. Common Polyatomic Ions
NH4+, ammonium OH, hydroxide
C2H3O2, acetate NO3, nitrate
CO32-, carbonate NO2, nitrite
HCO3, hydrogen carbonate* PO43-, phosphate
CN, cyanide PO33-, phosphite
SCN, thiocyanate SO42-, sulfate
ClO4, perchlorate SO32-. sulfite
ClO3, chlorate
ClO2, chlorite *hydrogen carbonate can also go by bicarbonate
ClO, hypochlorite

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 5. 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.

Table 5. Everyday Ionic Compounds
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

Example 1: Naming Ionic Compounds

Name the following ionic compounds, which contain a metal that can have more than one ionic charge:

  1. Li2S
  2. Li2SO4
  3. Li2SO3
  4. Fe2O3
  5. CuSe
  6. GaN
  7. CrCl3
  8. Ti2(SO4)3

Check Your Learning

Write the formulas of the following ionic compounds:

  1. chromium(III) phosphide
  2. mercury(II) sulfide
  3. manganese(II) phosphate
  4. copper(I) oxide
  5. chromium(VI) fluoride

Molecular (Covalent) Compounds

The bonding characteristics of inorganic molecular 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 molecular 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.

Table 5. Nomenclature Prefixes
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-

When only one atom of the first element is present, the prefix mono– is usually deleted 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.

Table 6. Names of Some Molecular Compounds Composed of Two Elements
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

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.)

Example 2: Naming Covalent Compounds

Name the following covalent compounds:

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

Check Your Learning

Write the formulas for the following compounds:

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

Key Concepts and Summary

Chemists use nomenclature rules to clearly name compounds. Ionic and molecular compounds are named using somewhat-different methods. Binary ionic compounds typically consist of a metal and a nonmetal. The name of the metal is written first, followed by the name of the nonmetal with its ending changed to –ide. For example, K2O is called potassium oxide. If the metal can form ions with different charges, a Roman numeral in parentheses follows the name of the metal to specify its charge. Thus, FeCl2 is iron(II) chloride and FeCl3 is iron(III) chloride.

Some compounds contain polyatomic ions; the names of common polyatomic ions should be memorized. Molecular compounds can form compounds 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.


  1. Name the following compounds:
    1. CsCl
    2. BaO
    3. K2S
    4. BeCl2
    5. AlF3
  2. Name the following compounds:
    1. NaF
    2. Rb2O
    3. BCl3
    4. P4O6
    5. ICl3
  3. Write the formulas of the following compounds:
    1. rubidium bromide
    2. magnesium selenide
    3. sodium oxide
    4. calcium chloride
    5. gallium phosphide
    6. aluminum bromide
    7. ammonium sulfate
  4. Write the formulas of the following compounds:
    1. lithium carbonate
    2. sodium perchlorate
    3. barium hydroxide
    4. ammonium carbonate
    5. calcium acetate
    6. magnesium phosphate
    7. sodium sulfite
  5. 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
  6. Write the formulas of the following compounds:
    1. barium chloride
    2. magnesium nitride
    3. sulfur dioxide
    4. nitrogen trichloride
    5. dinitrogen trioxide
    6. tin(IV) chloride
  7. Each of the following compounds contains a metal that can exhibit more than one ionic charge. Name these compounds:
    1. Cr2O3
    2. FeCl2
    3. CrO3
    4. TiCl4
    5. CoO
    6. MoS2
  8. Each of the following compounds contains a metal that can exhibit more than one ionic charge. Name these compounds:
    1. NiCO3
    2. MoO3
    3. Co(NO3)2
    4. V2O5
    5. MnO2
    6. Fe2O3
  9. The following ionic compounds are found in common household products. Write the formulas for each compound:
    1. potassium phosphate
    2. copper(II) sulfate
    3. calcium chloride
    4. titanium dioxide
    5. ammonium nitrate
    6. sodium bisulfate (the common name for sodium hydrogen sulfate)
  10. The following ionic compounds are found in common household products. Name each of the compounds:
    1. Ca(H2PO4)2
    2. FeSO4
    3. CaCO3
    4. MgO
    5. NaNO2
    6. KI
  11. What are the IUPAC names of the following compounds?
    1. manganese dioxide
    2. mercurous chloride (Hg2Cl2)
    3. ferric nitrate [Fe(NO3)3]
    4. titanium tetrachloride
    5. cupric bromide (CuBr2)


binary compound: compound containing two different elements.

nomenclature: system of rules for naming objects of interest