4.6.1: Naming Ionic Compounds
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- 408987
- To use the rules for naming ionic compounds.
After learning a few more details about the names of individual ions, you will be one step away from knowing how to name ionic compounds. This section begins the formal study of nomenclature, the systematic naming of chemical compounds.
Naming Ions
Monatomic cations can be separated into two categories.
Type I- Ions whose charge does not vary.
Ions like Na+, Al3+, Ca2+ only form ions of a single charge magnitude. So, it is unambiguous and simplest to refer to them by their name, simply adding the word "ion". So a sodium ion is Na+, an aluminum ion is Al3+, etc.
Type II- Ions whose charge does vary.
Some elements lose different numbers of electrons, producing ions of different charge magnitude (Figure 3.3). Iron, for example, can form two cations, each of which, when combined with the same anion, makes a different compound with unique physical and chemical properties. Thus, we need a different name for each iron ion to distinguish Fe2+ from Fe3+. The same issue arises for other ions with more than one possible charge. The International Union of Pure and Applied Chemists (IUPAC) dictates that an ion’s positive charge is indicated in parentheses after the element name, followed by the word ion. This can be done with either arabic numerals for charge (e.g. Iron (3+) ion) or roman numerals for oxidation state (e.g. Iron (III) ion). In this case, either would be correct, but many textbooks and instructors continue to use only roman numerals, so we will adopt the same convention for cations with variable charge for the sake of consistency.
Sodium forms only a 1+ ion, so there is no ambiguity about the name sodium ion.
*Refer to section IR-5.3.2-3 of the IUPAC Red Book for more on cation and anion naming conventions.
Element | Charge | Name |
---|---|---|
iron | 2+ | iron(II) ion |
3+ | iron(III) ion | |
copper | 1+ | copper(I) ion |
2+ | copper(II) ion | |
tin | 2+ | tin(II) ion |
4+ | tin(IV) ion | |
lead | 2+ | lead(II) ion |
4+ | lead(IV) ion | |
chromium | 2+ | chromium(II) ion |
3+ | chromium(III) ion | |
gold | 1+ | gold(I) ion |
3+ | gold(III) ion |
Monatomic Anions
Like, type I cations, monatomic anions generally form ions of a single charge magnitude. To avoid confusion with cations, the ending of the element's name is changed to "-ide", before addition of the word anion. See the table below for examples.
Ion | Name |
---|---|
F− | fluoride ion |
Cl− | chloride ion |
Br− | bromide ion |
I− | iodide ion |
O2− | oxide ion |
S2− | sulfide ion |
P3− | phosphide ion |
N3− | nitride ion |
Name each ion.
- Ca2+
- S2−
- SO32−
- NH4+
- Cu+
Solution
- the calcium ion
- the sulfide ion
- the sulfite ion
- the ammonium ion
- the copper(I) ion
Name each ion.
- Fe2+
- Fe3+
- SO42−
- Ba2+
- HCO3−
- Answer a:
- iron(II) ion
- Answer b:
- iron(III) ion
- Answer c:
- sulfate ion
- Answer d:
- barium ion
- Answer e:
- hydrogencarbonate ion or bicarbonate ion
Write the formula for each ion.
- the bromide ion
- the phosphate ion
- the copper(II) ion
- the magnesium ion
Solution
- Br−
- PO43−
- Cu2+
- Mg2+
Write the formula for each ion.
- the fluoride ion
- the carbonate ion
- the tin(II) ion
- the potassium ion
- Answer a:
- F-
- Answer b:
- CO32-
- Answer c:
- Sn 2+
- Answer d:
- K+
Naming Binary Ionic Compounds with a Metal that Forms Only One Type of Cation
A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic nonmetal anion. The metal cation is named first, followed by the nonmetal anion as illustrated in Figure \(\PageIndex{1}\) for the compound BaCl2. The word ion is dropped from both parts.
Subscripts in the formula do not affect the name.
Name each ionic compound.
- CaCl2
- AlF3
- KCl
Solution
- Using the names of the ions, this ionic compound is named calcium chloride.
- The name of this ionic compound is aluminum fluoride.
- The name of this ionic compound is potassium chloride
Name each ionic compound.
- AgI
- MgO
- Ca3P2
- Answer a:
- silver iodide
- Answer b:
- magnesium oxide
- Answer c:
- calcium phosphide
Naming Binary Ionic Compounds with a Metal That Forms More Than One Type of Cation
If you are given a formula for an ionic compound whose cation can have more than one possible charge, you must first determine the charge on the cation before identifying its correct name. For example, consider FeCl2 and FeCl3 . In the first compound, the iron ion has a 2+ charge because there are two Cl− ions in the formula (1− charge on each chloride ion). In the second compound, the iron ion has a 3+ charge, as indicated by the three Cl− ions in the formula. These are two different compounds that need two different names. By the Stock system, the names are iron(II) chloride and iron(III) chloride (Figure \(\PageIndex{2}\)).
Name of cation (metal) + (Roman Numeral in parenthesis) + Base name of anion (nonmetal) and -ide | |
---|---|
Name each ionic compound.
- Co2O3
- FeCl2
Solution
Explanation | Answer | |
---|---|---|
a |
We know that cobalt can have more than one possible charge; we just need to determine what it is.
|
cobalt(III) oxide |
b |
Iron can also have more than one possible charge.
|
iron(II) chloride |
Name each ionic compound.
- AuCl3
- PbO2
- CuO
- Answer a:
- gold(III) chloride
- Answer b:
- lead(IV) oxide
- Answer c:
- copper(II) oxide
Naming Ionic Compounds with Polyatomic Ions
The process of naming ionic compounds with polyatomic ions is the same as naming binary ionic compounds. The cation is named first, followed by the anion. One example is the ammonium sulfate compound in Figure \(\PageIndex{6}\).
Write the proper name for each ionic compound.
- (NH4)2S
- AlPO4,
- Fe3(PO4)2
Solution
Explanation | Answer |
---|---|
a. The ammonium ion has a 1+ charge and the sulfide ion has a 2− charge. Two ammonium ions need to balance the charge on a single sulfide ion. The compound’s name is ammonium sulfide. |
ammonium sulfide |
b. The ions have the same magnitude of charge, one of each (ion) is needed to balance the charges. The name of the compound is aluminum phosphate. |
aluminum phosphate |
c. Neither charge is an exact multiple of the other, so we have to go to the least common multiple of 6. To get 6+, three iron(II) ions are needed, and to get 6−, two phosphate ions are needed . The compound’s name is iron(II) phosphate. |
iron(II) phosphate |
Write the proper name for each ionic compound.
- (NH4)3PO4
- Co(NO2)3
- Answer a:
- ammonium phosphate
- Answer b:
- cobalt(III) nitrite
Figure \(\PageIndex{1}\) is a synopsis of how to name simple ionic compounds.
Name each ionic compound.
- ZnBr2
- Al2O3
- (NH4)3PO4
- AuF3
- AgF
- Answer a:
- zinc bromide
- Answer b:
- aluminum oxide
- Answer c:
- ammonium phosphate
- Answer d:
- gold(III) fluoride or auric fluoride
- Answer e:
- silver fluoride
Summary
- Ionic compounds are named by stating the cation first, followed by the anion.
- Positive and negative charges must balance.
- Some anions have multiple forms and are named accordingly with the use of roman numerals in parentheses.
- Ternary compounds are composed of three or more elements.