3.5: Ionic Compounds- Formulas and Names
Last updatedAug 25, 2020
3.4: An Atomic-Level Perspective of Elements and Compounds
3.6: Molecular Compounds- Formulas and Names
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6.9: Binary Ionic Compounds and Their Properties
6.18: Ionic Compounds Containing Polyatomic Ions
Learning Objectives
Derive names for common types of inorganic compounds using a systematic approach
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 in great detail). 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. The rules for organic compounds, in which carbon is the principle element, will be treated in a later chapter on organic chemistry.
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 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 Only Monatomic Ions
The name of a binary compound containing monatomic ions 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 3.5.2 .
Table 3.5.1 : Names of Some Ionic Compounds
NaCl, sodium chlorideNa2O, sodium oxide
KBr, potassium bromideCdS, cadmium sulfide
CaI2, calcium iodideMg3N2, magnesium nitride
CsF, cesium fluorideCa3P2, calcium phosphide
LiCl, lithium chlorideAl4C3, aluminum carbide
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 3.5.2 .
Table 3.5.2 : Names of Some Polyatomic Ionic Compounds
KC2H3O2, potassium acetate(NH4)Cl, ammonium chloride
NaHCO3, sodium bicarbonateCaSO4, calcium sulfate
Al2(CO3)3, aluminum carbonateMg3(PO4)2, magnesium phosphate
Ionic Compounds in Your Cabinets
Ionic CompoundUse
NaCl, sodium chlorideordinary table salt
KI, potassium iodideadded to “iodized” salt for thyroid health
NaF, sodium fluorideingredient in toothpaste
NaHCO3, sodium bicarbonatebaking soda; used in cooking (and as antacid)
Na2CO3, sodium carbonatewashing soda; used in cleaning agents
NaOCl, sodium hypochloriteactive ingredient in household bleach
CaCO3 calcium carbonateingredient in antacids
Mg(OH)2, magnesium hydroxideingredient in antacids
Al(OH)3, aluminum hydroxideingredient in antacids
NaOH, sodium hydroxidelye; used as drain cleaner
K3PO4, potassium phosphatefood additive (many purposes)
MgSO4, magnesium sulfateadded to purified water
Na2HPO4, sodium hydrogen phosphateanti-caking agent; used in powdered products
Na2SO3, sodium sulfitepreservative
Table 3.5.3 : Names of Some Transition Metal Ionic Compounds
Transition Metal Ionic CompoundName
FeCl3iron(III) chloride
Hg2Omercury(I) oxide
HgOmercury(II) oxide
Cu3(PO4)2copper(II) phosphate
Naming Ionic Compounds
Name the following ionic compounds, which contain a metal that can have more than one ionic charge:
Fe2S3
CuSe
GaN
CrCl3
Ti2(SO4)3
Solution
The anions in these compounds have a fixed negative charge (S2−, Se2− , N3−, Cl−, and SO2−4 ), and the compounds must be neutral. Because the total number of positive charges in each compound must equal the total number of negative charges, the positive ions must be Fe3+, Cu2+, Ga3+, Cr4+, and Ti3+. These charges are used in the names of the metal ions:
iron(III) sulfide
copper(II) selenide
gallium(III) nitride
chromium(III) chloride
titanium(III) sulfate
Exercise 3.5.1
Write the formulas of the following ionic compounds:
(a) chromium(III) phosphide
(b) mercury(II) sulfide
(c) manganese(II) phosphate
(d) copper(I) oxide
(e) chromium(VI) fluoride
Answer
(a) CrP; (b) HgS; (c) Mn3(PO4)2; (d) Cu2O; (e) CrF6
Explanation:
