Naming Chemical Compounds: Ionic, Molecular, and Acids

Naming Binary Ionic Compounds

Overview

Binary ionic compounds are formed from a metal cation and a nonmetal anion. The naming conventions depend on the predictability of the cation's charge and the identity of the anion.

• Metal cations from groups 1A, 2A, and 3A have predictable charges.

• Anions from groups 5A, 6A, and 7A also have predictable charges.

• Some transition metals only have one charge (e.g., Ag+, Zn2+, Cd2+).

Predictable Charges

Group	Ion	Example
1A	+1 cation	Na+
2A	+2 cation	Mg2+
3A	+3 cation	Al3+

Group	Ion	Example
5A	-3 anion	N3-
6A	-2 anion	O2-
7A	-1 anion	Cl-

Naming Rule

• Name of cation + base name of anion + -ide

Charge Balance

The sum of the positive charges must exactly cancel the sum of the negative charges in the compound.

Formula	Name
MgCl2	magnesium chloride
Na3N	sodium nitride
Al2O3	aluminum oxide
AgCl	silver chloride

Naming Ionic Compounds Containing Polyatomic Ions

Overview

Polyatomic ions are groups of atoms that act as a single ion. They must be memorized and treated as a unit when naming compounds.

Common Polyatomic Ions

Formula	Name
C2H3O2-	acetate
CO32-	carbonate
HCO3-	hydrogen carbonate (bicarbonate)
OH-	hydroxide
NO3-	nitrate
NO2-	nitrite
PO43-	phosphate
PO33-	phosphite
NH4+	ammonium
SO42-	sulfate
SO32-	sulfite

Naming Rule

• Name of cation + name of polyatomic ion

Examples

Formula	Name
NaNO3	sodium nitrate
Mg(OH)2	magnesium hydroxide
NH4Cl	ammonium chloride
Pb(C2H3O2)2	lead(II) acetate

Naming Molecular Compounds

Overview

Molecular compounds are composed only of nonmetals. Their composition is not predictable using charge, so Greek prefixes are used to indicate the number of atoms of each element present.

Naming Rule

• Prefix + name of first element + prefix + base name of second element + -ide

Common Prefixes

Prefix	Number
mono	1
di	2
tri	3
tetra	4
penta	5
hexa	6
hepta	7
octa	8
nona	9
deca	10

Exceptions

• The first prefix is omitted if the first element is "mono".

Examples

Formula	Name
NO	nitrogen monoxide
NO2	dinitrogen monoxide
CO2	carbon dioxide
SF4	sulfur tetrafluoride
N2H4	dinitrogen tetrahydride

Naming Binary Acids

Overview

Binary acids consist of hydrogen and a nonmetal (not polyatomic). The acid is named based on the nonmetal.

Naming Rule

• hydro + base name of nonmetal + -ic + acid

Examples

Formula	Name
HCl (aq)	hydrochloric acid
HF (aq)	hydrofluoric acid
H2S (aq)	hydrosulfuric acid

Exception

• This rule applies only when the acid is dissolved in water (aqueous, aq). If not dissolved, name it using ionic compound rules.

Formula	Name
HCl (aq)	hydrochloric acid (dissolved in water)
HCl (g)	hydrogen chloride (pure substance)

Naming Acids with Oxygen in the Anion (Oxyacids)

Overview

Oxyacids contain hydrogen and a polyatomic ion with oxygen (oxyanion). The naming depends on the ending of the oxyanion.

Naming Rule 1: If the anion ends with -ite

• base name of oxyanion + -ous + acid

Examples

Formula	Name
HNO2	nitrous acid
H3PO3	phosphorous acid
H2SO3	sulfurous acid

Naming Rule 2: If the anion ends with -ate

• base name of oxyanion + -ic + acid

Examples

Formula	Name
HNO3	nitric acid
H3PO4	phosphoric acid
HC2H3O2	acetic acid

Tip

• "-ate" and "acid" rhyme as "-ic".