Long before chemists knew the formulas for chemical compounds, they developed a system of nomenclature that gave each compound a unique name. Today we often use chemical formulas, such as NaCl, C₁₂H₂₂O₁₁, and Co(NH₃)₆(ClO₄)₃, to describe chemical compounds. But we still need unique names that unambiguously identify each compound.

Common Names

Some compounds have been known for so long that a systematic nomenclature cannot compete with well-established common names. Examples of compounds for which common names are used include water (H₂O), ammonia (NH₃), and methane (CH₄).

Naming Ionic Compounds

(Metals with Non-metals)

The names of ionic compounds are written by listing the name of the positive ion followed by the name of the negative ion.

NaCl			sodium chloride
(NH4)2SO4			ammonium sulfate
NaHCO3			sodium bicarbonate

We therefore need a series of rules that allow us to unambiguously name positive and negative ions before we can name the salts these ions form.

Naming Positive Ions

Monatomic positive ions have the name of the element from which they are formed.

Na+	sodium		Zn2+	zinc
Ca2+	calcium		H+	hydrogen
K+	potassium		Sr2+	strontium

Some metals form positive ions in more than one oxidation state. One of the earliest methods of distinguishing between these ions used the suffixes -ous and -ic added to the Latin name of the element to represent the lower and higher oxidation states, respectively.

Fe2+	ferrous		Fe3+	ferric
Sn2+	stannous		Sn4+	stannic
Cu+	cuprous		Cu2+	cupric

Chemists now use a simpler method, in which the charge on the ion is indicated by a Roman numeral in parentheses immediately after the name of the element.

Fe2+	iron(II)		Fe3+	iron (III)
Sn2+	tin(II)		Sn4+	tin(IV)
Cu+	copper(I)		Cu2+	copper(II)

Polyatomic positive ions often have common names ending with the suffix -onium.

H3O+	hydronium
NH4+	ammonium

Naming Negative Ions

Negative ions that consist of a single atom are named by adding the suffix -ide to the stem of the name of the element.

F-	fluoride	O2-	oxide
Cl-	chloride	S2-	sulfide
Br-	bromide	N3-	nitride
I-	iodide	P3-	phosphide
H-	hydride	C4-	carbide

Common Polyatomic Negative Ions

		-1 ions
HCO3-	bicarbonate		HSO4-	hydrogen sulfate (bisulfate)
CH3CO2-	acetate		ClO4-	perchlorate
NO3-	nitrate		ClO3-	chlorate
NO2-	nitrite		ClO2-	chlorite
MnO4-	permanganate		ClO-	hypochlorite
CN-	cyanide		OH-	hydroxide
		-2 ions
CO32-	carbonate		O22-	peroxide
SO42-	sulfate		CrO42-	chromate
SO32-	sulfite		Cr2O72-	dichromate
S2O32-	thiosulfate		HPO42-	hydrogen phosphate
		-3 ions
PO43-	phosphate		AsO43-	arsenate
BO33-	borate

Naming Polyatomic Ions

At first glance, the nomenclature of the polyatomic negative ions in the table above seems hopeless. There are several general rules, however, that can bring some order out of this apparent chaos.

The name of the ion usually ends in either -ite or -ate. The -ite ending indicates a low oxidation state. Thus,the NO₂^- ion is the nitrite ion.

The -ate ending indicates a high oxidation state. The NO₃^- ion, for example, is the nitrate ion.

The prefix hypo- is used to indicate the very lowest oxidation state. The ClO- ion, for example, is the hypochlorite ion.

The prefix per- (as in hyper-) is used to indicate the very highest oxidation state. The ClO₄^- ion is therefore the perchlorate ion.

There are only a handful of exceptions to these generalizations. The names of the hydroxide (OH^-), cyanide (CN^-), and peroxide (O₂^2-) ions, for example, have the -ide ending because they were once thought to be monatomic ions.

Naming Simple Covalent Compounds

( Non-metals with non-metals )

Oxidation states also play an important role in naming simple covalent compounds. The name of the atom in the positive oxidation state is listed first. The suffix -ide is then added to the stem of the name of the atom in the negative oxidation state.

HCl	hydrogen chloride
NO	nitrogen oxide
BrCl	bromine chloride

As a rule, chemists write formulas in which the element in the positive oxidation state is written first, followed by the element(s) with negative oxidation numbers.

The number of atoms of an element in simple covalent compounds is indicated by adding one of the following Greek prefixes to the name of the element.

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

The prefix mono- is seldom used because it is redundant. The principal exception to this rule is carbon monoxide (CO).

Naming Acids

Simple covalent compounds that contain hydrogen, such as HCl, HBr, and HCN, often dissolve in water to produce acids. These solutions are named by adding the prefix hydro- to the name of the compound and then replacing the suffix -ide with -ic. For example, hydrogen chloride (HCl) dissolves in water to form hydrochloric acid; hydrogen bromide (HBr) forms hydrobromic acid; and hydrogen cyanide (HCN) forms hydrocyanic acid.

Many of the oxygen-rich polyatomic negative ions in Table 2.1 form acids that are named by replacing the suffix -ate with -ic and the suffix -ite with -ous.

Acids containing ions ending with ide often become				hydro -ic acid
Cl-	chloride		HCl	hydrochloric acid
F-	fluoride		HF	hydrofluoric acid
S2-	sulfide		H2S	hydrosulfuric acid
Acids containing ions ending with ate usually become				-ic acid
CH3CO2-	acetate		CH3CO2H	acetic acid
CO32-	carbonate		H2CO3	carbonic acid
BO33-	borate		H3BO3	boric acid
NO3-	nitrate		HNO3	nitric acid
SO42-	sulfate		H2SO4	sulfuric acid
ClO4-	perchlorate		HClO4	perchloric acid
PO43-	phosphate		H3PO4	phosphoric acid
MnO4-	permanganate		HMnO4	permanganic acid
CrO42-	chromate		H2CrO4	chromic acid
ClO3-	chlorate		HClO3	chloric acid
Acids containing ions ending with ite usually become				-ous acid
ClO2-	chlorite		HClO2	chlorous acid
NO2-	nitrite		HNO2	nitrous acid
SO32-	sulfite		H2SO 3	sulfurous acid
ClO-	hypochlorite		HClO	hypochlorous acid

Complex acids can be named by indicating the presence of an acidic hydrogen as follows.

NaHCO3	sodium hydrogen carbonate (also known as sodium bicarbonate)
NaHSO3	sodium hydrogen sulfite (also known as sodium bisulfite)
KH2PO4	potassium dihydrogen phosphate

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