Compounds between nonmetals are molecular, not ionic. Therefore, a different system for naming them is used. In a compound formed between two nonmetals, the element that occurs first in this series is written and named first:
Si, B, P, H, C, S, I, Br, N, Cl, O, F
The name of the second element retains the -ide ending as though it were an anion. A Latin or Greek prefix (mono-, di-, tri-, and so on) is attached to the name of each element to indicate the number of atoms of that element in the molecule. The prefix mono- is rarely used for nam-ing the first element. Some common prefixes and their numerical equivalences are as shown in the margin table.
P r a C T i C e 6 . 4
Write the name for each of the following compounds using the Stock System:
(a) PbI2 (b) SnF4 (c) Fe2O3 (d) CuO P r a C T i C e 6 . 5
Write formulas for the following compounds:
(a) tin(IV) chromate (c) tin(II) fluoride (b) chromium(III) bromide (d) copper(I) oxide e x a m P l e 6 . 4
Name the compound FeS.
SOlutiOn
This compound follows the rules for a binary compound.
1, 2. It is a compound of Fe, a metal, and S, a nonmetal, and Fe is a transition metal that has more than one type of cation. In sulfides, the charge on the S is 22. Therefore, the charge on Fe must be 12, and the name of the positive part of the compound is iron(II).
3. We have already determined that the name of the negative part of the compound will be sulfide.
The name of FeS is iron(II) sulfide.
S2– Fe2+
Here are some examples of compounds that illustrate this system:
N2O3
di nitrogen tri oxide
Indicates two Indicates three nitrogen atoms oxygen atoms
CO carbon monoxide N2O dinitrogen monoxide
CO2 carbon dioxide N2O4 dinitrogen tetroxide PCl3 phosphorus trichloride NO nitrogen monoxide SO2 sulfur dioxide N2O3 dinitrogen trioxide P2O5 diphosphorus pentoxide S2Cl2 disulfur dichloride CCl4 carbon tetrachloride S2F10 disulfur decafluoride rules For NamiNg biNary ComPouNds CoNTaiNiNg TWo NoNmeTals 1. Write the name for the first element using a prefix if there is more than one
atom of this element.
2. Write the stem of the second element and add the suffix -ide. use a prefix to indicate the number of atoms for the second element.
6.5 • Naming Compounds Containing Polyatomic Ions 109 These examples illustrate that we sometimes drop the final o (mono) or a (penta) of the prefix
when the second element is oxygen. This avoids creating a name that is awkward to pronounce.
For example, CO is carbon monoxide instead of carbon monooxide.
Phosphorus pentachloride
Learning objective
key term polyatomic ion
More ions are listed on the back endpapers.
Table 6.5 Names, Formulas, and Charges of Some Common Polyatomic Ions
Name Formula Charge Name Formula Charge
Acetate C2H3O2- -1 Cyanide CN- -1
Ammonium NH4+ +1 Dichromate Cr2O72- -2
Arsenate AsO43- -3 Hydroxide OH- -1
Hydrogen carbonate HCO-3 -1 Nitrate NO-3 -1
Hydrogen sulfate HSO-4 -1 Nitrite NO-2 -1
Bromate BrO3- -1 Permanganate MnO-4 -1
Carbonate CO2-3 -2 Phosphate PO43- -3
Chlorate ClO-3 -1 Sulfate SO2-4 -2
Chromate CrO2-4 -2 Sulfite SO32- -2
6.5 NaMiNg CoMpouNds CoNtaiNiNg polyatoMiC ioNs
Recognize names, formulas, and charges of polyatomic ions, name compounds con-taining polyatomic ions, and write formulas from names of compounds concon-taining polyatomic ions.
A polyatomicion is an ion that contains two or more elements. Compounds containing poly-atomic ions are composed of three or more elements and usually consist of one or more cations combined with a negative polyatomic ion. In general, naming compounds containing polyatomic ions is similar to naming binary compounds. The cation is named first, followed by the name for the negative polyatomic ion.
To name these compounds, you must learn to recognize the common polyatomic ions (Table 6.5) and know their charges. Consider the formula KMnO4. You must be able to recog-nize that it consists of two parts KMnO4. These parts are composed of a K+ ion and a MnO4
-P r a c T i c e 6 . 6 Name these compounds:
(a) Cl2O (b) SO2 (c) CBr4 (d) N2O5 (e) NH3 (f) ICl3 P r a c T i c e 6 . 7
Name these compounds:
(a) KBr (b) Ca3N2 (c) SO3 (d) SnF2 (e) CuCl2 (f) N2O4 e x a m P l e 6 . 5
Name the compound PCl5. SoLution
1. Phosphorus and chlorine are nonmetals, so the rules for naming binary compounds containing two nonmetals apply. Phosphorus is named first. Therefore, the compound is a chloride.
2. No prefix is needed for phosphorus because each molecule has only one atom of phos-phorus. The prefix penta- is used with chloride to indicate the five chlorine atoms.
(PCl3 is also a known compound.)
The name for PCl5 is phosphorus pentachloride.
ENHANCED EXAMPLE
110 chapter 6 • Nomenclature of Inorganic Compounds
P r a c t i c e 6 . 8 Name these compounds:
(a) NaNO3 (b) Ca3(PO4)2 (c) KOH (d) Li2CO3 (e) NaClO3
ion. The correct name for this compound is potassium permanganate. Many polyatomic ions that contain oxygen are called oxy-anions and generally have the suffix -ate or -ite. Unfortu-nately, the suffix doesn’t indicate the number of oxygen atoms present. The -ate form contains more oxygen atoms than the -ite form. Examples include sulfate (SO2-4 ), sulfite (SO2-3 ), nitrate (NO3-), and nitrite (NO-2).
e X a M P L e 6 . 6
What is the minimum number of different elements in a compound that contains a poly-atomic ion?
SOLUTION
Most polyatomic ions are anions and contain two different elements. Adding a cation to a polyatomic ion to form a compound means the minimum is three different elements in a polyatomic compound.
Inorganic compounds are also formed from more than three elements (see table 6.6). In these cases, one or more of the ions is often a polyatomic ion. Once you have learned to recognize the polyatomic ions, naming these compounds follows the patterns we have already learned. First identify the ions. Name the cations in the order given and follow them with the names of the anions. Study the following examples:
tabLe 6.6 Names of Selected Compounds That Contain More Than One Kind
of Positive Ion
Formula Name of compound KHSO4 potassium hydrogen sulfate Ca(HSO3)2 calcium hydrogen sulfite NH4HS ammonium hydrogen sulfide MgNH4PO4 magnesium ammonium phosphate NaH2PO4 sodium dihydrogen phosphate Na2HPO4 sodium hydrogen phosphate KHC2O4 potassium hydrogen oxalate KAl(SO4)2 potassium aluminum sulfate Al(HCO3)3 aluminum hydrogen carbonate
Ken Karp
Potassium permanganate crystals are dark purple.
ruLes for NaMiNg coMPouNds coNtaiNiNg PoLyatoMic ioNs 1. Write the name of the cation.
2. Write the name of the anion.
Only four of the common negatively charged polyatomic ions do not use the ate–ite sys-tem. These exceptions are hydroxide (OH−) hydrogen sulfide (HS−) peroxide (O22-), and cya-nide (CN−). Care must be taken with these, as their endings can easily be confused with the -ide ending for binary compounds (Section 6.4).
There are three common positively charged polyatomic ions as well—the ammonium, the mercury(I) (Hg22+), and the hydronium (H3O+) ions. The ammonium ion (NH+4) is frequently found in polyatomic compounds, whereas the hydronium ion (H3O+) is usually associated with aqueous solutions of acids (Chapter 15).
Compound Ions Name
NaHCO3 Na+; HCO-3 sodium hydrogen carbonate
NaHS Na+; HS- sodium hydrogen sulfide
MgNH4PO4 Mg2+; NH+4; PO3-4 magnesium ammonium phosphate NaKSO4 Na+; K+; SO24- sodium potassium sulfate
P r a c t i c e 6 . 9 Name these compounds:
(a) KHCO3 (b) NaHC2O4 (c) BaNH4PO4 (d) NaAl(SO4)2
ENHANCED EXAMPLE
6.6 • Acids 111
6.6 aCIdS
use the rules to name an acid from its formula and to write the formula for an acid from its name.
While we will learn much more about acids later (see Chapter 15), it is helpful to be able to recognize and name common acids both in the laboratory and in class. The simplest way to recognize many acids is to know that acid formulas often begin with hydrogen.