Naming Inorganic
Compounds
Ionic compounds
Chemical nomenclature
The names and chemical formulas of compounds are essential in chemistry.
The system used in naming substances is called chemical nomenclature, from the Latin
words “nomen" (name) and “calare" (to call).
Many important substances that have been known for a long time, such as water (H2O) and
ammonia (NH3), do have traditional names (called common names). For most substances,
however, there are a set of rules that leads to a unique name for each substance.
The rules for chemical nomenclature are based on the division of substances into categories.
The major division is between organic and inorganic compounds. Organic compounds
contain carbon-carbon bonds and/or carbon-hydrogen bonds, often in combination with
oxygen, nitrogen, or other elements. All others are inorganic compounds. Early chemists
associated organic compounds with plants and animals and inorganic compounds with the
nonliving portion of our world.
In this section we will see the basic rules for naming three categories
of inorganic compounds:
• ionic compounds
• molecular compounds
• acids.
Compounds
7 |Ionic
IONS AND
IONIC COMPOUNDS
S
nucleus of an atom is unchanged by chemical processes, but some atoms can readMany atoms gain or lose electrons to end
up
with
the same number of electrons as
gain or lose electrons. If electrons are removed from or added
to
an
atom,
a
charged
The net charge on an ion is represented by a superscript. The superscripts
the closest
noble
in the
periodic
We
might
deduce that this is because their
An gas
ion with
a positive
charge istable.
cation
(pronounced
ticle called
an ion is formed.
and 3+ ,afor
instance,
mean a net charge resulting from the loss of one, two, and
T-ion);
a negatively charged
ion is an anion
electronic
configuration
is (AN-ion).
very stable.
Nearby
elements
can obtain
stable
electrons,
respectively.
The superscripts
-, 2 - , these
and 3- same
represent
net charges res
To see how ions form, consider the sodium atom, which has 11 protons and 11 elecfrom
thehas
gain
of one, two, and three electrons, respectively. Chlorine, with 17 p
arrangements
electrons.
ns. This
atom easily loses by
one losing
electron. or
The gaining
resulting cation
11 protons and 10
and 17 electrons, for example, can gain an electron in chemical reactions, produci
trons, which means it has a net charge of 1+ .
-
Cl ion:
11p!
11e"
11p
!
"
10e
17p"
Loses an
electron
Na atom
17e!
17p"
18e!
Gains an
electron
Na! ion
Cl atom
Cl! ion
In general, metal atoms tend to lose electrons to form cations and nonmetal atoms t
gain electrons to form anions. Thus, ionic compounds tend to be composed of metals b
electron
from an atom of Na leaves it with the same
with nonmetals, as in NaCl.
For example, the loss of one
number of electrons as in a Ne atom.
SAMPLE EXERCISE 2.7
Writing Chemical Symbols for Ions
Similarly, when Cl gains an electron,
upsymbol,
withincluding
18, thesuperscript
same number
of number, for (a) the ion
Give it
theends
chemical
indicating mass
22 protons, 26 neutrons, and 19 electrons; (b) the ion of sulfur that has 16 neutron
electrons as in Ar.
18 electrons.
SOLUTION
(a) The number of protons is the atomic number of the element. A periodic table or
elements tells us that the element with atomic number 22 is titanium (Ti). The mass nu
(protons plus neutrons) of this isotope of titanium is 22 + 26 = 48. Because the ion has
more protons than electrons, it has a net charge of 3+ : 48Ti3+.
(b) The periodic table tells us that sulfur (S) has an atomic number of 16. Thus, each ato
Ionic Compounds
When elemental Na is allowed to react with elemental Cl, an electron transfers from
Na to Cl, forming a Na+ ion and a Cl- ion.
Because objects of opposite charge attract, the Na+ and the Cl- ions bind together to
form the compound sodium chloride (NaCl). Sodium chlorideSECTION
(table 2.7
salt),
isand
an Ionic
example
Ions
Compounds
of an ionic compound, a compound made up of cations and anions.
11p!
11e#
Loses an
electron
10e#
Na! ion
Na atom
17p!
11p!
e!
17e#
18e#
17p!
Gains an
electron
Cl atom
Cl# ion
(a)
(b)
(c)
! FIGURE
2.21 compounds
Formation of an ionic
compound.
(a) The transfer
of an
electron from
a Na
In general
ionic
are
combinations
of
metals
and
nonmetals.
+
atom to a Cl atom leads to the formation of a Na ion and a Cl ion. (b) Arrangement of these
In contrast,
generally
composed of nonmetals only, as in
ions in solid molecular
sodium chloride,compounds
NaCl. (c) A sample are
of sodium
chloride crystals.
H2O.
posed of nonmetals only, as in H2O.
Ionic Compounds
The periodic table is useful for remembering ionic charges, especially those of elements
on the left and right sides of the table. The charges of these ions relate in a simple way to
their positions in the table:
+ ions
•
The
group
1A
elements
(alkali
metals)
form
1
+
2+
zinc, and scandium are Ag , Zn , and
+ ions
•
The
group
2A
elements
(alkaline
earths)
form
2
ou would place these ions in this table.
• The group 6A elements form 2- ions
e number of electrons as a noble-gas
• The group 7A elements (halogens) form 1- ions
1A
H!
2A
Li!
Na! Mg 2!
Transition metals
7A 8A
"
3A 4A 5A 6A H N
O
" B
3"
2"
F
N O
L
2"
" E
3!
S
Cl
Al
2"
Br
2"
I
K! Ca2!
Se
Rb! Sr2!
Te
Cs! Ba2!
"
"
G
A
S
E
S
Predictable charges of some common ions. Notice that the red stepped line
that divides metals from nonmetals also separates cations from anions.
Hydrogen forms both 1+ and 1- ions.
Ionic Compounds
Practice exercise
Identifying Ionic and Molecular Compounds
• Which of these compounds would you expect to be ionic: N O, Na O, CaCl , SF ?
2
2
2
4
SOLUTION
We predict that Na2O and CaCl2 are ionic compounds because they are composed of
a metal combined with a nonmetal. We predict (correctly) that N2O and SF4 are
molecular compounds because they are composed entirely of nonmetals.
• Which of these compounds are molecular: CBr , FeS, P O , PbF ?
4
4
6
2
Compounds
e Ionic
ions in
ionic compounds are arranged in three-dimensional struct
Chemical
formulas
give Because
only the relative
of atoms of
each type in of
a NaCl
2.21(b)
shows
for that
NaCl.
there number
is no discrete
“molecule”
molecule
empirical
formulas
write
onlyare
ancalled
empirical
formula
for .this substance. This is true for mos
mpounds.
E.g. The empirical formula for hydrogen peroxide H2O2 is HO. The empirical formula for
C2H4empirical
is CH2.
canethylene
write the
formula for an ionic compound if we know the
ons. This is true because chemical compounds are always electrically n
We can write the empirical formula for an ionic compound if we know the charges of
uently,
theThis
ions
in an
ionicchemical
compound
alwaysare
occur
inelectrically
such a ratio
that t
the ions.
is true
because
compounds
always
neutral
(if not differently
specified).
charge
equals the
total negative charge. Thus, there is one Na + to one C
2+
Ba
Cl
Cl), one
to
two
(giving BaCl ), and so forth.
E.g. There is one Na+ to one Cl- (giving NaCl),2 one Ba2+ to two Cl- (giving BaCl2), and so on.
you consider these and other examples, you will see that if the charges
charges
the cation
anion areon
equal,
theion
subscript
on the
eachcharges
ion is 1. Ifare
the no
andIf the
anion
areon
equal,
the and
subscript
each
is 1. If
are not equal, the charge on one ion will become the subscript on the
rgecharges
on one
ion (without its sign) will become the subscript on the other i
other ion.
e, the ionic compound formed from Mg (which forms Mg 2+ ions) and N
2+
For example,
N3- ions)
is Mgthe
3Nionic
2: compound formed from Mg (which forms Mg ions) and N (which
forms N3- ions) is Mg3N2:
Mg 2 !
N 3"
Mg3N2
Ionic Compounds
Practice exercise
• Write the empirical formulas for (a) glucose (molecular formula C H
6
12O6); (b)
nitrous oxide, a substance used as an anesthetic and commonly called laughing gas
(molecular formula N2O).
SOLUTION
(a) The subscripts of an empirical formula are the smallest whole-number ratios. The
smallest ratios are obtained by dividing each subscript by the largest common factor, in
this case 6. The resultant empirical formula for glucose is CH2O.
(b) Because the subscripts in N2O are already the lowest integral numbers, the
empirical formula for nitrous oxide is the same as its molecular formula, N2O.
• Give the empirical formula for diborane, whose molecular formula is B H .
2
6
Names and Formulas of Ionic Compounds
Cations
a. Cations formed from metals have the same name as the metal:
Na
2+
sodium ion
Zn
2+
zinc ion
Al
3+
aluminum ion
b. Some metals can form cations with different charges. In these cases the positive charge
is indicated by a Roman numeral in parentheses following the name of the metal:
Fe
2+
iron (II) ion
Cu
+
copper (I) ion
Fe
3+
iron (III) ion
Cu
2+
copper (II) ion
An older method still widely used for distinguishing between differently charged ions
of a metal uses the endings -ous and -ic added to the root of the element’s Latin name:
Fe
2+
ferrous ion
Cu
+
coprous ion
Fe
3+
ferric ion
Cu
2+
copric ion
c. Common polyatomic cations formed from nonmetal atoms have names that end in -ium:
NH4+ ammonium ion
H3O+ hydronium ion
m the cations, and the nonmetals form the anions.
Names and Formulas of Ionic Compounds
Cations
metal atoms have the same name as the metal:
Ions of the same element that have different charges have different properties, such as
Zn2+ zinccolors.
ion
Al3+ aluminum ion
different
ions with different charges, the positive charge is indicated by
arentheses following the name of the metal:
E.g.
+
(II)Both
ion substancesCu
copper(I)
ion are compounds of iron.
shown
on the right
2+ left is Fe O , which contains Fe2+ and
(III)The
ionsubstance on
Cuthe
copper(II)
3 4ion
Fe3+ ions.
The substance on the right is Fe2O3, which contains Fe3+ ions.
ment that have different charges have different properties,
s (! FIGURE 2.23).
form cations with different charges are transition metals,
n the middle of the periodic table, from group 3B to group " FIGURE 2.23 Different ions of the
rm only one cation (only one possible charge) are those of same element have different properties.
2A, as well as Al3+ (group 3A) and two transition-metal Both substances shown are compounds of
on the left is Fe3O4,
and
Zn2+ (group 2B). Charges are not expressed when iron. The substance
Note
2+
3+
which
contains
Fe
and
Fe
ions. The
owever,
there is any
doubt
in cations
your mind
metal
Mostiftransition
metals
form
withwhether
differenta charges.
substance on the right is Fe2O3+
3, which
The metals
that form
only one
arethe
those
of group
and group
and two
cation,
use a Roman
numeral
tocation
indicate
charge.
It is 1Acontains
3+2A, as well as Al
Fe
ions.
transition
Ag+ (group 1B) and Zn2+.
even
thoughmetal
it mayions:
be unnecessary.
b. What does the -ium ending on the name ammonium ion tell you about the
composition of the ion?
Names and Formulas of Ionic Compounds
Cations
List of common Cations
TABLE 2.4 • Common Cations*
Charge
Formula
Name
Formula
Name
1+
H!
Li+
hydrogen ion
lithium ion
NH 4 !
Cu +
ammonium ion
copper(I) or cuprous ion
Na!
sodium ion
K!
potassium ion
Cs +
cesium ion
Ag!
silver ion
Mg 2!
magnesium ion
Co2+
cobalt(II) or cobaltous ion
Ca2!
calcium ion
Cu2!
copper(II) or cupric ion
Sr 2+
strontium ion
Fe 2!
iron(II) or ferrous ion
Ba2+
barium ion
Mn2+
manganese(II) or manganous ion
Zn2!
zinc ion
Hg 22+
mercury(I) or mercurous ion
Cd2+
cadmium ion
Hg 2!
mercury(II) or mercuric ion
Ni2+
nickel(II) or nickelous ion
Pb2!
lead(II) or plumbous ion
Sn2+
tin(II) or stannous ion
Cr 3+
chromium(III) or chromic ion
Fe 3!
iron(III) or ferric ion
2+
3+
Al3!
aluminum ion
*The
ionsoften
we useused
mostions
often
this
course are in boldface. Learn them first.
The
most
areinin
boldface
Names and Formulas of Ionic Compounds
Anions
a. The names of monatomic anions are formed by replacing the ending of the name of
the element with -ide:
O2- oxide ion
H- hydride ion
N3- nitride ion
A few polyatomic anions also have names ending in -ide:
OH- hydroxide ion
O22- peroxide ion
CN- cyanide ion
b. Polyatomic anions containing oxygen are called oxyanions.
The -ate is used for the most common oxyanion, and -ite is used for an oxyanion that has the
same charge but one O atom fewer:
NO3- nitrate ion
SO42- sulfate ion
NO2- nitrite ion
SO32- sulfite ion
ion (one more O atom than chlorate)
Names and perchlorate
Formulas
of Ionic Compounds
chlorate ion
ClO4 ClO3!
ClO2 ClO-
Anions
chlorite ion (one O atom fewer than chlorate)
hypochlorite ion (one O atom fewer than chlorite)
Prefixes are used when the number of oxyanions of an element is more than four, as
with
the
halogens.
These
rules
are summarized in ! FIGURE 2.24.
The prefix per- indicates one more O atom than the oxyanion ending in -ate; hypoGIVE IT SOME THOUGHT
indicates
one O atom fewer than the oxyanion ending in -ite:
What information is conveyed by the endings -ide, -ate, and -ite in the name of
an anion?
ClO
perchlorate ion (one more O atom than chlorate)
4
" FIGURE 2.25 can help you remember the charge and number of oxygen atoms in
the various oxyanions. Notice that C and N, both period 2 elements, have only three O
chlorate
ClO33elements
atoms each, whereas the period
P, S, and Clion
have four O atoms each. Beginning at the lower right in Figure 2.25, note that ionic charge increases from right to left,
from 1- for CIO4- to 3 - for
PO423-- . In the
second period
charges
also increase
ClO
chlorite
ion the
(one
O atom
fewerfrom
than chlorate)
2right to left, from 1 - for NO3 to 2- for CO3 . Notice also that although each of the
anions in Figure 2.25 ends ClO
in -ate,- the ClO
per- prefix.
4 ion also has a ion
hypochlorite
(one O atom fewer than chlorite)
GO FIGURE
Procedure
namingbyanions.
Name
the anionfor
obtained
removing one oxygen atom from the perbromate
ion,The
BrO4first
. part of the element’s name, such as “chlor” for chlorine or “sulf” for sulfur, goes in the blank.
Simple
anion
Oxyanions
_________ide
(chloride, Cl!)
"O atom
per______ate
!
(perchlorate, ClO4 )
_________ate
(chlorate, ClO3!)
Common or
representative
oxyanion
!O atom
_________ite
(chlorite, ClO2!)
!O atom
hypo____ite
(hypochlorite, ClO!)
62
Names and Formulas of Ionic Compounds
Anions
Common
The composition
and charges of common oxyanions are related to their
CHAPTERoxyanions.
2 Atoms, Molecules,
and Ions
location in the periodic table.
Maximum of 3 O atoms
in period 2.
Period 2
Group 4A
Group 5A
CO32–
Carbonate ion
NO3–
Nitrate ion
PO43–
Period 3
Phosphate ion
Group 6A
2–
SO4
Sulfate ion
Group 7A
–
ClO4
Perchlorate ion
Charges increase right
to left.
Maximum of 4 O atoms
in period 3.
! FIGURE 2.25 Common oxyanions.
he composition and charges of common
Practice
exercise
xyanions
are related
to their location in the
eriodic table.
GIVE IT SOME THOUGHT
Predict the formulas for the borate ion and silicate ion, assuming they contain a
single B and Si atom, respectively, and follow the trends shown in Figure 2.25.
the formula for (a) the selenate ion and (b) the selenite ion. (Sulfur and selenium are
• Predict
EXERCISE
2.11 Determining the Formula of an Oxyanion
both in group 6A and form SAMPLE
analogous
oxyanions.)
from Its Name
SOLUTION
2-.
the selenate ion and (b) the
Basedanalogous
on the formulaselenate
for the sulfate
ion,ispredict
the formula
for4(a)
(a) The sulfate ion is SO42-. The
ion
therefore
SeO
ion. (Sulfur and selenium are both in group 6A and form analogous oxyanions.)
(b) The ending -ite indicatesselenite
an oxyanion
with the same charge but one O atom fewer than the
corresponding oxyanion that ends
in -ate. Thus, the formula for the selenite ion is SeO32- .
SOLUTION
(a) The sulfate ion is SO42-. The analogous selenate ion is therefore SeO42- .
(b) The
-ite indicatesto
an oxyanion
same chargeion.
but one
O atom
fewer
than the
bromate
ionending
is analogous
that forwith
thethechlorate
Write
the
formula
corresponding oxyanion that ends in -ate. Thus, the formula for the selenite ion is SeO32 - .
formula for the
• The
for the hypobromite and bromite ions.
PRACTICE EXERCISE
Names and Formulas of Ionic Compounds
Anions
c. Anions derived by adding H+ to an oxyanion are named by adding as a prefix the word
hydrogen or di-hydrogen, as appropriate:
CO32- carbonate ion
PO43- phosphate ion
HCO3- hydrogen carbonate ion
H2PO4- dihydrogen phosphate ion
Note
Each H+ added reduces the negative charge of the parent anion by one.
An older method for naming some of these ions uses the prefix bi-. Thus, the HCO3- ion is
commonly called the bicarbonate ion, and HSO4- is sometimes called the bisulfate ion.
hydroxide.
(c) You must determine the charge of Fe in this compound because an iron atom can
form
- Ionic Compounds
more than one cation. Because the compound contains three chloride ions, Cl , the cation
must be Fe 3+, the iron(III), or ferric, ion. Thus, the compound is iron(III) chloride or ferric
chloride.
of ionic compounds consist of the cation name followed by the anion name:
Names and Formulas of Ionic Compounds
Names
PRACTICE EXERCISE
Name the ionic compoundsMgBr
(a) NH24Br,
(b) Cr2O3, (c)bromide
Co(NO3)2.
magnesium
Answers: (a) ammonium CaCl
bromide,
(b) chromium(III) oxide, (c) cobalt(II) nitrate
2 calcium chloride
NaF sodium fluoride
TABLE 2.5 • Common Anions*
List of common Cations
Charge
Formula
Name
Formula
Name
1-
H-
hydride ion
acetate ion
F!
Cl !
Br!
I!
CNOH !
fluoride ion
chloride ion
bromide ion
iodide ion
cyanide ion
hydroxide ion
CH 3COO!
(or C2H3O2 - )
ClO3 ClO4!
NO3!
MnO4 -
2-
O 2!
O22S2!
oxide ion
peroxide ion
sulfide ion
CO32!
CrO42Cr2O72SO42!
carbonate ion
chromate ion
dichromate ion
sulfate ion
3-
N3-
nitride ion
PO43!
phosphate ion
*The ions we use most often are in boldface. Learn them first.
chlorate ion
perchlorate ion
nitrate ion
permanganate ion
Names and Formulas of Ionic Compounds
Ionic Compounds
Practice exercise
Name the ionic compounds (a) K2SO4, (b) Ba(OH)2, (c) FeCl3.
•
SOLUTION
In naming ionic compounds, it is important to recognize polyatomic ions and to determine the
charge of cations with variable charge.
(a) The cation is K+, and the anion is SO42-, the sulfate ion, making the name potassium sulfate.
(b) The cation is Ba2+, and the anion is OH-, the hydroxide ion: barium hydroxide.
(c) You must determine the charge of Fe because an iron atom can form more than one cation.
Because the compound contains three chloride ions, Cl-, the cation must be Fe3+, the iron(III), or
ferric, ion. Thus, the compound is iron(III) chloride or ferric chloride.
• Name the ionic compounds (a) NH Br, (b) Cr O , (c) Co(NO ) .
4
2
3
3 2