Experiment 3 - Nomenclature of Compounds
INTRODUCTION
Naming Ionic Compounds
An ion is a single atom or a group of atoms with a net charge. A positively charged ion is
called a cation. A negatively charged ion is called an anion. An ion occurs when the
total numbers of electrons and protons are not equal. For example, a neutral magnesium
atom has 12 protons and 12 electrons. The magnesium ion, however, has 12 protons, but
only 10 electrons. The net charge is 2+ and the formula for the ion is written as Mg2+.
This is an example of a monatomic ion (single atom ion). Groups of covalently bonded
atoms can also have a net charge. For example, the hydroxide ion consists of one atom
each of oxygen and hydrogen. The group as a whole has 9 protons and 10 electrons
giving a net charge of 1-. The formula of this polyatomic ion ("many" atom ion) is
written as OH-. Most polyatomic ions are anions; the ammonium ion, NH4+, is an
important polyatomic cation.
Naming monatomic ions follows some simple rules. The ionic forms of some transition
metals and the names and formulas of the polyatomic ions, however, must be memorized.
There are some helpful hints and trends for the monatomic ions to make learning easier.
Metals tend to form cations and nonmetals tend to form anions. The common charges of
monatomic main group ions are related to their position on the periodic table. All
elements in Group IA (the first column - the alkali metals) form cations with a 1+ charge.
All elements in Group IIA (the second column - the alkaline earth metals) form cations
with a 2+ charge. Metals in Group IIIA form cations with a 3+ charge. Elements in
Group VIIA (the halogens) form anions with a 1 charge. Elements in Group VIA (the
chalcogens) form anions with a 2 charge.
Most main group elements have only one monatomic ionic form. In such cases, the
monatomic cations are simply named with the element name followed by the word "ion".
For example Na+ is the sodium ion. The monatomic anions are named by changing the
suffix of the element name to "ide" and adding the word "ion". For example F- is the
fluoride ion and O2- is the oxide ion.
Most transition metals (and even some heavier main group elements), however, have
more than one possible monatomic ionic form. For example, iron has two ionic forms,
Fe2+ and Fe3+. These two forms are distinguished in the written names of the ions by
writing the charge as a Roman numeral in parentheses after the name of the element
(without a space). Thus Fe2+ is the iron(II) ion (pronounced "iron two ion"). This ion
was formerly called the ferrous ion from the Latin name for iron, ferrum. Many other
transition metals also have old names that are still used. For more examples see Table
3.1.
Ionic compounds consist of oppositely charged ions present in a ratio to yield a
uncharged substance. Usually only two kinds of ions are present. Occasionally three
kinds are present, but this is uncommon.
The nomenclature of the ionic compounds is simple. By convention, the name of the
cation is written first and the name of the anion is written second. The word "ion" is
omitted and the names are separated by a space. The same order (cation first, anion
second) applies to writing the chemical formulas of ionic compounds. The chemical
formula, however, also indicates the smallest whole number ratio between the ions as
subscripts. For example, table salt is composed of a 1:1 ratio of Na+ and Cl- ions. Its
name is sodium chloride and the formula is NaCl. (Note that omitted charge numbers
and subscripts imply a "1".)
Table 3.1
Name of Ion
Formula of Ion
Classic Name of Ion
copper(I) ion
Cu+
cuprous ion
copper(II) ion
2+
cupric ion
iron(II) ion
Fe2+
ferrous ion
iron(III) ion
Fe
3+
lead(II) ion
Pb2+
Cu
ferric ion
plumbous ion
4+
lead(IV) ion
Pb
plumbic ion
Au+
gold(I) ion
gold(III) ion
Au
aurous ion
3+
auric ion
Consider the following examples:
Example 1: Write the formula for magnesium chloride.
Solution: The cation is Mg2+, and the anion is Cl. When the anion and the cation are
combined, the charges must "cancel". In other words, the total number of positive
charges has to be the same as the total number of negative charges. The smallest whole
number ratio of cations to anions to cancel the charges can be found mathematically
using the least common multiple method. However, an easier way of accomplishing this
is to write the cation followed by the anion, and criss-cross the numbers where the
number (without its sign) of the charge on the cation becomes the subscript for the anion,
and the number of the charge on the anion becomes the subscript for the cation as shown
below:
Mg2+
Cl1

Mg1Cl2

MgCl2
Note that in writing the final formula, if there is no subscript, then the number is
understood to be 1.
Example 2: Write the formula for calcium phosphate.
Solution: The cation is Ca2+ and the anion is PO43. Use the criss-cross method to
determine the subscripts.
Ca2+
PO43

Ca3(PO4)2
Note that the polyatomic ion is in parentheses. Whenever you have more than one of a
polyatomic ion in a formula, enclose the polyatomic ion formula in parentheses and put
the subscript outside the parentheses.
Example 3: Write the formula for tin(IV) sulfide.
Solution: The cation is Sn4+ (remember the Roman numeral tells what the charge is on
the cation) and the anion is S2. Use the criss-cross method to determine the subscripts.
Sn4+
S2

Sn2S4
 SnS2
Notice that both of the subscripts are divisible by two. In writing formulas for ionic
compounds, you need to write the empirical formula with respect to the cation and the
anion. In other words, the subscripts must be in the lowest whole number ratio. In order
to accomplish this for tin(IV) sulfide divide the subscripts by 2 to get SnS2.
Example 4: Write the formula for iron(III) phosphate.
Solution: The cation is Fe3+ and the anion is PO43. You can do the criss-cross method if
you wish, but that is not necessary here. Since the numbers on the charges are the same
for the positive and the negative charge, you only need one of each (in other words, no
subscripts). Therefore, the formula is FePO4.
Example 5: Name the compound K2S.
Solution: The key to naming the compound is to identify the cation and anion in the
compound. If you cannot find a distinct anion and a distinct cation in the compound,
then the compound is not ionic. Another way to determine if the compound is NOT ionic
is that all of the atoms involved are non-metals or metalloids (unless it begins with NH4).
When attempting to identify the cation and the anion, remember that the cation is always
written first, and except for ammonium (NH4+), the cation only consists of one atom.
Here, the cation is K+, and the anion is S2. In this case, the charges are deduced by the
positions of potassium and sulfur on the periodic chart.
Once you have identified the cation and the anion, simply name the cation first, then the
anion (do not write the word "ion" in the name of the ionic compound). The name of K2S
is potassium sulfide.
Example 6: Name the compound Ba(NO3)2 .
Solution: Let's identify the cation and the anion. There are more than two different
atoms here. Since "NH4" is not in the formula, it is the anion that is polyatomic. The
cation is Ba2+, and the anion is NO3. The charge on barium must be 2+ since it is an
alkaline earth metal. The charge on nitrate must be memorized. The name of this
compound is barium nitrate.
Example 7: Name the compound CuBr2.
Solution: Identifying the anion is easy. The anion is Br. Since the cation is a transition
metal (or heavy main group metal), we need to look at the charge from the anion to help
us determine the charge on the cation. Each Br anion has a 1- charge. Since there are
two bromide ions, the total negative charge is 2-. This means that the cation(s) must have
a total charge of 2+. Since there is only one cation, the charge for each cation is 2+.
Therefore, the cation must be Cu2+, and the compound is named copper(II) bromide, or
cupric bromide.
A list of ions is provided for you to help you with this exercise.
Cations with only one possible charge:
Li+
Na+
K+
Mg2+
Ca2+
Sr2+
lithium ion
sodium ion
potassium ion
magnesium ion
calcium ion
strontium ion
Al3+
NH4+
Zn2+
Ag+
Ni2+
Ba2+
aluminum ion
ammonium ion
zinc(II) ion
silver(I) ion
nickel(II) ion
barium ion
Cations which can have more than one charge:
Fe2+
Fe3+
Cu+
Cu2+
Co2+
Co3+
iron(II) ion or ferrous ion
iron(III) ion or ferric ion
copper(I) ion or cuprous ion
copper(II) ion or cupric ion
cobalt(II) ion or cobaltous ion
cobalt(III) ion or cobaltic ion
Sn2+
Sn4+
Pb2+
Pb4+
Mn2+
Mn3+
tin(II) ion or stannous ion
tin(IV) ion or stannic ion
lead(II) ion or plumbous ion
lead(IV) ion or plumbic ion
manganese(II) ion or manganous ion
manganese(III) ion or manganic ion
Anions:
H
F
Cl
Br
I
O2
O22
S2
N3
NO3
NO2
CO32
hydride ion
fluoride ion
chloride ion
bromide ion
iodide ion
oxide ion
peroxide ion
sulfide ion
nitride ion
nitrate ion
nitrite ion
carbonate ion
C2H3O2
MnO4
SO42
HSO4
SO32
Cr2O72
CrO42
PO43
C2O42
CN
OH
ClO3
acetate ion
permanganate ion
sulfate ion
hydrogen sulfate ion
sulfite ion
dichromate ion
chromate ion
phosphate ion
oxalate ion
cyanide ion
hydroxide ion
chlorate ion
HCO3
ClO
hydrogen carbonate ion
hypochlorite ion
So far, only the naming of ionic compounds has been illustrated. Now let's discuss how
to name compounds where there are no distinct cations or anions, the covalent
compounds.
Naming Covalent Compounds
Covalent compounds are composed of atoms joined by the sharing of pairs of electrons,
called covalent bonds. Binary compounds, substances composed of two nonmetallic
elements, are the simplest covalent compounds. Usually, one of the elements is more
electronegative than the other. Fluorine is the most electronegative element. Generally,
the closer an element is to fluorine on the periodic table, the greater its electronegativity.
Thus, electronegativity tends to increase as you go up any given column or to the right on
any given row in the periodic table (ignoring noble gases).
In writing formulas and names, the less electronegative element is written first, followed
by the more electronegative element. In the name, the more electronegative element is
written as if it were an anion, so it has the "ide" suffix. The names of the elements are
separated by a space.
By the law of multiple proportions two elements may combine in more than one ratio to
form different compounds. For example, carbon and oxygen can form either CO or CO2.
These compounds must have different names. We use Latin prefixes to indicate the
number of atoms of each element type as shown in the table below (Table 3.2):
Table 3.2
Number of Atoms
1
2
3
4
5
6
7
8
9
10
Prefix
monoditritetrapentahexaheptaoctanonadeca-
If there is only one atom of the first element in the name / formula, the prefix "mono-" is
omitted. So CO is carbon monoxide and CO2 is carbon dioxide. Note the "o" in "mono"
is dropped before the word oxide.
Some compounds have a "common" name. For example, trihydrogen nitride is ammonia,
and dihydrogen monoxide is called water.
Example 8: Write the formula for carbon tetrafluoride.
Solution: Since the first atom has no prefix, there is only 1 carbon atom. The prefix
"tetra-", followed by fluoride, means there are four fluorine atoms. Therefore the formula
is written CF4.
Example 9: Write the formula for dinitrogen trioxide.
Solution: The first atom has a prefix "di-" meaning two, so there are two nitrogen atoms
that are written first. The prefix "tri-" means there are three oxygen atoms, so the
formula is written N2O3.
Example 10: Name the compound, IF3.
Solution: There is only one iodine, so we do not need the "mono-" prefix. For fluorine,
the prefix "tri-" is used since there are three of them. Also, fluorine is written like the
anion, fluoride. So the name of this compound is iodine trifluoride.
Example 11: Name the compound, P4S6.
Solution: There are four P atoms written first, so the first part of the name is
tetraphosphorus. The prefix "hexa-" is used since there are six of the second atom. Since
the second atom is sulfur, it is written as the anion, sulfide. So the name of this
compound is tetraphosphorus hexasulfide.
The tricky part about naming these compounds is classifying them as ionic or covalent.
Once you do that, you know which set of rules to apply. Use the following guidelines to
help you choose.
Composed of only nonmetals or metalloids Covalent
and no ammonium, NH4+ , is present
Metal and nonmetal
Ionic
Ammonium and a nonmetal
Ionic
Metal and a polyatomic ion
Ionic
Ammonium and a polyatomic ion
Ionic
Name contains prefixes like "mono",
"di", "tri", etc.
Covalent
Name has no prefixes and is not a
special name like water.
Ionic
If you can distinguish metals from nonmetals and can recognize the polyatomic ion
groups as formulas and names, the easier this will be.
Experiment 3 - Nomenclature of Compounds
REPORT
Name________________________________________
Section _________
25 bottles are set out for you with either the name or the formula on the label. Record this on the
table below. Then fill in the rest of the missing information. All of the compounds on this table
are ionic. See the example below.
#
EX.
1
Name
Formula
Cation
Anion
Color
sodium chloride
NaCl
Na+
Cl-
white
copper(II) bromide
2
PbCr2O7
3
Cr2(SO4)3
4
barium nitrate
5
silver(I) acetate
6
KClO3
7
CuCl
8
KMnO4
9
potassium nitrite
10
MnCl2
11
Ni(C2H3O2)2
12
ammonium sulfate
13
chromium(III) chloride
14
cobalt(II) nitrate
15
Na3PO4
16
potassium Iodide
17
cobalt(II) sulfate
18
iron(III) chloride
19
20
K2CO3
ammonium phosphate
21
ZnSO4
22
mercury(II) chloride
23
copper(I) sulfide
24
25
NaBr
iron(II) sulfate
Again, fill in the blanks in the table. This is a list of compounds that are found in the "real
world". Some of the compounds below are ionic and others are covalent. You must be able to
tell the difference.
#
Name
1
Formula
Real World Use
AgCl
self tint sunglasses
2
dinitrogen monoxide
laughing gas
3
diphosphorus pentoxide
drying agent
4
lithium carbonate
medication for bipolar disorder
5
6
NaHCO3
tin (II) fluoride
baking soda
fluoride in toothpaste
7
Sr(NO3)2
used in road flares
8
CaCO3
Antacids
9
magnesium sulfate
10
11
Epsom salts
CuSO4
cobalt(II) chloride
root eater
humidity indicator
12
(NH4)2CO3
smelling salts
13
SO2
pollutant in coal burning plants
14
barium sulfate
15
nasty drink before X-rays
K2CrO4
used in breathalyzers
16
dihydrogen monosulfide
rotten eggs smell
17
chromium(III) oxide
dye used to color money
18
lead(II) sulfate
used in lead acid batteries
19
Fe2O3
Rust
20
MgO
magnesium supplement
21
sodium hypochlorite
Bleach
22
calcium sulfate
plaster of Paris
23
CO2
bubbles in soft drinks
24
FeSO4
generic iron supplement
25
manganese(IV) oxide
used in flashlight batteries
26
tetraphosphorus trisulfide
used in match heads
27
silicon dioxide
sand (life's a beach)