Writing Chemical Formulas and
Naming Chemical Compounds
Bell Work Mar 8, 2006
Transition metals
Metalloids
Nobel Gasses
Metals lose electrons
and become positive
ions .
Non-metals except Nobel Gasses gain electrons & become negative ions.
Main Group Elements are elements in group 1
& group 2; and groups 13 – 18.
Groups are columns. The rows are called periods.
Metals Non-metals
Metals (H is a non-metal)
Non-metals (includes the Noble Gases)
1+ 2+
Metals lose electrons to become
positive ions (cations)
Non-metals (excluding the Noble Gases) gain electrons to become negative ions (anions)
Stable Ion Charges 3+ 4- 3- 2- 1-
No
ions
form
Binary Ionic Compounds
• Binary ionic compound consists of two
different ions.
• The overall formula must show that the
compound is neutral
A+ + B- AB
For example, the formula for the
compound formed by the aluminum
ion, Al3+ and the oxide ion O2- is
determined as follows:
Al3+ O2-
Multiplying the charge by the subscript: Al3+ (2 x 3+ = 6+)
O2- (3 x 2- = 6-)
shows that the charge on Al3+ equals the opposite charge of O2-
Added together the charges equal zero. Therefore, the charges are balanced.
5. Check the subscripts to see they are at the smallest possible whole-number ratio and can not be further simplified.
The correct formula is therefore written as:
Al2O3
2 3 You must have the same number of positive
& negative charge in the final formula.
Determining Binary Ionic Compound Formulas
1. Write the ions with their charges side by side.
2. Always write the cation first, then the anion.
3. Cross over the charges by using the absolute
value of the ion charge as the sub-script for
each atom in the formula. Drop the sign.
Al3+ O2-
3 2
4. Check that the charges are balanced. •Multiply the charge by the subscript to see the absolute value of the charges are equal
4. If needed, simplify the formula to the lowest whole number ratio.
= Al2O3
Determining Binary Ionic Compound Formulas
Simply stated, cross over the charges
and the crossed over charges become
the formula subscripts:
Al3+ O2-
Becomes
Al2O3
Do not put the + or – sign in the compound’s formula
3 2
What is wrong with this formula:
Al4O6
• This formula is not in its smallest
possible whole number ratio.
• Al4O6 would be simplified by dividing by
the subscripts by a factor of two.
Al2O3 is the correct formula.
Common Monatomic ions
Naming binary compounds The nomenclature, or naming system, of binary ionic
compounds involves combining the names of the
compound’s positive and negative
The name of the cation is given first,
followed by the name of the anion.
NaCl
For most simple ionic compounds, the ratio of the
ion’s charge is not indicated in the compound’s name
because it is understood based on the relative
charges of the compound’s ions.
Name of cation Name of anion
Naming monatomic ions • Naming main group cations
• Main group elements that form monatomic cations
retain their elemental name.
Na+ is sodium,
Al3+ is aluminum.
Naming main group monatomic anions
The ending of the element’s name is dropped.
Then the ending -ide is added to the root name.
Fluorine becomes fluoride. Iodine becomes iodide.
Oxygen becomes oxide. Sulfur becomes sulfide.
Naming polyatomic bianary compounds
o The name of the cation is given first, followed
by the name of the anion.
AgNO3
is silver nitrate
oWhen more than one polyatomic ion is present
in a compound, the formula for the entire
polyatomic ion is surrounded by parentheses.
aluminum sulfate is Al2(SO4)3
Al 3+ (SO4) 2-
Silver (Ag+) Nitrate (NO3)-
Ag(NO3) = AgNO3
Polyatomic ions
NH4+
NO3-
OH-
SO42-
PO43-
MnO4-
CrO42-
Transition Metals With Multiple Charges • Many Transition Metals elements can form more than
one ion.
• For example copper can form : Cu+ & Cu2+
• Like all monatomic cations, Cu retains its elemental name, copper.
• If more than one charge is possible, the charge is indicated with a Roman Numeral.
• This called the Stock System.
• Cu+ is named copper(I)
• Cu2+ is named copper(II)
• Names of metals that commonly form only one cation such as zinc (Zn) and silver (Ag) do not include a Roman numeral.
If the element
only forms one
ion (see silver) do
not use a roman
numeral.
Sample Problem 7-3
• Write the formula for tin(IV) sulfate.
• Write the symbols for the ions side by side.
Write the cation first.
• Sn4+ (SO4 )2−
• Cross over the charges to give subscripts. Add
parentheses around the polyatomic ion if
necessary.
• Simplify the formula
Sn2(SO4)4 Sn(SO4)2
SAMPLE PROBLEM ONE
• Write the formulas for the binary ionic
compounds formed between the following
elements:
• a. zinc and iodine b. zinc and sulfur
• Write the symbols for the ions side by side. Write
the cation first.
• a. Zn2+ I−
• b. Zn2+ S2−
Cross over the charges to give subscripts. a. ZnI2
B. Zn2S2 = ZnS
Always simplyfy the formulas of ionic compounds
Writing Formulas with Transitions Metals • Write the formula for chromium (III) fluoride
• Write the symbols for the ions side by side.
Write the cation first.
• Cr3+ F−
• Cross over the charges to give subscripts.
• Cr13+ F3
−
• The formula is therefore CrF3.
• Check to see if the subscripts can be simplified.
Practice problems #3: Write the formulas for:
a. magnesium and iodine
b. potassium and sulfur
c. aluminum and chlorine
d. zinc & bromine
e. cesium and sulfur
f. calcium and nitrogen
g. chromium (II) and oxygen
Answers
a. MgI2
b. K2S
c. AlCl3
d. ZnBr2
e. Cs2S.
f. Ca3N2
g. CrO
Stock System of Nomenclature Some elements, such as iron, form two or more
cations with different charges. To distinguish the
ions formed by such elements, the Stock system
of nomenclature is used.
This system uses a Roman numeral to indicate
an ion’s charge. The numeral is enclosed in
parentheses and placed immediately after the
metal name.
Names of metals that commonly form only one
cation do not include a Roman numeral.
Naming binary compounds
The name of the cation is given first,
followed by the name of the anion.
FeCl3
Name of cation Name of anion
iron (II or III)? chloride
iron (III) chloride
`
If you look copper up on you ion sheet you will see
copper can form two ions.
But how do you determine if the copper in
CuCl2 is copper (I), Cu1+ , or copper (II), Cu2+ ?
How do you determine the Roman numeral to use
in naming the compound below
CuCl2
Cu2+ Cl1-
Check the ion table to see if there is indeed a
Cu2+.
Under the 2+ section of the d-block elements we find
Cu2+ and this ion is named copper (II).
The Roman numeral II means a charge of 2+
The compound is copper(II) chloride
Cross the subscripts back to the charge numbers
Cu1Cl2
Cation (positive) anion (negative)
` If you look iron up on you ion sheet you will not find
Fe1+ or S1-
How do you determine the Roman numeral to use
in naming the compound below
FeS
Fe1S1
Fe1+ S1-
`
If you look iron up on you ion sheet you
will see iron can form two ions.
But how do you determine if the
copper in FeS is iron (II), Fe 2+, or
iron (III), Fe3+ ?
How do you determine the Roman numeral to use
in naming the compound below
FeS
There is no main group element that commonly
forms more than one monatomic anion.
When a binary compound contains a d-
block element, do the following:
1. Determine the charge of the anion.
In this case our anion is S.
S is a non-metal.
All non-metal ions have a negative charge.
Look up S on the ion table and you will see
that it has a charge of minus two and is
written S2-.
2. Multiply the charge of the anion times
the subscript.
This will give you the total charge of the
anion.
In this case the formula is FeS. To find the
total charge of the anion multiply -2 x 1.
The total anion charge is negative two.
3. The charge of the cation must balance
with the charge of the anion.
For FeS, the total charge of the anion is 2-.
Therefore, the charge of the cation must
be 2+
4. Check the ion table to see if there is
indeed a Fe2+.
Under the 2+ section of the d-block
elements we find Fe2+ .
We also see that Fe2+ is named iron(II).
The Roman numeral II means a charge of
+2
Our compound is properly named iron(II)
sulfide
• Some d-block elements only form one ion
such as zinc.
• From the ion table we find zinc ion is Zn2+.
• If a d-block element only forms one ion,
such as zinc forming Zn2+, we do not
include the Roman numeral as part of
the compound’s name.
• The correct name for ZnCl2 is
• zinc chloride
Examples
a. FeF2
b. FeN
c. ZnS
d. CdO
e. CrN
f. NiBr2
Chromium(III) nitride
iron(III) nitride
Zinc sulfide
cadmium oxide
iron(II) fluoride
nickel(II) bromide
Stock System of Nomenclature Some elements, such as iron, form two or more
cations with different charges. To distinguish the
ions formed by such elements, the Stock system
of nomenclature is used.
This system uses a Roman numeral to indicate
an ion’s charge. The numeral is enclosed in
parentheses and placed immediately after the
metal name.
Names of metals that commonly form only one
cation do not include a Roman numeral.
Naming polyatomic bianary compounds
o The name of the cation is given first, followed
by the name of the anion.
AgNO3
is silver nitrate
oWhen more than one polyatomic ion is present
in a compound, the formula for the entire
polyatomic ion is surrounded by parentheses.
aluminum sulfate is Al2(SO4)3
Al 3+ (SO4) 2-
Silver (Ag+) Nitrate (NO3)-
Ag(NO3)
Polyatomic ions
NH4+
NO3-
OH-
SO42-
PO43-
MnO4-
CrO42-
Sample Problem 7-3
• Write the formula for tin(IV) sulfate.
• Write the symbols for the ions side by side.
Write the cation first.
• Sn4+ (SO4 )2−
• Cross over the charges to give subscripts. Add
parentheses around the polyatomic ion if
necessary.
• Simplify the formula
Sn2(SO4)4 Sn(SO4)2
Practice problems #7: writing formulas
for polyatomic compounds
1. Write formulas for the
following ionic
compounds:
a. sodium carbonate
b. copper(II) nitrate
c. calcium nitrite
d. calcium sulfate
e. ammonium phosphate
f. potassium perchlorate
g. aluminum phosphate
a. Na2CO3
b. Cu(NO3)2
c. Ca(NO2)2
d. CaSO4
e. (NH4)3PO4
f. KClO4
g. AlPO4
Answers
2. Give the names for
the following
compounds:
a. Ca(OH)2
b. KClO3
c. NH4OH
d. FeCrO4
e. Fe3(PO4)2
f. Fe2(SO4)3
a. calcium hydroxide
b. potassium chlorate
c. ammonium hydroxide
d. iron (II) chromate
e. iron (II) phosphate
f. iron (III) sulfate
naming polyatomic compounds
Answers
Naming Binary Molecular Compounds
o Unlike ionic compounds, molecular
compounds are composed of individual
covalently bonded units, or molecules.
o Chemists use two nomenclature systems
to name binary molecules.
o The naming system that we will study is
based on prefixes
Naming Binary Molecular Compounds molecular compounds are composed of non-metals Examples: H2O, CO2 , CO, CCl4 , P2F3
The rules for the prefix system of nomenclature of
binary molecular compounds are as follows.
1. The element more to the left on the P-table is written first.
• This element keeps its Periodic-table name.
• C in CO2 is called “carbon
The first element is given a prefix only if there are
more than 1 of them in the formula (example P2F3)
The C in CO2 is not mono carbon, just carbon.
2. The second element is named by combining:
(a) a prefix indicating the number of atoms in the formula
(b) the name of the second element ends with “ide”
The “O” in CO2 is called “oxide”.
Because there are two “O”s it is called carbon dioxide
.
• 3. The o or a at the end of a prefix is usually
dropped when the word following the prefix
begins with another vowel, e.g., monoxide or
pentoxide.
• The prefix system is illustrated below ( write 1- 4)
• The prefix system is illustrated further in
Table 7-4, which lists the
• names of the six oxides of nitrogen.
• Note the application of rule 1, for
• example, in the name nitrogen dioxide for
NO2.
– No prefix is needed with nitrogen because only
one atom of nitrogen, the less-electronegative
element, is present in a molecule of NO2.
• On the other hand, the prefix di- in
dioxide is needed according to rule 2 to
indicate the presence of two atoms of
the more-electronegative element,
oxygen.
• Take a moment to review the prefixes in
the other names in Table 7-4.
SAMPLE PROBLEM 7-4
• a. Give the name for As2O5.
• b. Write the formula for oxygen
difluoride
SAMPLE PROBLEM 7-4 Answer
• a. A molecule of the compound contains two arsenic
atoms, so the first word in the name is “diarsenic.”
• The five oxygen atoms are indicated by adding the
prefix pent- to the word “oxide.”
• The complete name is diarsenic pentoxide.
• b. OF2
SAMPLE PROBLEM 7-4 b. The first symbol in the formula is that for oxygen.
Oxygen is first in the name because it is less
electronegative than fluorine.
Since there is no prefix, there must be only one
oxygen atom.
The prefix di- in difluoride shows that there are two
fluorine atoms in the molecule.
The formula is OF2.
PRACTICE #7, PROBLEMS 1. Name the following binary molecular
compounds:
a. SO3
b. ICl3
c. PBr5
d. PF5
e. XeF4
f. CCl4
1. Answers:
a. sulfur trioxide
b. iodine trichloride
c. phosphorus pentabromide
d. phosphorus pentafluorode
e. xenon tetrafluoride
f. carbon tetrachloride
Practice 2. Write formulas for the
following compounds:
a. carbon tetraiodide
b. phosphorus trichloride
c. dinitrogen trioxide
d. dinitrogen pentoxide
a. Cl4
b. PCl3
c. N2O3
d. N2O3
Answers