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