Naming CompoundsWriting Formulas

and Equations

Larry Scheffler

Lincoln High School

The chemical formula represents the composition of each molecule.

In writing the chemical formula, in almost all cases the element farthest to the left of the periodic table is written first.

So for example, the chemical formula of a compound that contains one sulfur atom and six fluorine atoms is SF6.

If the two elements are in the same group, the symbol of the element of that is lower in the group (i.e. heavier) is written first e.g. IF3.

Naming Compounds

Ionic compounds are combinations of positive

and negative ions.

In writing the chemical formula, the positive ion is written first, It is then followed by the name of the negative ion.

Monatomic anions end in ide. Special endings

apply for polyatomic ions

ExamplesNaCl Sodium chloride

BaF2 Barium Fluoride

ZnO Zinc Oxide

Naming Ionic Compounds

Names of Polyatomic Ions with Oxygen

Polyatomic ions usually contain oxygen in addition to another element. Normally, they have a negative charge.  They end in either "ate" or "ite" depending on the number of oxygen atoms present.

ClO- hypochlorite

ClO2- chlorite

ClO3- chlorate

ClO4- perchlorate

NO2- Nitrite 

NO3- Nitrate 

PO33- phosphite 

PO43- phosphate 

SO32-

SO42-

sulfite sulfate 

Polyatomic Ion -- Exceptions

Most polyatomic ions contain oxygen– Their names end in “ite” or “ate”.

There are several exceptions:OH- hydroxide

CN- cyanide

SCN- thiocyanate

Elements with Multiple CationsWhen an element can form more than one cation, a Roman numeral is used to distinguish the oxidation state of the compound.Iron, Tin, Lead, Copper, and are common elements with more than one cation. Examples

1. PbSO4  =  lead (II) sulfate   This compound is formed from Pb2+ and 

SO42-

2. Pb(SO4)2 =  lead (IV) sulfate   This compound is formed from Pb4+ and 

SO42-

3. Fe(OH)2  =  iron (II) hydroxide   This compound is formed from Fe2+

and  OH- 4. Fe(OH)3  =  iron (III)  hydroxide    This compound is

formed from Fe3+ and  OH-

Examples of Ionic Compounds

1. NaCl = Sodium chloride

2. ZnF2 = Zinc fluoride3. KOH = Potassium hydroxide

4. Ca(NO3)2 = Calcium nitrate

5. BaSO3 = Barium Sulfite

6. Al2(SO4) 3 = Aluminum sulfate

7. Ca3(PO3)2 = Calcium phosphite

8. NH4Cl = Ammonium chloride

9. (NH4)2CO3 = Ammonium carbonate

When naming covalent compounds, the name of the first element in the formula is unchanged.

The suffix “-ide” is added to the second element.

Often a prefix to the name of the second element indicates the number of the element in the compound

Covalent molecules use prefixes; Ionic molecules use Roman Numerals (most of the time).

Examples:

SF6 – sulfur hexafluorideP4O10 – tetraphosphorous decoxideCO – carbon monoxideCO2 – carbon dioxide

Naming Covalent Compounds

Greek Prefixes:Mono-Di-Tri-Tetra-Penta-Hexa-Hepta-Octa-Nona-Deca-

Naming Covalent Compounds

Covalent molecules with multiple possibilities

Example of exceptions:

(not normally done, but you may come across)– A Roman Numeral is used to indicate the state of the more

positive element

1. N2O   =  Nitrogen (I) oxide   Since oxygen has a 2- charge, the nitrogen must be 1+ to  balance the charges.    Also known as dinitrogen monoxide

2. N2O3 =  Nitrogen (III) oxide    Since oxygen has a 2- charge, the nitrogen must be 3+ to balance the charges.  Also  known as dinitrogen trioxide

The binary compounds of hydrogen are special cases. They were discovered before a convention was adopted and hence their original names have stayed.

Hydrogen forms binary compounds with almost all non-metals except the noble gases. Examples

HF - hydrogen fluorideHCl - hydrogen chloride H2S - hydrogen sulfide

Water H2O is not called dihydrogen monoxide

Binary compounds of Hydrogen

• When many hydrogen compounds are dissolve in water they take on the form of an acid. Special rules apply to acids.

• The “ide” suffix becomes “hydro” “-ic”, • The “ite” suffix becomes “ous” • The “ate” suffix becomes “ic”

Acids

Cl- Chloride HCl Hydrochloric Acid

NO2- Nitrite HNO2 Nitrous Acid

NO3- Nitrate HNO3 Nitric Acid

SO32- Sulfite H2SO3 Sulfurous Acid

SO42- Sulfate H2SO4 Sulfuric Acid

PO33- Phosphite H3PO3 Phosphorous Acid

PO43- Phosphate H3PO4 Phosphoric Acid

CO32- Carbonate H2CO3 Carbonic Acid

Writing Formulas for Ionic Compounds

Write the positive ion (cation) first, then the negative ion.The positive charges must balance the negative charges. Use subscripts to show how many times each ion must appear in order for the charges to balance. A subscript is not used if the ion appears only once. (Crisscross Method)Use parenthesis around polyatomic ions that appear more than once in the formula

Examples

1. Na+ and Cl- = NaCl

2. Zn2+ and Br- = ZnBr2

3. K+ and OH- = KOH

4. Ca2+ and OH- = Ca(OH)2

5. Fe2+ and SO42- = FeSO4

6. Fe3+ and SO42- = Fe2(SO4) 3

7. Ca2 + and PO43- = Ca3(PO4)2

8. NH4+ and Cl- = NH4Cl

9. NH4+ and CO3

2- = (NH4)2CO3

Diatomic Molecules

Certain elements exist as diatomic molecules in nature

The “Seven at 7”– Starting at #7, in the

shape of a 7 (sort of), there are 7 elements

H2 Hydrogen

N2 Nitrogen

F2 Fluorine

O2 Oxygen

I2 Iodine

Cl2 Chlorine

Br2 Bromine

Diatomic Molecules Certain elements exist as diatomic molecules in nature

The “Seven at 7”– Starting at #7, in the

shape of a 7 (sort of), there are 7 elements

H2 Hydrogen Have

N2 Nitrogen No

F2 Fluorine Fear

O2 Oxygen Of

I2 Iodine Ice

Cl2 Chlorine Cold

Br2 Bromine Bunnies