Naming Ionic and Molecular Compounds and Writing Formulas

A compound is made of two or more elements.

The name should tell us how many and what type of atoms there are.

We have already examined two types of compounds:

A. Ionic compounds


I. Naming Monoatomic Ions


- Representative metal - if the charge is always the same (Group A) just write the name of the metal. (It does not change)

- Transition metals can have more than one type of charge. Indicate the charge with roman numerals in parenthesis.


Name these:

Na+1                                                    Al+3                                                      Fe+3

Fe+2                                                     Pb+2                                                     Li+1


Write Formulas for these:

Potassium ion                                             Magnesium ion                           Copper (II) ion

Chromium (VI) ion                                    Barium ion                                   Mercury (II) ion



      Change the element ending to ide

      Ie. Fluorine (F) would become Fluoride (F-1)


Name these:

Cl -1                                                         N -3                                                      Br -1                

O -2                                                                        Ga+3                                                    S -2


Write Formulas for these:

Sulfide ion                                           Iodide ion                               Phosphide ion


II. Naming Polyatomic Ions

Polyatomic ions are groups of atoms that stay together and have a charge.

You must memorize the following common polyatomic ions:


Nitrate NO3-1                           Sulfate  SO4-2                             Phosphate PO4-3

Nitrite  NO2-1                           Sulfite SO3-2                               Phosphite  PO3-3    


Carbonate CO3-2                       Hydroxide OH-1                              Ammonium NH4+1


III. Naming Binary Ionic Compounds

Binary Compounds consist of 2 elements. Simply name the two ions.

Representative elements:

Ex. 1.         NaCl = Na Cl - = sodium chloride

Ex. 2        MgBr2 = Mg+2  Br - = magnesium bromide


Transition metals:

-   Need to first figure out their charges, then indicate the charge in brackets following the name of the metal.

-   The compound must be neutral (same total number of + and charges).

-   Use the anion to determine the charge on the positive ion.

 Examples: Naming Binary Ionic Compounds Containing Transition Metals

1.      Write the name of CuO              - Need the charge of Cu

                - The charge of O is -2

                - Therefore copper must be +2

                - Name: Copper (II) chloride


  2.   Name CoCl3                                 - Cl is -1 and there are three of them = -3

                - Co must be +3

                - Name:  Cobalt (III) chloride


3.      Name Cu2S                               - Since S is -2, the Cu2 must have an overall charge of +2, so each ion is +1.

                - Name: Copper (I) sulfide


4.      Fe2O3                                         - Each O is 2;  therefore 3 x -2 = -6 charge

                - 3 Fe must  = +6, so each Fe ion is +2.

                - Name: Iron (III) oxide


Write the names of the following binary ionic compounds:

1.      KCl

2.      Na3N

3.      CrN

4.      Sc3P2

5.      PbO

6.      PbO2

7.      Na2Se


IV. Naming Ternary Ionic Compounds

Ternary Compounds consist of 3 or more elements. Name the two ions. At least one of the ions will be polyatomic note: the suffix of the polyatomic ion does not change.  Example: K3PO3 is potassium phosphite


Write the names of the following ternary ionic compounds:

8. NaNO3      
9. CaSO4
10. CuSO3
11. (NH4)2O
12. LiCN
13. Fe(OH)3
14. (NH4)2CO3
15.  NiPO4


V. Writing Chemical Formulas

A chemical formula shows the kind and number of atoms in the smallest piece of a substance.


- The overall charge of the compound is zero.

- Balance the charges by adding subscripts.

- Put polyatomic ions in brackets when using a subscript.

    Example: Write the formula for calcium chloride.

                        Calcium is Ca+2,  Chloride is Cl-1,

                        Ca+2 Cl-1  would have a +1 charge, therefore another Cl-1 is needed

                        Chemical formula: CaCl2


Write the chemical formula of the following compounds:

1.      Lithium sulfide

2.      tin (II) oxide

3.      tin (IV) oxide

4.      Magnesium fluoride

5.      Copper (II) sulfate

6.      Iron (III) phosphide

7.      gallium nitrate

8.      Iron (III) sulfide

9.      Ammonium chloride

10.  ammonium sulfide

11.  barium nitrate


Things to look for

- If cations have brackets in the name, the number is their charge.

- If anions end in -ide they are probably off the periodic table (Monoatomic)

- If anion ends in -ate or -ite it is polyatomic


B. Molecular compounds

        - Made of molecules.

        - Made by joining nonmetal atoms together.


I. Naming Molecular Compounds

The name of a molecular compound indicates the type and number of atoms. Prefixes are used to tell you the number. You must memorize the following prefixes.



1 mono-    6 hexa-
2 di-   7 hepta-
3 tri-      8 octa-
4 tetra-   9 nona-
5 penta-   10 deca-



To name a molecular compound, write two words:

Prefix + Name of first nonmetal           +          Prefix + Name of second nonmetal + ide



- dont write mono- if there is only one of the first element.

         - dont use the following double vowels when writing names (oa oo)



Name these molecular compounds:

1.  N2O

2.  NO2

3.  Cl2O7

4.  CBr4

5.  CO2

6.  BaCl2


Write formulas for these:

1. diphosphorus pentoxide

2. tetraiodide nonoxide

3. sulfur hexaflouride

4. nitrogen trioxide

5. Carbon tetrahydride

6. phosphorus trifluoride

7. aluminum chloride