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Class IX

Writing Chemical Formula of a Chemical Compound

How to write chemical formula, Steps to write chemical Formula
There are some RULES to be obeyed while writing the chemical formula of a chemical compound. These are as follows –



1.  When a compound consists of metal and non – metal, the name or symbol of the metal is written first.

E.g. – In Barium chloride, BaCl2
Barium is a Metal, hence written first.
Whereas, Chlorine is a Non – metal , hence written after metal.

2.    In compounds formed with polyatomic ions, the number of ions present is indicated by enclosing the formula of ion in a bracket and writing number of ions outside the bracket.

E.g. – In Calcium hydroxide, Ca(OH)2
Hydroxide ion, OH– is a polyatomic ion, so the symbol can’t be altered.
Here, the subscript shows that there are 2 hydroxide (OH–) ions are present in a molecule of calcium hydroxide.

3.     The valencies or charges on the ion must be balanced.

E.g. – In Calcium hydroxide, Ca(OH)2
Calcium is present as Ca2+ ion, hence it have +2 charge.
Hydroxide is present as OH–, hence it have –1 charge.
In order to maintain electrical neutrality of the compound,
2 OH– ions should be present, which have 2 x –1= –2 charge.
So, the total charge becomes ZERO or balanced.


 STEPS TO WRITE THE CHEMICAL FORMULA
1     1.    Write the symbol of the constituent elements / ions.
E.g. – In Magnesium chloride,
The constituent ions are magnesium (Mg2+) ion and chloride (Cl–) ion. Write symbols as shown WITHOUT showing charges.

Symbol       Mg              Cl
2      2.  Write the valencies of the constituent elements / ions below them.
E.g. – Write the valencies below the symbols as shown below –
         
              Symbol        Mg              Cl

              Valency        2                1

3      3.   Crossover the valencies of the combining atoms / ions and write the chemical                 formulae.
E.g. – Write the valencies below the symbols as shown below –
                    Hence, the chemical formula of Magnesium chloride is MgCl2
 Some other examples –

Practice Questions

I.            Write down the formulae of following chemical compounds –
                1.  Sodium oxide
               2. Potassium iodide
               3.  Lead iodide
               4.  Potassium nitrate
               5.  Copper sulphate
               6.  Silver bromide
               7.   Zinc nitrate
               8.   Barium chloride
               9.    Copper chloride
               10.   Aluminium sulphate

II.            Write the names represented by the following chemical formulae –
                                   AgNO3
                    BaSO4
                  AgCl

                                    Ca(NO3)2
                      H2S
                  NaNO3
                         NaHCO3
                         ZnCO3
                         FeO
                  CaCO3

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