Question

What are the general rules for writting a chemical formula?Explain with example
Class 9 question mizoram ​

Answer 1

Explanation:

Rules for writing a chemical formula

Write down the symbols of the elements / ions, which combine to form a molecule of the compound, side by side. While writing the formula of a compound containing a metal and a non-metal, the symbol of the metal is written first followed by that of the non-metal. Two metals do not join up to form compounds.

Write the valency of each element / ion.

Interchange the valencies of the elements / ions and write as the subscript. Write them close together and ignore ‘1’ to obtain the formula.

The radical / polyatomic ions must be written in brackets, before the subscript is written. In case the number of polyatomic ion is one, the bracket is not required. For example – NaOH.

Whenever applicable, write simple ratios of the valencies.

The final formula must be written without any sign of the charge.

The following examples will help you to acquire better knowledge about writing the formulae of compound.

Water contains the elements hydrogen and oxygen.

(a) Write the symbol. H O

(b) Write the valency of each element 1 2

(c) Interchange the valencies of the elements and write as the subscript. H2 O1

(d) Write them close together and ignore ‘1’ to obtain the formula. H2O

Hydrogen sulphide contains the elements hydrogen and sulphur.

(a) Write the symbol. H S

(b) Write the valency of each element 1 2

(c) Interchange the valencies of the elements and write as the subscript. H2 S1

(d) Write them close together and ignore ‘1’ to obtain the formula. H2S

Hydrogen chloride contains the elements hydrogen and chlorine.

(a) Write the symbol. H Cl

(b) Write the valency of each element 1 1

(c) Interchange the valencies of the elements and write as the subscript. H1 Cl1

(d) Write them close together and ignore ‘1’ to obtain the formula. HCl

Carbon tetrachloride contains the elements carbon and chlorine.

(a) Write the symbol. C Cl

(b) Write the valency of each element 4 1

(c) Interchange the valencies of the elements and write as the subscript. C1 Cl4

(d) Write them close together and ignore ‘1’ to obtain the formula. CCl4

Carbon dioxide contains the elements carbon and oxygen.

(a) Write the symbol. C O

(b) Write the valency of each element 4 2

(c) Interchange the valencies of the elements and write as the subscript. C2 O4

(d) Write simple ratios of the valencies. C1 O2

(e) Write them close together and ignore ‘1’ to obtain the formula. CO2