What are the general rules for writing a chemical formula? Explain with examples.
Answers
Answer:
Three rules for writing chemical formula
- Identify the cation and anion present in the chemical for which the formula must be written. Write down the ions with their respective charges.
- Balance the charges by criss-cross multiplication such that the cation receives the charge of anion and vice-versa.
- Write the cation first followed by the anions.
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Answer:
General rules
- 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.
Examples
1) 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
2 ) 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