What are the rules for writing down the chemical formula of a compound? Form the formula of ‘Hydrogen chloride’ and ‘Sodium carbonate’ by crossing over their valencies.
Answers
Answer:
Chemical formula is a symbolic representation of a molecule’s composition. It indicates –
(a) the elements that are present in the compound and
(b) the ratio of these elements in the compound.
A chemical formula is also called molecular formula. The symbols in the molecular formula tell us about the elements and the subscript tells us how many atoms of that element are present in one molecule.
For example, a molecule of water (H2O), consists of two hydrogen atoms and one oxygen atom. It is therefore represented by the chemical formula H2O. Lack of a subscript on the O indicates one atom of O per water molecule. Sodium chloride (common salt) contains equal number of sodium and chlorine atoms and is represented by the formula, NaCl. Sulphuric acid, H2SO4 contains three elements – hydrogen, oxygen and sulphur in ratio 2 : 4 : 1.
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.
Explanation:
The valency of sodium is +1 and that of carbonate ion is −2. To form chemical formula valency will cross multiply and will result in Na2CO3
The valencies of both hydrogen and chlorine are 1. When we cross-multiply the valencies, we get H
1
Cl
1
. So, we can write the chemical formula as HCl.
H Cl
1 × 1
H
1
Cl
1
HCl
