CHEMICAL FORMULAE
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A chemical formula represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule.
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While writing the formula of a molecular compound the less electronegative non-metal element is written on the left hand side whereas the more electronegative non-metal element is written on the right hand side.
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In naming the molecular compounds, the name of less electronegative non-metal is written as such but the name of more electronegative non-metal is changed to have the ending ‘ide’.
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When there are more than one atoms of an element in a molecular compound, then ‘the number of atom’ is indicated by using appropriate ‘prefixes’ in the formula.
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While writing the formula of ionic compounds as a convention, the cation (positive ion) is written on the left hand side and the anion (negative ion) is written on the right had side.
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The number of cations and anions is adjusted in such a way that the total number of positive valencies of cations becomes equal to the total number of negative valencies of anions
Formulae of compounds:
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The chemical formula of a compound is a symbolic representation of compound which tells about the number of atoms of each element present in the compound.
For example:
H2O 2 atoms of Hydrogen and 1 atom of Oxygen
SO2 2 atoms of Sulphur and 2 atoms of Oxygen
NH3 1 atom of Nitrogen and 3 atoms of Hydrogen
HCl 1 atom of Hydrogen and 1 atom of chlorine
C2H5OH 2 atoms of carbon, 6 atoms of Hydrogen and 1 atom of oxygen
CCl4 4 atoms of chlorine and 1 atom of carbon
Writing chemical formula of a molecular compound:
Step 1:
Write the symbols of the elements present in the compound.
Example:
Aluminium chloride
Aluminium [Al] Chlorine [Cl]
Step 2:
Write the valency of each element.
(Cross over of valency of combining atoms)