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Writing Chemical Formula

For writing the chemical formula of any compound, the following should be known:

(a) symbol of the elements,
(b) formula of the radicals,
(c) valency of the element and radical.

This activity should be done in groups. Prepare two sets of placard one with the symbols and the other set with the valencies of the elements. Each student should hold two placards one in the right hand and the other in the left hand. The right hand card should contain symbol placard and left hand should contain valency placard. The students should criss-cross their valencies keeping the symbol in the place. This will give the formula of a compound.

Steps involved in writing chemical formula:

Write the symbol of the constituent elements or radicals present in the compound.

Example : Silver Chloride – AgCl


Example : Ag1Cl1
Criss-cross the valencies and write them at the bottom of the element or radical as subscripts.


Example : Ag1Cl1 or AgCl
If the valency number is divisible by a common number, divide and simplify the formula.


Example : C2O4 can be simplified as CO2.If the radical is also a part of the compound, and two or more units of the radical are involved, then the radical part of the compound is enclosed in brackets and the number representing the units of the radical (valency of the metal) is shown on the outside of the bracket on the right side. 


Example : In zinc nitrate, valency of zinc is 2 and that of nitrate radical is 1. The formula of the compound is Zn(NO3)2.

In any formula, write the metallic portion of the compound first, and then the non-metallic portion.
 Mg NO3
 Mg2 NO31
 Mg(NO3)2Þ Mg(NO3)2
Formula Þ Mg(NO3)2

Formula for sodium chloride
The charges on Na and Cl atoms are shown as follows:

In order that the total positive charge may be equal to the total negative charge in the molecule, the formula for sodium chloride will be NaCl.


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