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CBSE Class 9 Answered

in the rules while writing the chemical formulae why we should symbol of the fist and then symbol.please explain in detial
Asked by | 28 Feb, 2013, 10:38: PM
Expert Answer
Rule 1: CRISS-CROSS rule 

1. write the correct symbols of the elements and radicals 
2. Above each symbol, write the correct valence or oxidation numbers. 
Ex. Al+3 O-2 
3. To obtain the proper subscript, criss-cross the valence or oxidation 
number,and drop the algebraic sign. 
Ex. Al+3O-2 became Al2O3 


Rule 2: When the subscript is number 1, subscript is not written. 
Ex. Sodium Chloride- Na+1 Cl+1 --- NaCl 


Rule 3: When the oxidation # of both elements are numerically equal but grater than 1, the subscript are not also written. 
Ex. Calcium Oxide~ Ca+2O-2 ----- CaO 


Rule 4: All radicals tke more than once( the subscript is 2 or more) must be 
enclosed in Pahrentheses()


Ex. Ammonium Sulfate~~ NH4+1 SO4-2-----(NH4)2 SO


RULE 4: All subscripts must be reduced to lowest term (except for molecular or 
covalent copound).
 

Ex. Tin (IV) Chloride ~ Sn+4Cl-1------ SnO2

Answered by | 02 Mar, 2013, 07:55: PM
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