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 SO4 
RULE 4: All subscripts must be reduced to lowest term (except for molecular or 
covalent copound). 

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

Write the symbol for the atom or polyatomic ion with the positive oxidation number first. 

Ex. Al+3 

Follow this with the symbol of the atom or polyatomic ion with a negative oxidation number. 

Ex. I-1 

Use subscripts to make the sum of the oxidation numbers equal to the charge of ions. 

Ex. Al+3 and I-1 

We write AlI3

When a subscript is used with an ion containing two or more elements, the ion must be enclosed in parentheses () before writing the subscript. 

Ex. Mg+2 and NO3-1 

is written as Mg (NO)