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
Ex. Tin (IV) Chloride ~ Sn+4Cl-1------ SnO2
Write the symbol for the atom or polyatomic ion with the positive oxidation number first.
Follow this with the symbol of the atom or polyatomic ion with a negative oxidation number.
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)