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To use this method, the absolute value of the oxidation number of the first ion is used as the subscript of the second, and vice versa. The subscripts are then reduced by the greatest common factor. For instance, to find the formula for aluminium oxide:
- The oxidation number of Al is +3 and oxygen is -2. Criss cross the absolute values to give Al2O3
To find the formula for magnesium oxide:-
- The oxidation number of Mg is +2 and oxygen is -2. Criss cross the absolute values to give Mg2O2
- In this example there is a common factor of 2 so divide by 2 to give MgO.
It is a helpful method just cross them and put them down without changes.
- Sharon Bewick; Jonathan Edge; Therese Forsythe; Richard Parsons. CK12 Chemistry. CK-12 Foundation. pp. 348–349. GGKEY:J8HDA9JYCCN. Retrieved 25 March 2011.
- Formula Writing, SparkNotes
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