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Criss-cross method for writing chemical formulas, variable valencies of transition metals, and radical charges for Grade 9 students.
Chemical formula writing in Grade 9 Chemistry uses the criss-cross method of valencies.
Write the symbols side by side with valencies below ($Al^{3+}$ and $O^{2-}$). Cross the numbers to get $Al_2O_3$. For polyatomic radicals, always enclose the radical in parentheses before applying subscript numbers (e.g., $Ca(OH)_2$ and $Al_2(SO_4)_3$).
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