Question

how do you find formulas for compounds?

Answer 1

Example,
If you were trying to find the formula for sodium carbonate

Sodium = Na+, Carbonate (which is a polyatomic ion) = CO3^2-

Since the sum of the oxidation numbers must be zero, we need two sodium atoms for one carbonate.

Na2CO3


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use the charges of the ions.

cancel out the charges by finding a common multiple of the + and - charges