Prediction of formulae

For an element in its highest oxidation number (and sometimes others), it is possible to predict the formula of its orthoacid from coordination number considerations. The condensed oxoacid formulae are then easily derived by subtracting the appropriate number of water molecules. 

Third- and fourth-row elements prefer four-coordination in their oxoanions. For example, using the oxidation number approach, we can see that when phosphorus(V) coordinates four 0 ions to form a complex ion of stoichiometry PO4, the resultant charge on this oxoanion is (+5 - 8) = -3. Thus, the formula of the corresponding neutral oxoacid is H3PO4. 

Sulfur(VI) coordinates four 0 ions to form a complex ion of stoichiometry SO4, with resultant charge (+6 - 8) = -2, so the oxoacid is H2SO4. 

The chlorine(VII) oxoanion, with stoichiometry CIO4, has charge (+7 - 8) = -1, so only one proton is required for formation of the neutral oxoacid HCIO4. 

For oxoanions of the second period, (e.g. carbonate, COf ) three-coordination is the norm as it allows for the formation of a planar assembly with extensive n bonding. 

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