Question

formula and valency of aluminate

Answer 1

=> AlO₂ is the formula of aluminate.
=> -1 is the combining capacity or valency of aluminate.
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Answer 2

The formula for aluminate ion is $\bold{\mathrm{AlO}_{2}}$.  

Here Al is bonded with two O atoms with -1 charge on overall ion. Hence the formula can be written as $\left[\mathrm{AlO}_{2}\right]^{-1}$.

Here Al is bonded with two O atoms; one of the O is bonded with double bond and another is bonded with single bond hence the valency of Al in aluminate ion will be 3 here as it is bonded with three bonds.