Here's why:

* Aluminate is an anion (negatively charged ion) formed from aluminum and oxygen.

* Aluminum has a +3 charge (Al³⁺) and oxygen has a -2 charge (O²⁻).

* To balance the charges, you need two oxygen atoms to combine with one aluminum atom. This gives you the formula AlO₂⁻.

Note: Aluminate can also exist as a complex ion, like [Al(OH)₄]⁻, but the fundamental radical is AlO₂⁻.