In the differences between Boron and Aluminium, it is given that boron forms stable Borates and Aluminium forms unstable aluminates .
Please explain the stability of the 2 compounds.
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Firstly, in both borate and aluminate; oxygen is covanlently bounded to boron and aluminium. Oxygen being more electronagative than any element in group 13 from which boron and aluminum belong to will always have delta (-).
Now, about the stability of both, as you go down the group 3, size increase down the group but positive charge (+3) remain the same, This implies that charge density decrease down the group 3. Charge density is about charge of an ion divided by the square of the ionic radius. Therefore, it implies that the polarizing ability of the metal decreases down the group 3. (This also means that, the metallic potential of Boron is greater than that of aluminium** taking into account polarizing ability***). Therefore, comparing their oxyanion compounds (borates and aluminates )which is stabilize mainly by polarising ability of each of the elements ( boron and aluminium) then, Borates will be more stability than aluminate.