Question

In aluminate ion, the coordination number of aluminium is:

• A. 2
• B. 4
• C. 6
• D. 1

Answer 1

The correct option is C 6

Aluminate ion isolated is $Al{O}_2^{-}$.
When in solution, this is surrounded by 4 water molecules, two water molecules react with the oxygen to give overall 4 $O{H}^{-}$ groups. These four hydroxyl groups and two water molecules are held by coordinate bonds in the complex.
Hence the compound has 6 coordinate bonds.