Question

In Aluminate ion, the coordination number of Aluminium is :
A. $2$
B. $4$
C. $6$
D. $1$

Answer 1

Hint: We know that coordination number refers to the number of attached atoms, molecules and ions to the central metal atom. Aluminate ions are composed of more than one element so we can say that it is a polyatomic ion. It contains an aluminium atom and two oxygen atoms. The overall charge on aluminate ions is $- 1$. Aluminate ions have the chemical formula $Al{O_2}^{ - 1}$.

Complete answer:
We can get the coordination number of aluminate ions by observing its solution in water, means by calculating the number of bonds aluminate ions make in its aqueous solution, as we know that coordination number is the number of ions are atoms surrounding the centre metal atoms. In the solution aluminate ion is surrounded by four water molecules. Two water molecules react with oxygen to give four hydroxide groups. Now these two molecules of water and four hydroxide ions are held by coordination bonds in the complex. The compound has six coordinate bonds. Hence the coordination number of aluminium is $6$. So here option C is the correct answer to this problem that is $6$.

Note: Aluminate ion reacts with metals and gives useful compounds. Aluminates are used in a variety of purposes like in cements, aluminizing processes. We have also calculated the coordination number of aluminate ions by calculating the coordinate bond that aluminates make in the water solution. It exists in aqueous solution as ${[Al{(OH)_4}{({H_2}O)_2}]^ - }$. Hence coordination number of aluminate ions is six because it is surrounded by four water molecules which leads to formation of coordinated bonds of two water molecules and four hydroxyl groups.