Question

what is the structure of K3CrO8 AND WHAT IS THE OXIDATION NUMBER OF Cr here ....

Answer 1

The structure of K3CrO8 is Tetragonal.

Let oxidation number of Cr = x

3 + x + 8 * ( - 2 ) = 0

3 + x - 16 = 0

x = + 13

x = + 5

but since chromium can have the max of only + 5 oxidation state. 
This contrary is solved by the bonding pattern as shown in structure.
The Cr is attached to only 5 oxygen through single bond hence oxidation state is + 5.

Answer 2

Answer:

The structure of K3CrO8 is Tetragonal.

Let oxidation number of Cr = x

3 + x + 8 * ( - 2 ) = 0

3 + x - 16 = 0

x = + 13

x = + 5

but since chromium can have the max of only + 5 oxidation state.

This contrary is solved by the bonding pattern in structure.

The Cr is attached to only 5 oxygen through single bond hence oxidation state is + 5.