Give the decreasing order of the basic properties of oxides.
a. `Tl_(2)O` , b. `Al_(2)O_(3)` , c. `Tl_(2)O_(3)` , d. `Ga_(2)O_(3)`

Explanation

Anions are same, so check the charges on the cation. The lesser the charge on the ions, the more ions or more basic in the nature.
a.
`overset(+1 xx 2)(Tl_(2))overset(-2)(O)` , b. `overset(+3 xx 2)(Al_(2))overset(-2 xx 3)(O_(3))`, c. `overset(+3 xx 2)(Tl_(2))overset(-2 xx 3)(O_(3))` , d. `overset(+3 xx 2)(Ga_(2)) overset(-2 xx 3)(O_(3))`
Charges on the cations in b., c., and d. are same so check the sizes of the cations. Large cation, more ionic and more basic. All of them belong to group 13. Therefore, size of `Ti^(3+) gt Ga^(3+) gt Al^(3+)`.
Hence decreasing basic oxides :
`Tl_(2)O gt Tl_(2)O_(3) gt Ga_(2)O_(3) gt Al_(2)O_(3) (a gt c gt d gt b)`