Name the oxometal anions of the first series of the transition metals in which the metal exhibits the oxidation state equal to its group number.

Explanation

In `Cr_(2)O_(7)^(2-)` and `CrO_(4)^(2-)` the oxidation state of `Cr` is `+6(2X+7)(-2)=-2` and `x+4(-2)=-2`. The group number is equal to the number of electrons in `(n-1)` d and ns Sub-shells for d-block elements. So group no. `=5+1=6`
In `MnO_(4)^(-)` the oxidation state of `Mn` is +`7 (2x+7)(-2)=-1`. the group number is eqyal to the number of electron in `(n-1)`d and ns Sub-Shells. for d-block elements., SO group no. `5+2=7`