How would you account for the following?
(i)The atomic radii of the metals of the third (5d) series of transition elements are virtually the same as those of the corresponding members of the second (4d) series.
(ii)The `E^(@)` value for the `Mn^(3+)//Mn^(2+)` couple is much more positive than that for `Cr^(3+)//Cr^(2+)` couple or `Fe^(3+)//Fe^(2+)` couple.
(iii)The highest oxidation state of a metal is exhibited in its oxide or fluoride.

Answer

(i) it the second transition series , the number of shells increases and so does the atomic size, However, in the third series `(5d series)` , the atomic size is more or less the same . This is known as lanthanide contraction. The is due to the poor shielding effect by f-electrons.
(ii) Oxygen and fluorine act as strong oxidising agent because of their high electronegativities and small sizes . Hence , they bring out the highest oxidation state form various metals. In other words, a metal exhibits highest oxidation state in its oxides and fluorines. For example , in `OsF_(6)` and `V_(2)O_(5)`, the oxidation states of `Os` and `V` are `+6` and `+5` respectively .