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 member of the second `(4d)` serie
(ii) Because oxygen and fluorine are highly electronegative elements and small in size.

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 .