i. The oxidation states of an element depends on its electronic configuration and the number of unpaired electrons.

ii. The electronic configuration of Fe, Co and Ni are as follows:

iii. Hence, the expected oxidation states of Fe, Co and Ni shall be +8, +9 and +10 respectively.

iv. Fe by the loss of two electrons from 4s subshell forms Fe²⁺ and further loss of one more electron from 3d-orbital forms Fe³⁺, which is very stable due to half filled orbital, 3d⁵ . Hence, there is no further loss of electrons from the 3d subshell and thus Fe does not show +8 oxidation.

v. Co forms Co²⁺ due to the loss of two electrons from 4s subshell and further loss of one more electron from 3d-orbital forms Co³⁺. In cobalt, +2 and +3 oxidation states are more stable and it does not lose electrons further. Hence, Co does not show +9 oxidation state.

vi. Ni forms Ni²⁺ by the loss of two electrons from 4s-orbital and by the loss of one more electron from 3dorbital, it forms Ni³⁺. The amount of energy required for the ionization of Ni to Ni²⁺ is less when compared to that required for the ionization of Ni to Ni⁴⁺. Hence, Ni²⁺ oxidation state is comparatively more stable. There is no further loss of electrons and therefore, Ni does not show +10 oxidation state.