(1) Transition elements (also known as transition metals) are elements that have partially filled d orbitals. IUPAC defines transition elements as an element having a d subshell that is partially filled with electrons, or an element that has the ability to form stable cations with an incompletely filled d orbital. The electronic configuration of transition element is (n-1)d5 ns1 or (n-1)d10 ns1.
These elements show variable oxidation state because their valence electrons in two different sets of orbitals, that is (n-1)d and ns. The energy difference between these orbitals is very less, so both the energy levels can be used for bond formation. Thus, transition elements have variable oxidation states.
(2) Mn(Z-25) has the highest number of unpaired electrons in the d-subshell and it shows high oxidation state(+7).
(3) Scandium (Sc) only exhibits a +3 oxidation state in these series.