Oxidation State

The very first element of the 3d transition series, scandium, is atypical among transition metals in exhibiting only a single oxidation state, +3. Every other element of the series shows variable (multiple) oxidation states, a direct consequence of the (n-1)d and ns orbitals lying so close together in energy that electrons can be lost from either (or both) depending on the chemical environment and the oxidising/reducing conditions present -- unlike, say, an alkali metal, whose single valence electron is essentially the only one ever accessible for bonding.

Across the 3d series specifically, the total number of distinct oxidation states available to an element first rises, then falls, as one moves from scandium to copper: it rises because more electrons become available (from both the d and s sub-shells) as atomic number increases through the first half of the series, and it then decreases again toward the end of the series as the number of paired d electrons increases (paired electrons are, in general, harder to remove selectively than unpaired ones, and once the d sub-shell approaches its stable filled or half-filled configuration, fewer intermediate oxidation states remain energetically accessible). This produces the classic pattern where the first and last elements of the series show comparatively few oxidation states, while elements near the middle show the most. Concretely: scandium, the first element, has only one oxidation state, +3; manganese, roughly at the midpoint, has the most of any 3d element, six distinct states ranging from +2 all the way to +7; and copper, the last element commonly discussed, is restricted to just +1 and +2.

The relative stability among the different oxidation states available to a given 3d metal correlates closely with the extra stability conferred by half-filled and fully-filled electronic configurations. A textbook example: Mn²⁺, with a 3d⁵ (exactly half-filled) configuration, is markedly more stable than Mn⁴⁺, with a 3d³ configuration -- part of why manganese's +2 state is so persistently favoured in aqueous chemistry despite the metal's formal capacity to reach +7.

For the heavier 4d and 5d series, the accessible oxidation states range more widely still, from +3 (for yttrium and lanthanum, the first elements of their respective rows) up to the very high value of +8 (reached by ruthenium and osmium, roughly at the midpoints of the 4d and 5d rows respectively -- the highest oxidation state found anywhere among the d-block elements). The very highest oxidation states of the 4d and 5d elements are typically only realised in compounds with the most strongly electronegative elements -- oxygen, fluorine and chlorine -- for example RuO₄, OsO₄ and WCl₆. As a broad rule moving down a group within the d-block, the stability of the higher oxidation state increases while the stability of the lower oxidation state decreases -- the reverse of the pattern familiar from p-block groups. …