Compare the stability of +2 oxidation state for the elements of the first transition series.
The stability of the +2 oxidation state varies significantly across the first transition series.
Scandium does not exhibit the +2 oxidation state; scandium(II) is virtually unknown and Sc shows only the +3 oxidation state. In the early part of the series, titanium(IV) is more stable than Ti(III) or Ti(II), indicating that higher oxidation states are favoured for early elements.
The elements which give the greatest number of oxidation states occur in or near the middle of the series. Manganese exhibits all oxidation states from +2 to +7. The +2 state exists but is not the only stable state for these middle elements.
Towards the end of the series, the +2 oxidation state becomes more prominent and stable. Copper exhibits both +1 and +2 oxidation states. Zinc exhibits only the +2 oxidation state, which is highly stable because no d electrons are involved.
The variability of oxidation states arises from incomplete filling of d orbitals. The stability of +2 oxidation state increases from left to right in the series, with early elements favouring higher oxidation states and later elements stabilizing in the +2 state.
Explanation
The answer is constructed from the textbook context which states that Sc(II) is virtually unknown, Ti(IV) is more stable than Ti(III) or Ti(II), Mn exhibits all states from +2 to +7, and Zn shows only +2 state with no d electrons involved. The table shows +2 is common from Ti to Zn. The trend of increasing +2 stability from left to right is derived from these stated facts.