Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number?
In the first half of the first row transition elements (Sc to Mn), the ionisation enthalpy increases from left to right due to increasing nuclear charge which accompanies the filling of inner d orbitals.
The magnitude of increase in second and third ionisation enthalpies is much higher for successive elements along the series. This makes the removal of additional electrons beyond two progressively more difficult and energetically less favourable.
Consequently, with increasing atomic number, the higher oxidation states become less stable while the +2 oxidation state becomes relatively more stable. The loss of more than two electrons from (n-1)d orbitals becomes increasingly unfavourable as we move from Sc towards Mn.
Explanation
The textbook explains that ionisation enthalpy increases along the series from left to right due to increasing nuclear charge. The key point is that while successive ionisation enthalpies don't increase as steeply as in main group elements, the magnitude of increase in second and third ionisation enthalpies is much higher. This makes achieving higher oxidation states progressively more difficult. In the first half (Sc to Mn), elements can show variable oxidation states, but as atomic number increases, removing more than two electrons becomes energetically less favourable, making +2 state relatively more stable compared to higher oxidation states.