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, the +2 oxidation state becomes increasingly stable as we move from scandium (Sc) to manganese (Mn). This trend can be attributed to several factors, including electron configuration, the nature of d-orbitals, and the overall stability of the resulting ions.

Understanding Electron Configuration

The transition metals are characterized by the filling of d-orbitals. As we progress through the series from Sc to Mn, the electron configuration changes, leading to different stability in oxidation states. For example:

• Scandium (Sc) has the electron configuration [Ar] 3d¹ 4s². When it loses two electrons to form Sc²⁺, it achieves a stable noble gas configuration of [Ar].
• Titanium (Ti) follows with [Ar] 3d² 4s². The +2 state (Ti²⁺) is also stable due to the loss of the 4s electrons, leaving behind a half-filled d-subshell.
• As we reach manganese (Mn), with the configuration [Ar] 3d⁵ 4s², the +2 state (Mn²⁺) is particularly stable because it results in a half-filled d-subshell, which is energetically favorable.

Role of d-Orbitals

The stability of the +2 oxidation state is influenced by the electronic structure of the d-orbitals. As we move across the series, the d-orbitals become more filled, and the energy difference between the d and s orbitals decreases. This leads to a greater tendency for the metals to lose their 4s electrons first, resulting in a stable +2 state.

Influence of Crystal Field Stabilization Energy

Another important factor is the crystal field stabilization energy (CFSE). Transition metals in the +2 oxidation state often experience a significant stabilization when they form complexes, particularly in octahedral or tetrahedral geometries. The presence of ligands can further stabilize the +2 state by splitting the d-orbitals, enhancing the overall stability of the ion.

Example of Stability Trend

To illustrate this trend, consider the following:

• Sc²⁺ is less stable than Ti²⁺ because Sc has only one d-electron, while Ti has two, allowing for more effective pairing and stabilization.
• As we reach Mn²⁺, the half-filled d-subshell (3d⁵) provides maximum stability due to exchange energy and symmetry, making it one of the most stable oxidation states in this series.

Conclusion on Stability Trends

In summary, the increasing stability of the +2 oxidation state in the first half of the first-row transition elements is primarily due to the favorable electron configurations, the role of d-orbitals, and the influence of crystal field stabilization. As we progress from Sc to Mn, the combination of these factors leads to a more stable +2 state, making it a common and important oxidation state for these metals.