CBSE Class 12 Chemistry Chapter 16: d and f Block Elements NCERT Solutions

To determine the electronic configuration of ions, we first write the configuration of the neutral atom and then remove electrons from the outermost shell(s). For transition metals, electrons are typically removed from the 4s orbital before the 3d orbital.

1. Cr³⁺: The electronic configuration of neutral Chromium (Cr) is [Ar] 3d⁵ 4s¹. To form Cr³⁺, we remove one electron from the 4s orbital and two electrons from the 3d orbital. Thus, the configuration is [Ar] 3d³.
2. Pm³⁺: Promethium (Pm) is a lanthanide. Its electronic configuration is [Xe] 4f⁵ 6s². To form Pm³⁺, we remove two 6s electrons and one 4f electron. Thus, the configuration is [Xe] 4f⁴.
3. Cu⁺: The electronic configuration of neutral Copper (Cu) is [Ar] 3d¹⁰ 4s¹. To form Cu⁺, we remove the single electron from the 4s orbital. Thus, the configuration is [Ar] 3d¹⁰. This fully filled d-orbital configuration makes it very stable.
4. Ce⁴⁺: Cerium (Ce) has the electronic configuration [Xe] 4f¹ 5d¹ 6s². To form Ce⁴⁺, all four valence electrons are removed. Thus, the configuration is [Xe].
5. Co²⁺: The electronic configuration of neutral Cobalt (Co) is [Ar] 3d⁷ 4s². To form Co²⁺, we remove two electrons from the 4s orbital. Thus, the configuration is [Ar] 3d⁷.
6. Lu²⁺: Lutetium (Lu) has the electronic configuration [Xe] 4f¹⁴ 5d¹ 6s². To form Lu²⁺, we remove two electrons from the 6s orbital. Thus, the configuration is [Xe] 4f¹⁴ 5d¹.
7. Mn²⁺: The electronic configuration of neutral Manganese (Mn) is [Ar] 3d⁵ 4s². To form Mn²⁺, we remove two electrons from the 4s orbital. Thus, the configuration is [Ar] 3d⁵. This half-filled d-orbital configuration is very stable.
8. Th⁴⁺: Thorium (Th) is an actinoid. Its electronic configuration is [Rn] 6d² 7s². To form Th⁴⁺, all four valence electrons are removed. Thus, the configuration is [Rn].

The stability of an ion is often related to its electronic configuration, particularly the stability of its d-orbitals. Half-filled and fully-filled orbitals are known to have extra stability due to symmetry and exchange energy. Let's examine the electronic configurations of Mn²⁺ and Fe²⁺:

• The electronic configuration of Mn²⁺ is [Ar] 3d⁵. This configuration features a half-filled d-orbital (d⁵), which is a particularly stable arrangement.
• The electronic configuration of Fe²⁺ is [Ar] 3d⁶. This configuration has one electron more than a half-filled d-orbital.

When Mn²⁺ is oxidized to Mn³⁺, its configuration changes from the stable 3d⁵ to 3d⁴. This transition involves moving away from a stable state, making it less favorable. Conversely, when Fe²⁺ (3d⁶) is oxidized to Fe³⁺, its configuration changes to 3d⁵. This transition from a 3d⁶ configuration to a stable, half-filled 3d⁵ configuration is energetically favorable. Therefore, Fe²⁺ is more readily oxidized to Fe³⁺ than Mn²⁺ is to Mn³⁺, indicating that Mn²⁺ compounds are more stable towards oxidation to the +3 state.

In the first half of the first-row transition elements (from Scandium, Sc, to Manganese, Mn), the +2 oxidation state generally becomes more stable as the atomic number increases. This trend can be understood by looking at the electronic configurations of these elements and their +2 ions.

The +2 oxidation state arises from the loss of the two 4s electrons. As we move across the period from Sc (atomic number 21) to Mn (atomic number 25), electrons are progressively added to the 3d subshell:

• Sc (+2 state): [Ar] 3d¹
• Ti (+2 state): [Ar] 3d²
• V (+2 state): [Ar] 3d³
• Cr (+2 state): [Ar] 3d⁴
• Mn (+2 state): [Ar] 3d⁵

The stability of an ion is influenced by its electronic configuration. A half-filled d-orbital (d⁵) configuration is particularly stable. As we progress from Sc²⁺ to Mn²⁺, the number of d-electrons increases, and the electronic configuration approaches the highly stable half-filled state (d⁵) in Mn²⁺. This increasing stability of the d-electron configuration contributes to the increasing stability of the +2 oxidation state for these elements.

While other oxidation states are also possible and often more stable for some elements (like Cr and Mn), the stability of the +2 state specifically shows a clear increasing trend in this segment of the transition series due to the filling of the 3d orbitals towards the stable d⁵ configuration.