As we move down in groups 13,14 and 15, inertness of ns2 electrons of valence shell increases (inert pair effect). Thus in metals present at the bottom of the groups 13,14, and 15 lower oxidation state becomes more important, viz. Pb (+2 state rather +4), Bi (+3 state rather +5) and Ti (+1 rather than+3).

The Stability of Lower Oxidation States in Aqueous Solution

Introduction:
In aqueous solutions, the stability of an ion refers to its tendency to exist in a particular oxidation state. The stability of different oxidation states can be affected by factors such as the electronic configuration, size, and charge of the ion.

Explanation:
Among the given pairs of ions, the lower oxidation state in aqueous solution is more stable in the case of Ti and Ti3.

1. Ti and Ti3:
- Titanium (Ti) has an atomic number of 22 and its electronic configuration is [Ar] 3d2 4s2.
- In its lower oxidation state of +2, Ti loses two electrons from its 4s orbital to form Ti2+ ion, which has the electronic configuration [Ar] 3d2.
- On the other hand, in its higher oxidation state of +3, Ti loses three electrons from its 3d orbital to form Ti3+ ion, which has the electronic configuration [Ar].
- The 3d subshell is more stable when it is half-filled or fully-filled. Therefore, the Ti2+ ion with the 3d2 configuration is more stable than the Ti3+ ion with the 3d0 configuration.
- This stability arises from the symmetric filling of the 3d subshell, which minimizes electron-electron repulsion.

2. Cu and Cu2:
- Copper (Cu) has an atomic number of 29 and its electronic configuration is [Ar] 3d10 4s1.
- In its lower oxidation state of +1, Cu loses one electron from its 4s orbital to form Cu+ ion, which has the electronic configuration [Ar] 3d10.
- In its higher oxidation state of +2, Cu loses two electrons from its 4s and one electron from its 3d orbital to form Cu2+ ion, which has the electronic configuration [Ar] 3d9.
- The 3d subshell is more stable when it is fully-filled. Therefore, the Cu+ ion with the 3d10 configuration is more stable than the Cu2+ ion with the 3d9 configuration.
- This stability arises from the fully-filled 3d subshell, which maximizes electron-electron pairing and minimizes electron-electron repulsion.

3. Cr2 and Cr3:
- Chromium (Cr) has an atomic number of 24 and its electronic configuration is [Ar] 3d5 4s1.
- In its lower oxidation state of +2, Cr loses two electrons from its 4s orbital and one electron from its 3d orbital to form Cr2+ ion, which has the electronic configuration [Ar] 3d4.
- In its higher oxidation state of +3, Cr loses three electrons from its 4s orbital and one electron from its 3d orbital to form Cr3+ ion, which has the electronic configuration [Ar] 3d3.
- The 3d subshell is more stable when it is half-filled or fully-filled. Therefore, both Cr2+ and Cr3+ ions have partially filled 3d subshells, but the Cr3+ ion has a lower electron count in the 3d subshell.
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