Which of the following will exhibit only +3 oxidation state?

Ac

The correct answer is option 2. Ac.

Let us explore each element in detail and understand why Actinium (Ac) is the only element in the list that exhibits only the +3 oxidation state, while the others can show multiple oxidation states.

1. Thorium (Th)

Thorium is an actinide and typically exhibits the +4 oxidation state as its most stable form.

For example, in thorium dioxide (\(ThO_2\)), thorium is in the +4 oxidation state.

Though +3 oxidation state can occur under specific conditions, Thorium is most stable in the +4 oxidation state.

Oxidation states: +3, +4 

Thorium can exist in both +3 and +4 oxidation states, so it is not restricted to only +3.

2. Actinium (Ac)

Actinium is also an actinide element but it only exhibits the +3 oxidation state in all its compounds. This is because Actinium's electron configuration results in a stable loss of three electrons to achieve a noble gas-like configuration (\( [Rn] 6d^1 7s^2 \)). When Ac loses these three electrons, it forms the \(Ac^3+{}\) ion, which is very stable.

Oxidation state:+3 only 

Actinium does not form any stable compounds in any other oxidation state, making +3 its only oxidation state.

3. Neptunium (Np)

Neptunium, another actinide, is unique in that it exhibits a wide range of oxidation states, from +3 to +7. It can exist in various oxidation states due to the involvement of its 5f, 6d, and 7s electrons in bonding. Some common oxidation states include:

+3: Present in neptunium(III) chloride (\(NpCl_3\)).

+4: Present in neptunium(IV) oxide (\(NpO_2\)).

+5: In the pentavalent state, found in compounds like neptunium(V) oxide (\(NpO_2^+\)).

+6: Found in neptunium(VI) oxide (\(NpO_2^{2+}\)).

+7: Present in neptunium(VII) oxide (\(NpO_4^{3-}\)).

Oxidation states: +3, +4, +5, +6, +7 

Neptunium can exist in multiple oxidation states, so it does not exhibit only +3.

4. Curium (Cm)

Curium is another actinide that mainly exhibits the +3 oxidation state, which is the most stable. However, under specific conditions, it can also show the +4 oxidation state. In Curium(III) oxide (\(Cm_2O_3\)), curium is in the +3 oxidation state. In Curium(IV) oxide (\(CmO_2\)), curium is in the +4 oxidation state, though this is less common and less stable than the +3 state.

Oxidation states: +3, +4 

Curium can exist in both +3 and +4 oxidation states, so it does not exhibit only +3.

Only Actinium (Ac) exhibits the +3 oxidation state exclusively, with no other stable oxidation states.

Thus, the correct answer is 2. Actinium (Ac).