Actinide Oxidation States Overview

Actinides are a series of chemical elements from Actinium (Ac) to Lawrencium (Lr). They are part of the f-block elements. While many actinides show multiple oxidation states, the most common oxidation state for most actinides is +3.

However, some actinides can exhibit other oxidation states, such as +2, +4, +5, +6, and +7. The question asks for a specific pair of actinides that *only* show the +3 oxidation state among the given options.

Analyzing Specific Actinides and Their Oxidation States

Let's look at the typical oxidation states for the actinides mentioned in the options:

• Actinium (Ac): The most stable and common oxidation state is +3. It is the only known stable oxidation state for Actinium.
• Lawrencium (Lr): It is predicted and found to primarily exist in the +3 oxidation state due to its electronic configuration (\( \text{[Rn] 5f}^{14} \text{ 7s}^{2} \text{ 7p}^{1} \)), which favors the loss of three electrons. It is considered to show only the +3 state.
• Nobelium (No): While +3 is a possible oxidation state, +2 is found to be more stable in aqueous solutions. Thus, Nobelium shows both +2 and +3 oxidation states.
• Curium (Cm): The most common oxidation state is +3. However, Curium can also exhibit the +4 oxidation state, for example, in \( \text{CmF}_{4} \).
• Berkelium (Bk): Similar to Curium, Berkelium commonly exists in the +3 oxidation state but can also show the +4 oxidation state, for instance, in \( \text{BkO}_{2} \) and \( \text{BkF}_{4} \).

We can summarize the common oxidation states in a table:

Evaluating the Given Pairs

Now let's examine each pair provided in the options based on their oxidation states:

• Option 1: Ac and Lr

  Actinium (Ac) shows only +3.

  Lawrencium (Lr) shows only +3.

  Both elements in this pair show only the +3 oxidation state.

• Option 2: Ac and No

  Actinium (Ac) shows only +3.

  Nobelium (No) shows +2 and +3.

  Since Nobelium shows +2 in addition to +3, this pair does not consist of elements showing +3 only.

• Option 3: Cm and Bk

  Curium (Cm) shows +3 and +4.

  Berkelium (Bk) shows +3 and +4.

  Both elements show +4 in addition to +3, so this pair is incorrect.

• Option 4: Cm and Lr

  Curium (Cm) shows +3 and +4.

  Lawrencium (Lr) shows only +3.

  Since Curium shows +4, this pair is incorrect.

Conclusion

Based on the analysis of common oxidation states, the pair of actinides in which both elements show only the +3 oxidation state is Actinium (Ac) and Lawrencium (Lr).