Understanding Europium Oxidation States in Water

Europium (Eu) is a chemical element that belongs to the lanthanide series, also known as the inner transition metals or f-block elements. Lanthanides are generally known to exhibit a common oxidation state of +3 in their compounds, especially in aqueous solutions.

However, some lanthanides, including Europium, show other stable oxidation states in aqueous solution. This is related to the stability of certain electronic configurations in the 4f subshell.

Let's consider the oxidation states of Europium:

• Eu(III): This is the most common and stable oxidation state for Europium, consistent with the general trend for lanthanides. The electronic configuration of neutral Eu is $[Xe]4f^76s^2$. In the +3 oxidation state, it loses the two $6s$ electrons and one $4f$ electron, resulting in $[Xe]4f^6$.
• Eu(II): Europium is one of the few lanthanides that can exist in a relatively stable +2 oxidation state in aqueous solution. This stability is attributed to the half-filled $4f$ subshell configuration. When Eu loses two $6s$ electrons, it attains the configuration $[Xe]4f^7$, which is particularly stable due to Hund's rule (maximum multiplicity, unpaired electrons). Eu(II) compounds, like $\text{EuSO}_4$, are known and soluble in water, forming $\text{Eu}^{2+}$ ions.
• Eu(IV): The +4 oxidation state for Europium is exceedingly rare and highly unstable, especially in aqueous solution. Achieving the +4 state would require removing an electron from the stable $4f^7$ configuration, which is energetically unfavorable.

Therefore, in aqueous solutions, the predominantly observed and stable oxidation states for Europium are +2 and +3.

Let's examine the given options based on this information:

• Option 1: +2 and +3 - This includes the two known stable oxidation states of Eu in aqueous solution.
• Option 2: +3 and +4 - This includes the unstable +4 state.
• Option 3: +2, +3 and +4 - This includes the unstable +4 state.
• Option 4: +3 only - This excludes the known stable +2 state.

Based on the stability and occurrence of Europium ions in water, the known oxidation states are +2 and +3.

Revision Table: Europium Oxidation States

Additional Information: Lanthanide Oxidation States

Lanthanides (elements from Lanthanum through Lutetium) are characterized by the filling of the 4f subshell. While the +3 oxidation state is the most common and stable for most lanthanides, variations occur:

• Elements like Cerium (Ce) can show +4 (stable due to $4f^0$ configuration).
• Elements like Europium (Eu) and Ytterbium (Yb) can show +2 (stable due to $4f^7$ and $4f^{14}$ configurations, respectively).
• Elements like Samarium (Sm), Europium (Eu), and Ytterbium (Yb) can also show +2 in addition to +3.
• Promethium (Pm) only exists as +3.

These variations in oxidation states are important characteristics of the lanthanide series and influence their chemical properties.