Question

Which one of the lanthanoids given below is the most stable in divalent form?

• A. Ce (Atomic Number 58)
• B. Sm (Atomic Number 62 )
• C. Eu (Atomic Number 63)
• D. Yb (Atomic Number 70)

Answer 1

The Correct Option is C

 The question asks us to determine which lanthanoid element is the most stable in the divalent state. The lanthanoids presented are cerium (Ce), samarium (Sm), europium (Eu), and ytterbium (Yb). Let's analyze each to find out why europium (Eu) is the correct answer.

1. Lanthanoids typically prefer to exist in a +3 oxidation state due to the stability brought by their electronic configuration. However, certain lanthanoids exhibit stability in the +2 and/or +4 states owing to unique electronic structures.
2. A divalent lanthanoid implies that the element is stable in a +2 oxidation state, where it has lost two electrons.
3. Europium (Eu) has the atomic number 63, giving it an electronic configuration of \(\text{[Xe]} 4f^7 6s^2\). In a +2 oxidation state, it has the configuration \(\text{[Xe]} 4f^7\), which is a half-filled subshell consisting of seven electrons in the 4f orbital. This half-filled state is exceptionally stable owing to the symmetrical distribution of electrons.
4. Let's reason why the others are not as stable in the +2 state:
   • Cerium (Ce): With atomic number 58, the configuration in the +2 state would be \(\text{[Xe]} 4f^1\). The single electron in the 4f orbital does not confer any particular stability.
   • Samarium (Sm): Atomic number 62, so in the +2 state, \(\text{[Xe]} 4f^6\). Although a bit more stable than cerium, it lacks the symmetrical half-filled subshell of europium.
   • Ytterbium (Yb): Atomic number 70, in the +2 state, \(\text{[Xe]} 4f^{14}\\). While a filled 4f subshell is a stable configuration, the divalent state is less common and not as stable as europium’s half-filled configuration.
5. Thus, the half-filled 4f orbital of europium when it is in a divalent state imparts exceptional stability, making it the most stable among the given options.

Conclusively, Europium (Eu) is the most stable in its divalent form compared to the other lanthanoids given.