Question

The ion with 4f$^7$ configuration is:

• A. Pr$^{3+}$
• B. Lu$^{3+}$
• C. Eu$^{2+}$
• D. Ce$^{4+}$

Hint:

Subtract the ion charge from atomic number to determine electronic configuration. Eu$^{2+}$ is a half-filled 4f$^7$ stable configuration.

Answer 1

The Correct Option is C

Europium (Z = 63). Eu$^{2+}$ means 63 – 2 = 61 electrons. Configuration: [Xe] 4f$^7$ — hence correct.
Pr$^{3+}$ → 58 – 3 = 55 → 4f$^2$
Lu$^{3+}$ → 71 – 3 = 68 → 4f$^{14}$
Ce$^{4+}$ → 58 – 4 = 54 → [Xe] → no 4f electrons.