Lanthanoid ions with $$4f^{7} configuration are : $$(A)Eu^{2+}$$\text{ }$$ (B)Gd^{3+}$$\text{ }$$ (C)Eu^{3+}$$\text{ }$$ (D)Tb^{3+}$$\text{ }$$ (E)Sm^{3+}$$ Choose the correct answer from the options given below :

We need to identify lanthanoid ions with $$4f^7$$ configuration because the lanthanoids fill the 4f orbitals, so we examine which ions have exactly 7 electrons in the 4f subshell.

• (A) Eu²⁺: Eu (Z=63) has configuration [Xe] 4f⁷ 6s². Eu²⁺ loses two 6s electrons: [Xe] 4f⁷. This has 4f⁷ configuration.
• (B) Gd³⁺: Gd (Z=64) has configuration [Xe] 4f⁷ 5d¹ 6s². Gd³⁺ loses 6s² and 5d¹: [Xe] 4f⁷. This has 4f⁷ configuration.
• (C) Eu³⁺: Eu (Z=63) has [Xe] 4f⁷ 6s². Eu³⁺ loses 6s² and one 4f: [Xe] 4f⁶. This does NOT have 4f⁷.
• (D) Tb³⁺: Tb (Z=65) has [Xe] 4f⁹ 6s². Tb³⁺ loses 6s² and one 4f: [Xe] 4f⁸. This does NOT have 4f⁷.
• (E) Sm³⁺: Sm (Z=62) has [Xe] 4f⁶ 6s². Sm³⁺ loses 6s² and one 4f: [Xe] 4f⁵. This does NOT have 4f⁷.

The ions with 4f⁷ configuration are (A) Eu²⁺ and (B) Gd³⁺, so the correct answer is Option 3: (A) and (B) only.