Question

Which of the following elements belong to Lanthanide series?

• A. Ce
• B. Hg
• C. Th
• D. Mn

Answer 1

The correct option is A

Ce

Lanthanides

The 15 elements in the periodic table in which the differentiating electron enter the 4f orbital are called Lanthanides. They run from atomic number 57 (Lanthanum) to atomic number 71 (Lutetium). They are placed outside the main body of the periodic table and the general outer electronic configuration of lanthanides is 6s² 5d^0-1 4f^1-14.

---

Explanation for wrong answers

(B) The atomic number of Mercury (Hg) is 80. Therefore, the electronic configuration is given as:

Hg- [Xe] 4f¹⁴ 5d¹⁰ 6s²

It has a completely filled d-orbital and therefore is a d-block element. Therefore, option (B) is incorrect.

(C) The atomic number of Thorium (Th) is 90. The electronic configuration is given as:

Th- [Rn] 6d² 7s²

It belongs to the 5f or actinide series. Therefore, option (C) is incorrect.

(D) The atomic number of Manganese (Mn) is 25. The electronic configuration is:

Mn- [Ar] 3d⁵ 4s²

The differentiating electron enters the d-orbital. Therefor it is a d-block element. Option (D) is incorrect.

---

Explanation for correct answer

(A) The atomic number of Cerium (Ce) is 58. The electronic configuration is:

Ce- [Xe] 4f¹ 5d¹ 6s²

Therefore, the differentiating electron enters the 4f orbital with one electron in the d-orbital. Therefore, it belongs to the lanthanide series.

The correct option is (A) Ce.