Question

Which of the following element does not belong to Actinoids series?

• A. Uranium
• B. Thorium
• C. Plutonium
• D. Holmium

Answer 1

The correct option is D

Holmium

Explanation for correct option

(d) Holmium

• Actinoids series has elements with atomic number $89$ to $103$.
• Lanthanoid series has elements with atomic number $57$ to $71$.
• Atomic number of holmium is $67$.
• So, Holmium belongs to the Lanthanoid series.
• Hence, holmium doesn't belong to Actinoid series.

Explanation for incorrect options

(a) Uranium

• Atomic number of uranium is $92$.
• Therefore, it belongs to actinoids series.

(b) Thorium

• Atomic number of thorium is $90$.
• Therefore, it belongs to actinoids series.

(c) Plutonium

• Atomic number of plutonium is $94$.
• Therefore, it belongs to actinoids series.

Hence option (d) Holmium is correct answer.