Which of the following is not an actinoid? 

Concept:

Actinoids - 

1. Actinides or actinoids are the elements of f-block in periodic table having an atomic number between 90 to 103.
2. They are following the element Actinium and that's why called actinoids or actinides.
3. All actinoids are radioactive in nature.
4. General electectronic configuration of actinoids are [Rn] 5f^1-14 6d^0-1 7s².
5. Examples of actinoids are - Uranium, Neptunium, Thorium, etc.

Explanation:

Actinides or actinoids are the elements of f-block in periodic table having an atomic number between 90 to 103.

⇒ Terbium (Z = 65),

• Terbium(Tb) having atomic number 65 is not lying in the range of actinoid's atomic number.
• Although, terbium itself belongs to the f-block of the periodic table but it is a member of the lanthanide series of f-block.
• Thus, terbium is a lanthanoid, not an actinoid.

The rest of the given options are between atomic no. 90 to 103 and therefore are actinoids.

Hence, only terbium(Z=65) is not an actinoid.

∴ the correct answer is option 4.