Question

Which of the following elements does not belong to actinides series?
A) Uranium
B) Thorium
C) Plutonium
D) Holmium

Answer 1

We need to know that the lanthanides and Actinides belong to f block elements. Lanthanides and Actinides are sometimes known as inner transition metals. Only the first four elements present in Actinides are found naturally and others are artificially made. Since they belong to transition elements, thus they are classified as inner transition elements.

Complete answer:
We need to remember that the lanthanides are the $14$ elements from cerium (atomic number $58$) to lutetium (atomic number $71$). The actinides are the $15$ elements from thorium (atomic number $90$) to lawrencium (atomic number $103$). In lanthanides $4f$ shell is being filled while in actinides $5f$ shell is being filled. Lanthanides have similar properties as that of alkaline earth metals while actinides have radioactive elements present in them. There are total $15$ elements in Actinide series:
Actinium
Thorium
Protactinium
Uranium
Neptunium
Plutonium
Americium
Curium
Berkelium
Californium
Einsteinium
Fermium
Mendelevium
Nobelium
Lawrencium
Uses of Actinides: The actinides are valuable primarily because they are radioactive. These elements can be used as energy sources for applications as varied as cardiac pacemakers and generation of electrical energy for instruments on the moon. Uranium and plutonium have been employed in nuclear weapons and in nuclear power plants.

So, the correct answer is “Option B”.

Note:
As we know that the lanthanides have been widely used as alloys to impart strength and hardness to metals. The main lanthanide used for this purpose is cerium, mixed with small amounts of lanthanum, neodymium, and praseodymium. These metals are also widely used in the petroleum industry for refining crude oil into gasoline products.