Question

What are Lanthanides and Actinides? Of these two, which series is primarily composed of man-made elements? Why could these elements not be accommodated in the main body of the periodic table?

Answer 1

Step $1$: Define Lanthanides and Actinides-

• Lanthanides are the series of elements present at the bottom of the periodic table.
• These series consist of elements with atomic numbers 57 to 71, and these 15 elements are called Lanthanides.
• They were named Lanthanide because elements in this series have the properties of this group's Lanthum(La) first element.
• These elements are present in period 6 of the table.
• Lanthum (La) is the only Lanthanide present in the table's main body.
• These elements along with Scandium (Sc) and Yttrium (Y)] are also called rare earth elements.

Actinides:

• Same as Lanthanides, Actinides are the series of elements present at the bottom of the periodic table.
• These series consist of elements with atomic numbers 89 to 103, and these 15 elements are called Actinides.
• They were named Actinides because elements in this series have the properties of Actinium(Ac) first element of this group.
• These elements are present in period 7 of the table.
• Actinium (Ac) is the only actinide present in the table's main body.
• All these elements are radioactive.

Step $2$: Find the series consisting of Man-made elements-

• Actinides series is primarily composed of man-made elements.
• There are 15 elements in the series, from Actinium (Ac) to Lawrencium (Lr). In these, the first four elements [Actinium(Ac) to Uranium(U)] occur naturally.
• Elements with an atomic number greater than the Uranium (U) with atomic no. Ninety-two are produced artificially.

Step $3$: These elements cannot be accommodated in the main body of the periodic table-

• Lanthanides and actinides are also called inner transition metals. They are placed as a separate group in the periodic table because of their electronic configuration.
• These elements come under the periodic table's 3B and 4B groups, respectively.
• These 3B elements are the first elements to begin filling the d shell electrons, and 4B elements are the second; the next electrons are placed in the d shell.
• In the 3B group, from Lanthanum (La) to Hafnium(Hf) has one electron in d-shell, and the next electron gets to fill in the 4f shell.
• In the 4B group, Actinium(Ac) to Lawrencium(Lr) has two electrons in d-shell. The next electron gets to fill in the 5f shell.
• So these elements from atomic numbers 58 to 71 and 90 to 103 are placed in separate f-block.
• Hence, Lanthanides and Actinides elements are not accommodated in the main body of the periodic table.