Question

What are lanthanoids? What is the position of Actinides in the periodic table?

Answer 1

Lanthanide series consists of 14 elements following the lanthanum element. F-block elements consist of two series lanthanoids and Actinoids, they are placed in group 3. They occupy period 6 and period 7 of the periodic table respectively.

Complete step by step solution:
-Lanthanides are elements in which differentiating electrons enter (n-2) f -orbital.
-Since (n-2) f orbital lies comparatively with the inner to the penultimate shell, these elements are also called inner transition elements.
- Atomic number of lanthanum is 57, electronic configuration of lanthanum is $[Xe]4{{f}^{0}}5{{d}^{1}}6{{s}^{2}}$.
- Lanthanides have general electronic configuration $[Xe]4{{f}^{1-14}}5{{d}^{0-1}}6{{s}^{2}}$, f orbitals can accommodate 14 electrons. Cerium having atomic number 58 to Lutetium having atomic number 71 are lanthanoids.
-Lanthanum itself is a d-block element, elements following lanthanum are called lanthanides.
- In Lanthanide, the last electron enters into a 6s orbital, so lanthanides are placed in period 6 and Group 3 in periodic table.
-In lanthanide, differentiating electrons enter in a 4f orbital.
-fourteen elements following actinium are called Actinides.
-Actinoids include fourteen elements from Thorium having atomic number 90 to Lawrencium having atomic number 103.
- General electronic configuration of Actinides is $[Rn]5{{f}^{1-14}}6{{d}^{0-1}}7{{s}^{2}}$. As the valence electrons enter in 7s orbital, Actinides occupy position in period 7 in periodic table.

Hence, Actinides are placed in Group 3 and period 7 in the periodic table. Lanthanides are elements in which differentiating electrons enter (n-2) f -orbital.

Note: Lanthanide series consists of 14 elements which follow lanthanum. f block elements are placed at the bottom of the periodic table. f block elements are called inner transition series. Lanthanum closely resembles lanthanoids.