Question

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

Answer 1

Lanthanoids : Lanthanoids are the elements in which the extra electron (also called differentiating electron) enters $(n-2)$ f-orbital. Since $(n-2)$ f-orbital lies comparatively deep within the inner to the penultimate shell, these elements are also called inner-transition elements.
$La(Z=57)$ to $Lu(Z=71)\left(15elements\right)$ are considered as Lanthanoids.
The position of actinoides in periodic table:
The actinoid series contains elements with atomic number $89$ to $103$ and is present in the third group of periodic table. The series is the row present below the Lanthanide series which is located underneath the main body of the periodic table.