ABSTRACT
It has two sets of elements, each set comprising of 14 elements. The 4f -block elements are termed as lanthanons or lanthanides. The elements are differentiated from others by filling of antipenultimate 4f orbitals which has a capacity to fill 14 electrons. The element Lanthanum belonging to 5d block element is the parent member of lanthanides and hence, the name is Lanthanides. In earlier days, they were called rare earth elements, but many of these elements are not rare. For 5f-block elements, the parent actinium, Ac belongs to 5f series and itself Ac belongs to 6d block element series. The 14 elements belonging to 5f block series are called Actinides and named after the element actinium. Lanthanides are not considered to be rare, ceruim is as abundant as copper. The element promethium (Pm) does not exist in nature and all other elements in the series are more abundant than iodine.
