Actinide Series in Periodic Table
What elements are in the actinide family?
The f-block elements in which the extra electrons enter 5f orbitals of the (n-2) energy levels are known as 5f block elements or actinide or actinoids.
Thus according to the definition of actinides, only thirteen elements from Th-90 with electronic configuration 5f0 6d2 7S2 to No-102 with electron configuration 5f14 6d0 7s2 should be the member of actinoids series.
However, all the elements from Ac-89 to Lw-103 are considered as the member of the actinides series. Since all these fifteen elements contain similar physical properties and chemical bonding. The name of actinides is derived from the element actinium.
Discovery of Actinide series
The existence of the lanthanides series helped the discoverers to predict another series of elements contain heavy elements in the periodic table. Before 1940, prior to the discovery of transuranium elements, the naturally occurring radioactive decay isotopes namely, 90Th90, 91Pa, and 92U placed before 72Hf, 73Ta, and 74W in IVB, VB, and VIB groups in the periodic table.
These elements placed there because of their oxidation number or state. These elements show +4, +5, and +6 oxidation state. Therefore the properties of these elements resemble Hf, Ta, and W.
The elements, neptunium came in 1940, plutonium came in 1941, and the chemical properties of these two elements very much similar to the uranium. Therefore Seaborg in 1944 suggested that the heavier elements, an atomic number greater than 89 placed wrongly in the periodic table.
He has given an idea to constitute a second series of inner transition elements similar to that of lanthanides. The revised periodic table thus includes the elements after 89Ac as second inner transition elements called actinides.
