which series do La and Ac belong to. d-series or f-series. please explain with reference to electronic configuration. also is it true that one electron is filled in d subshell of penultimate shell before filling starts in f subshell of antepenultimate shell

Dear zahid,

They belog to f series...

According to IUPAC, lanthanide series consists of 15 elements and their atomic numbers range from 57 to 71. The series start from lanthanum and ends on lutetium. These are all f-block elements except for lutetium, which is d-block. Another series in the periodic tables  is actinide series, which comprises of 15 elements and their atomic numbers   lie between 89 to 103. The name of the series is given because of the name of the first element of the series, actinium which means, "radioactive".

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he general electronic configuration of thed-block elements is [Inert gas](n-1)d^1-10n s^1-2Thed-sub-shell is the penultimate (last but one) sub-shell and is denoted as(n-1) d-sub-shell. The number of s electrons may vary from one to two. Thes-sub-shell in the valence shell is represented as thenssub-shell. However, palladium (Pd) is an exception with no electron in thes-sub shell. In the periodic table, the transition metals are present in ten groups (3 to 12). Group-2 belongs to thes- block with anns²configuration.

The elements in group-3 have anns²(n-1)d¹configuration. The first transition series is present in the 4^thperiod, and starts after Ca (Z=20) of group-2 which has configuration [Ar]4s². The electronic configuration of scandium (Sc), the first element of group-3 with atomic number Z=21 is[Ar]4s²3d¹. As we move from left to right, electrons are added to the samed-sub-shell till it is complete. The element of group-12 in the first transition series is zinc (Zn) with configuration [Ar]4s²3d¹⁰. Since the electrons added fill the(n-1)dorbitals, the properties of thed-block elements are quite different from those ofsandpblock elements in which the filling occurs either insor inp-orbitals of the valence shell. The electronic configuration of the individual elements present in all the transition series are given below:

First (3d) Transition Series (Sc-Zn)

Second (4d) Transition Series (Y-Cd)

Third (5d) Transition Series (Lu-Hg)

Fourth (6d) Transition Series (Lr-Cn)

A careful look at the electronic configuration of the elements reveals that there are certain exceptions shown by Pt, Au and Hg.. These are either because of the symmetry or nuclear-electron and electron-electron force. The(n-1)dorbitals that are involved in the transition metals are very significant because they influence such properties as magnetic character, variable oxidation states, formation of colored compounds etc. The valences(ns)andp(np)orbitals have very little contribution in this regard since they hardly change in the moving from left to the right in a transition series. In transition metals, there is a greater horizontal similarities in the properties of the elements in a period in comparison to the periods in which thed-orbitals are not involved. This is because in a transition series, the valence shell electronic configuration of the elements do not change. However, there are some group similarities as well.

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