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Why the Thorium configuraton is different from the general configuration of ACTINOIDS???
expected electronic configuration of Th is [Rn]5f1 6d1 7s2
but observed electronic configuration has no electron in 5f orbital and 2 electrons in the 6d orbital because empty,half filled,and completely filled f orbital is more stable than any other configuration . Thus Th acquires this configuration to increase its stability.
observed electronic configuration of Th is [Rn]5f06d27s2
The lanthanides and the actinides make up the f block of the periodic table. The lanthanides are the elements produced as the 4f sub level is filled with electrons and the actinides are formed while filling the 5f sub level
The f-block elements in which the last electron or the differentiating electron enters in (n-2) f-orbitals and the general electronic configuration is (n-2)f1-14 (n-1)d0-1 ns2 consists of two series of inner transition elements-Lanthanides (the fourteen element following lanthanum)
The elements in which the last electron enters one of the 4f-orbitals are called 4f-block elements or first inner transition series. They are also called Lanthanides (or) lanthanones because they come immediately after lanthanum. · Actinides (the fourteen element following actinium)
The elements in which the last electron enters one of the 5f-orbitals are called 4f-block elements or second inner transition series. They are also called Actinides because they come immediately after actinium. The general electronic configuration is [Rn]5f1-14 6d0-1 7s2
Lanthanides (58Ce- 71Lu) - This series starts from Lanthanum ( atomic numbe r= 57) and continues up to Lu (Lutetium) as shown in the table above. All the elements of the lanthanide series resemble each other very closely due to the presence of the same number of electrons in the outermost and the penultimate shells. They are also called rare earth elements. Though Lanthanum is a d-block element it is included in the lanthanides series as it resembles them.
Some important characteristics are given below-
The atomic size or ionic radii of tri positive lanthanide ions show a steady and gradual decrease with the increase in atomic number from La to Lu. Although they show some irregularities, the ionic radii decrease steadily from La to Lu. This gradual decrease in the size with increasing atomic number is called lanthanide contraction.
The major cause for lanthanide contraction is due to the inappropriate shielding of the 4f electrons due to the improper shape of the f-orbitals. As the atomic number increases in the lanthanide series, for every proton in the nucleus the extra electron goes to fill the 4f-orbitals.
The 4f-electrons constitute inner shells and are rather ineffective in screening the nuclear charge. Thus, there is a gradual increase in the effective nuclear charge experienced by the outer electrons. So the attraction of the nucleus for the electrons in the outermost shell increases as the atomic number increases and the electron cloud shrinks.
This results in gradual decrease in the size of lanthanides with increasing atomic number. The decrease in size is not regular throughout the lanthanides. A rapid decrease is seen only in the first six elements compared to the rest of the elements.
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