Lanthanide contraction controls the atomic sizes of elements of 6th & 7th period.give reason.?



Answer:
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LANTHANOID CONTRACTION....

The electrons in the 4f-subshell, which is progressively filled from cerium (Z = 58) to lutetium (Z = 71), are not particularly effective at shielding the increasing nuclear charge from the sub-shells further out. The elements immediately following the lanthanides have atomic radii which are smaller than would be expected and which are almost identical to the atomic radii of the elements immediately above them. Hence hafnium has virtually the same atomic radius (and chemistry) as zirconium, tantalum as niobium etc. The effect of the lanthanide contraction is noticeable up to platinum (Z = 78), after which it is masked by a relativistic effect known as the inert pair effect.
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MORE DETAILED INFO....

All elements following the lanthanides in the periodic table are influenced by the lanthanide contraction. .

The period 6 elements have very similar radii compared with the elements of the period 5 elements in the same group.....

For example, the atomic radii of the metal zirconium, Zr, (a period 5 element) is 1.59 Ångström and that of hafnium, Hf, (a period 6 element) is 1.56 Ångström. The ionic radius of Zr4+ is 0.79 Ångström and that of Hf4+ is 0.78 Ångström.

The radii are very closely similar even though the number of electrons increases from 40 to 72 and the atomic mass increases from 91.22 to 178.49 g/mol. The increase in mass and the unchanged radii lead to a steep increase in density from 6.51 to 13.35 g/cm3.

Zirconium and hafnium therefore have very similar chemical behaviour, having closely similar radii and electron configuration.,......


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here's the consequence of Lanthanoid Contraction.....

U know that size of the atoms increases down the group...

where as the size of the atom lying in second transition
series is larger than the atoms of the elemnts lying in the fist transition series....

The size of the atom of the 3rd transition series is nearly same as that of the atoms of elements lying in the same group of the second transition series...

so the atomic size of the 6th and 7th period are nearly same..theyre also called as CHEMICAL TWINS..

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Hope this is the best answer uve got.
all the best.,,,,,,,,,,,,,,,,,
Elements with atomic numbers from 55 to 86 constiyute the 6th period and atomic number 87 to 116 constitute the 7th period of the periodic table . In 6th period 15 lanthanides are included while in 7th period 15 actinides are included.

Atomic radius tends to decrease on passing along a period of the periodic table from left to right, and to increase on descending a group. This is, in part, because the distribution of electrons is not completely random. The electrons in an atom are arranged in shells which are, on average, further and further from the nucleus, and which can only hold a certain number of electrons. Each new period of the periodic table corresponds to a new shell which starts to be filled up, and so the outermost electrons are further and further from the nucleus as a group is descended.

The second major effect which determines trends in atomic radius is the charge of the nucleus, which increases with the atomic number, Z. The nucleus is positively charged, and tends to attract the negatively-charged electrons. Passing along a period from left to right, the nuclear charge increases while the electrons are still entering the same shell: the effect is that the physical size of the shell (and hence of the atom) decreases in response.

The increasing nuclear charge is partly counterbalanced by the increasing number of electrons in a phenomenon known as shielding, which is why the size of atoms usually increases as a group is descended. However, there are two occasions where shielding is less effective: in these cases, the atoms are smaller than would otherwise be expected.


Lanthanide contraction:
The electrons in the 4f-subshell, which is progressively filled from cerium (Z = 58) to lutetium (Z = 71), are not particularly effective at shielding the increasing nuclear charge from the sub-shells further out. The elements immediately following the lanthanides have atomic radii which are smaller than would be expected and which are almost identical to the atomic radii of the elements immediately above them. Hence hafnium has virtually the same atomic radius (and chemistry) as zirconium, tantalum as niobium etc. The effect of the lanthanide contraction is noticeable up to platinum (Z = 78), after which it is masked by a relativistic effect known as the inert pair effect.

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