Lanthanide contraction

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1. 4.1.5 Lanthanide Contraction
Lanthanide contraction is a special
efect of lanthanides to which can be explained
on the
basis of their atomic and ionic raddi and more over due to the shielding effect of 4f electrons.
As we know adding shell (n)
to the nucleus atomic radius increases due to increase in
shiedling effect, and decrease in effective nuclear charge. But some pecularities
noticed in case of
lanthanoids. In case of lanthanoids, the atomic and ionic radii decrease steadily with increase in
atomic number, which is called as lanthanide contraction. This decrease in size is regular in case
of ions but not for atoms.
The atomic and ionic radi of Ln atoms and Ln-ions (Ln**, Ln, Ln**) are given in Table
4.1.5
The decrease in size of the ions are graphically represented in figure 4.1.2.

2. University Chemistn
134
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Table 4.1.5 :
Atomic and lonic Radii of Lanthanoids
lonic radii of
Ln" ions
Ln Elements Atomic
Lnt ions ions with atomic radius
ions (A) (A) (A)
()
number
1.06 La57 1.88
1.03 Ce58 1.82
0.92
1.01 Pr59 1.83
0.90
0.99 Nd60 1.82
0.98
Pm61
Sm62 1.80 1.11 0.96
Eu63 2.04 1.09 0.95
Gd64 1.80 0.94
Tb65 1.78
0.92
0.84
Dy66 1.77 0.91
Ho67 1.77
0.89
Er68 1.76
0.88
Tm69 1.75 0.94 0.87
Yb70
1.94 0.93 0.86
Lu71 1.73
0.85
Cause of Lanthanide Contraction
As we know in lanthanides, the additional electron enters 4f-sub-shell but not the
valence shell 6s. The electrons add to 4f orbitals have not much
penetrating power,
so the electrons are
away from nucleus. They don't shield the valence 6s electrons much from the nucleus. Ie
mutual shielding effect of 4f-electrons (i.e., the shielding effect of one electron by
the
otner
a
also tiny because the shape of f-sub-shell is
very much diffused. Thus, the effective nuclk
charge experienced by each 4f electrons increase with increase in nuclear charge.
This causes
u contraction in the whole 4f sub shell. So due to this imperfect shielding lanthanide contracu
occurs.
As a whole, the effect results from
poor shielding of nuclear charge by 4f electron3, the 6s electrons are drawn towards the nucleus thus reducing the size of the atomic radiu
Z
Ze (total nuclear charge)
-
a
(shielding effect)
Less the value of shielding effect more the value of nuclear charge.
Thus,
as
number increases, the effective nuclear
charge experienced by each 4f electrons incrcaThis
atome
h consequence of which successive contraction occurs as we move from Las, t0
to Lu
charge
shielding effect is less able to counter the decrease in radius caused by increasing nueic

3. Fi 4.1.2 rysal radii of In", n and Ln lans