The document summarizes lanthanide contraction, which is the decrease in atomic and ionic radii as atomic number increases across the lanthanide series of elements. It occurs due to the imperfect shielding of the 4f electrons, which have low penetrating power and do not effectively shield the outer 6s electrons from the increasing nuclear charge. As atomic number increases, the effective nuclear charge felt by the 4f electrons increases, pulling the electrons closer to the nucleus and causing successive contraction across the series from lanthanum to lutetium. A table of atomic and ionic radii is provided to illustrate this steady decrease with increasing atomic number.

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 sizeofthe ions are graphically represented in figure 4.1.2.

2. University Chemistn
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Table 4.1.5 :
AtomicandlonicRadiiofLanthanoids
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 4felectron3,
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