Actinide
contraction definition in chemistry
The atomic
and ionic radii of the actinide elements gradually decreases as we move from
thorium to lawrencium along actinide
series.
Thus,on
going from Th 3+to Lr 3+
ion ,the ionic radius decrease from
109 pm to 98 pm . There is also a steady
decrease in the radii of An 4+ ion [ An = actinide ] .
This steady
decrease in atomic and ionic radii with increasing atomic number of actinide
elements that is, positive nuclear charge , is called actinide contraction and
it is analogous to lanthanide
contraction.
Although ,in
case of actinide elements, the contraction of volume from one element to
another is higher than lanthanide
element.
Because of
the poor shielding or screening effect of 5f-orbital than 4f-orbital.
The actinide contraction is also known
as f
or 5f- contraction.
causes of actinide contraction
The common
electronic configuration of actinide elements is 5f 1-14 6d 0-1 7s 2 .
In actinide
series elements , the differentiating electrons entered into the inner 5f
–orbital by one at each step from
actinium to lawrencium along actinide series .
Now , the
shielding or screening effect of ‘5f’-orbital is very much less than ‘s’ , ‘p’
even than that of ‘d’-orbital . This is due to the shape of ‘5f’-orbital.
Hence the
effective nuclear charge increases by one at each step .
Consequently
, the inward pull experienced by the 5f electrons increases.
As a result
, the size of the entire 5f n shell reduces . The sum of the
successive reductions gives the actinide
contraction.
Consequences of actinide contraction
Most of the
actinide elements are man made, except Ac, Th , Pa , U. They are formed
by the nuclear reaction. These elements are called trans uranic elements.
However, due to actinide contraction , the ionic size
of the actinide elements are almost same . For this reason, separation of
actinide elements from each other is difficult .
For example,
separation of Ac, Th , Pa and U elements
to each other from uranium ores are not
so easy .
Again, due to actinide contraction, the ionic size from Ac 3+ to Lr 3+ along actinide series
gradually decreases.
As a result,
the covalency of the compounds of actinide metal elements increases from left
to right along actinide series.
That is, the
compounds of lawrencium are most covalent and the compounds of actinium are
least covalent .
Why is Ac ( OH )3 more
basic than Cf ( OH )3?
It has been
experimentally found that with increasing atomic number , the atomic and ionicsize of actinide elements gradually decreases from actinium to lawrencium due to actinium
contraction.
As a result,
the covalent properties of actinide metal hydroxide compounds regularly increases [according to Fajan’s
rule,]from actinium to lawrencium.
That is , Cf
( OH )3 is more covalent than Ac ( OH )3 . Now , with
increasing covalent character, their basic property decreases .
Consequently, Ac ( OH )3 is more basic
than Cf ( OH )3 .
Why is Actinide contraction
more effective than Lanthanide contraction?
Like lanthanide elements,
the atomic radius and ionic radius of actinide elements regularly decreases
with increase in atomic number.
In the case of actinide
series, the amount of volume contraction from one element to another is greater
than that of lanthanide elements.
This
is because the screening effect of the 5f subshell is less than
that of the 4f subshell.
It has been experimentally
found that 4f orbital is more penetrating than 5f sub shell towards nucleus.
For this reason, the
electrons in the 4f subshell are more covered by the electrons in the outer
subshell.
The 5f subshell, on the other hand, extends so far outward toward the outside of the atom that it exceeds the 6s and 6p subshell.
Consequently, the shielding effect of
the 5f subshell is less than that of the 4f subshell. Therefore, actinide
contraction is more effective than lanthanide contraction.
Actinide contraction definition in chemistry
causes of actinide contraction
Consequences of actinide contraction
Why is Ac ( OH )3 more basic than Cf ( OH )3
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