Lanthanoid Contraction

Lanthanoid Contraction:

In general, the atomic and ionic radii of an element increases with increase of atomic number. But in lanthanoid, with increase in atomic number the atomic and ionic radii of elements gradually decrease. This is called lanthanoid contraction.

Cause of Lanthanoid contraction:

The lanthanoid contraction is the result of a poor shielding effect of 4f electrons.

To understand it better, first let us understand what is shielding effect???

When inner-shell electrons shield the outer-shell electrons from the force of attraction exerted by positively charged nucleus, this phenomenon is called shielding effect.

 And the electrons present in inner-shells (between the nucleus and outer shell) are called shielding electron (because they “shield” the outer-shell electrons from the force of attraction exerted by the positively charged nucleus.)

Due to shielding effect, the outer-shell electrons are not affected by nuclear charge. It means that when the shielding is not as good, the positively charged nucleus has a greater attraction to the electrons.

The shielding effect exerted by the inner electrons decreases in following order:

s > p > d > f

As the atomic number of the members of lanthanoids series increases the positive charge on the nucleus increases by +1 unit and one more electrons enters in the same 4f subshell.

The f subshell has lowest shielding effect. Hence with increase of nuclear charge the electron present in outer-shell (i.e. 5d, 6s) is pulled slightly towards nucleus. As a result of the pull, the size of lanthanoids go on decreasing with increase in atomic number.

 

Though the contraction in size from one element to another is very small, but the net contraction over the fourteen elements from Ce to Lu is appreciable.

Effects of Lanthanoid Contraction:

Due to lanthanoid contraction, following properties of lanthanoids are affected.

1)   Basicity:

Due to lanthanoid contraction the size of tripositive lanthanoid ion (M+) regularaly decreases with increase in atomic number. This results into decrease in basic character.

The ionic character of M-OH bond decreases and covalent character of M-OH bond gradually increases. Therefore the basic strength of corresponding hydroxides decreases from La(OH)₃ to Lu(OH)₃. Thus La(OH)₃   is most basic and Lu(OH)₃  is least basic in nature.

 2)   Ionic radii of post lanthanoids:

The element which follow the lanthanoids in the third transition series are known as post lanthanoids. As a result of lanthanoid contraction, the atomic radii (size) of the electrons which follow lanthanium (Hf, Ta, W etc) are similar to that of the elements of previous period.

There is normal increase in size from Sc to Y to La.

This trend disappears after the lanthanoids and pairs of elements. Zr-Hf (group 4) Nb-Ta, (group 5) Mo-W (group 6) and Tc-Re (group 7) etc. have almost identical sizes. These atoms show similar number of valence electrons and similar properties. These pairs are called “Chemical twins”.

The elements of the second and third transition series resemble each other more closely than the elements of the first and second transition series.