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. 

The Lanthanoids

The Lanthanoids:

·        It is a series of 14 elements

·        They are also known as f block element

·        Elements from atomic number 58 to 71 come in this series

·        General electronic configuration of the element of this series is 4f^1-14, 5d^0-1, 6s²

1)  Position in Periodic table:

In periodic table these 14 elements are placed separately at the bottom of table.  But actually it follows lanthanum (Z=57) of the 6^th period and group 3. Thus:

Lanthanoids belong to group 3 and sixth period of the periodic table.

(Only for sake of convenience these elements are shown at the bottom of the periodic table as lanthanoid series)

2) Electronic Configuration:

The atom of these elememts have electronic configuration with 6s2 common but their occupancy of 4f level is variable.

In this series, electrons are progressively filled in 4f orbital. Therefore the series is also known as 4f series.

The electronic configuration of lanthanoid series are :

4f^1-14, 5d^0-1, 6s² (where n=1-14 with increasing atomic number)

3) Atomic and Ionic size:

With increase in atomic number, the atomic and ionic radii of the element of the lanthanoid series gradually decrease.

This is called lanthanoid contraction and it is a unique feature of the lanthanoid series.

4) Oxidation State:

The common oxidation state of lanthanoids is +3. It is due to the loss of two 6s electrons and one 5d or 4f electron.

However some of the lanthanoids also show either +2 or +4 oxidation state.

5) Physical Properties: 

Lanthanoids show following physical properties:

• ·        The hardness of the lanthanoid increases with atomic number
• ·        Their melting point ranges between 1000 to 1200 k
• ·        They show metallic structure and nature
• ·        Except La+3 and Lu+3, lanthanoids show coloured ions(because of f block)

  6) Chemical Properties:

Lanthanoids show following chemical properties:

·        When lanthanoids are heated around 2500⁰C with carbon, they form carbides

·        When lanthanoids are gently heated with the hydrogen gas, the metal combine with hydrogen and form hydrides

·        When lanthanoids are burnt in oxygen,, they form oxide

·        On heating with nitrogen, lanthanoids form nitrides

·        When lanthanoids are treated with mineral acid, they liberate H2 gas.

·        When lanthanoids are treated with water, they form consequently hydroxides and liberate hydrogen gas

·        On heating with sulphur, lanthanoids form corresponding sulphides