lanthanide contraction.

Atomic and ionic radii of lanthanides                                   ( Lanthanides contraction )

The distance between nucleus and outermost electron of an atom is called as atomic radius.

 The distance between nucleus and point of influence of an ion is called as Ionic radius.

Atomic radius decreases across period and increases down the group. however lanthanides and place in same group and same period. It has been observed that there is a very small decrease in the atomic and ionic radii of lanthanides. This small  and steady decrease in atomic and ionic radii of lanthanides elements with increase in atomic number is called as lanthanide contraction.

The trend of atomic and ionic radii are tabulated in following table and shown in figure.

Following observations can be made from the graph-

1. It can be seen that atomic radius decreases from La (1.83Å) to Lu (1.74Å) by only 0.09Å (9pm) within 14 elements. Also, ionic radius decreases by only 0.17 A (17pm). This is extremely small decrease as compared to s, p and d-block elements.
2. It can be seen that the trend of atomic radius from Ce to Gd gets repeated from Gd to Lu.This leads to repetition of many properties and called as gadolinium break.
3. Atomic radii of Eu and Yb are much higher than expected. This is due to the the fact that this element can donate only two electrons in metallic bonding due to half filled and completely filled 4f subshell respectively while others donate 3 electrons full stop hens metallic bonding is weak and atomic radius (metallic) is higher in these elements.

Cause of Lanthanide contraction

In case of lanthanides, electrons are successively filled in 4f subshell, which is pre-penultimate shell

Also with increase in atomic number, the nuclear force of attraction increases. The outermost shell is 6th shell, which is shielded by added electrons. however, the shapes of f-orbitals are such that the electron cloud is diffused. Also, penetration of f-orbitals towards nucleus is minimum. So shielding effect of f orbitals is very poor.Thus, the increase in attractive force of nucleus cannot be compensated why shielding effect. The net effect is decrease in atomic and Ionic radius due to increasing nuclear charge from La to Lu.

Consequences of lanthanide contraction

Properties of post lanthanide elements (similarity of second and third transition series)

Lanthanum has 5d¹ configuration and placed below Yttrium (4d¹). Hafnium has 5d² configuration and placed below Zirconium (4d²) in the periodic table. In between La and Hf, there are 14 elements (lanthanides) in which there is decrease in atomic and Ionic radius. Obviously the atomic radius of Hf is much smaller than expected. show the increase in atomic radius due to addition of one shell from Zr to Hf has been compensated by lanthanide contraction. Thus, Zr and Hf have similar atomic radius. Also their outermost electronic configuration in same . Hence, their chemical properties are extremely similar and they are called as twin elements are chemical twins.

Similar is the case with Nb - W, Ru - Os,Rh -Ir, Pd-Pt etc and there are also twin elements. Atomic radii (Å) of this elements as metallic radii are given in following table.

Densities of post  lanthanide elements

Density is a ratio of mass and volume. When you from Zr to Hf, the atomic weight increases by about two times (Zr =91.2 and Hf =178.5). However due to lanthanide contraction the atomic radius remains almost the same. Thus density of Hf (11.4) is almost twice as compared to Zr (6.4).

Similar is the case with all the post lanthanide elements which have very high density due to lanthanide contraction as shown below.

Basicity of hydroxides of lanthanides

All the lanthanides from hydroxide in their +3 oxidation states having general formula Ln(OH)3. The ionic size of Ln³+ ions decreases from La to Lu due to lanthanide contraction. As the ionic size decreases, the covalent character of Ln-O Bond goes on increasing (Fajan's rule). Hence, it  becomes difficult for the hydroxide to donate OH- ion and so the basics strength goes on decreasing from La (OH)3 to Lu(OH)3.

In short, La(OH)3 is the strongest base while Lu(OH)3 is the weakest base among the hydroxides.

Separation of lanthanides

Separation of lanthanides from the ores is a difficult task due to similarity in their properties. However due to lantern contraction there is small difference in their Ionic radii. This property can be used for their separation by using ion- exchange method.

Other consequences

• Due to slight decrease in atomic and ionic radii, electronegativity goes on increasing slowly from La to Lu.
• Reduction potential goes on increasing slightly from La to Lu. 
• complex formation tendency changes with change in ionic size.