please explain lanthanoid contraction with examples and reason for it

As the ionic radii of the elements in the lanthanide series keeps on decreasing more than we normally expect (this is not an exception, because ionic radii does decrease along a period, albeit in a less rapid fashion), it’s the elements just after the lanthanide series which bear the mark of being exceptional cases.

Because of lanthanide contraction, the elements just after the entire lanthanide series actually have ionic radii less than that of their predecessors in the same group. This ‘decrease in the size’ is the mother of exceptions that I had proclaimed of so zealously.

Much as this seems like a passing property of the lanthanide series, do not be deceived. For this very thing shall haunt the daylights out of you till you till you are done with the second paper of JEE Advanced. Even the most innocuous question in JEE Advanced can turn into a death trap because it has been laced with lanthanide contraction. It simply is because lanthanide contraction has got a mammoth’s footprint in inorganic chemistry. Before long, you’ll be spouting ‘lanthanide contraction’ to solve every problem which can’t be solved in less than 180 seconds (don’t do that).

Let me give you an example of how widespread lanthanide contraction (or it’s subsequent immediate effect) is. Consider this bit of information:

The atomic radius of the metal zirconium, Zr, (a period-5 transition element) is 159 pm and that of hafnium, Hf, (the corresponding period-6 element) is 156 pm. The ionic radius of Zr4+ is 79 pm and that of Hf4+ is 78 pm. The radii are very similar even though the number of electrons increases from 40 to 72 and the atomic mass increases from 91.22 to 178.49 g/mol.

Zirconium and hafnium therefore have very similar chemical behaviour, having closely similar radii and electron configurations. Radius-dependent properties such as lattice energies, solvation energies, and stability constants of complexes are also similar. Because of this similarity hafnium is found only in association with zirconium. Titanium, on the other hand, is in the same group but differs enough from those two metals that it is seldom found with them.

Thus, lanthanide contraction leads to formation of pairs of elements, from the same group, which have very similar properties between them. These elements are known as chemical twins. Zr-Hf is one example.