Why `Zr` and `Hf` or `Nb` and `Ta` exhibit similar properties?
Or Zirconium (atomic number40) and hafnium number 72) occur together in minerals and they exhibit similar properties. Give reasons.

Answer

Due to lanthanide contraction ,`Hf(Z=72)` has size similar to that of `Zr(Z=40)`. Hence, their properties are similar and therefore,occur together. For the same reason,`Nb` and `Ta` have similar size and hence similar properties.