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1. Zr and Hf have similar phyical and chemical properties .why?

This is because the atomic radii of  4d and 5d transition elements are nearly same.

This similarity in size is  consequence of lanthanide contraction. Because of this  lanthanide contraction the radii of Hf becomes nearly equal to that of Zr and as the properties of an element depends on its atomic radii, they show similar properties.