Answers

2016-03-09T17:45:56+05:30
Due to lanthanoid contraction.
after lanthanum electron filling take place in f subshell.
Electrons present in f subshell didn't do good shielding due to which with the increasing atomic number or increasing effective nuclear charge size gets constricted and size of Halfenium and Zirconium becomes almost equal.
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2016-03-09T18:24:54+05:30
Due to lanthanoid contraction in f block, sizes of zirconium and Hafnium becomes quite similar. Lanthanoid Contraction  is defined as the successive decrease in ionic and atomic radii with the increase in atomic number because of the poor sheilding of f orbital electrons. 
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