Atomic sizes generally increase as we come down a group of the periodic table. But in the 4th group of the periodic table, Zr and Hf have almost the same atomic sizes. Why?

Explanation

This is a consequence of lanthanoid contraction. Along the lanthanoids there is progressive decrease in size with increase in atomic number which is known as lanthanoid contraction.Since comes immediately after the lanthanoids, its size, is smaller than expected"and it is almost the same as that of Zr.