Question

Explain why:
$Zr$ and $Hf$ exhibit similar properties. (Atomic number of $Zr = 40$ and $Hf = 72$)

Answer 1

They belong to $d -$ block elements. By looking at their atomic numbers we can find the electronic configuration and then the position of these elements in the periodic table can be determined. And we know that the elements who belong to the same group almost exhibit similar properties. And the reason why they have similar properties is lanthanide contraction.

Complete step by step answer:
We know that both the elements given in the question to $d -$ block. In $d -$ block elements there are two types of contraction named as lanthanide contraction and actinide contraction. Due to these contractions there are changes in the properties of $d -$block elements.
Lanthanide contraction: Lanthanum group is from $57$ to $71$. These are $f -$block elements and comes just before the $Hf$. The Lanthanide group is placed separately in the periodic table. Lanthanide contraction is contraction which is caused due to these lanthanide group elements having filled$4f -$orbitals in them. In this contraction there is change in the decrease in atomic radii of $d -$block elements. As we move from left to right in a period of a periodic table there is a decrease in atomic radii. And if we move top to bottom in a group of a periodic table there is an increase in atomic radii. But due to these lanthanide contractions there is sudden decrease (more than expected) in atomic radii. Hence the atomic radii of $Zr$ and $Hf$ are almost similar. So their properties are also similar due to lanthanide contraction.
Actinide contraction: Actinide group is from $90$ to $103$. These are $f -$ block elements and comes just before the $Rf$. As like the lanthanide group, the actinide group is also placed separately in the periodic table. Actinide contraction is contraction which is caused due to these actinide group elements having filled $5f -$orbitals along with filled $4f -$ orbitals in them. In this contraction there is change in the decrease in atomic radii of $d -$ block elements. But this actinide contraction is more than that of lanthanide contraction. As in lanthanide contraction the contraction is only because of $4f -$ orbitals but in actinide contraction the contraction is because of both $4f$ and $5f -$ orbitals.As we move from left to right in a period of a periodic table there is decrease in atomic radii. And if we move top to bottom in a group of a periodic table there is an increase in atomic radii. But due to these actinide contractions there is sudden decrease (more than expected) in atomic radii.

Note:
There is one more term which is actinides. But here for $Hf$ lanthanide contraction is the cause not actinide contraction. Actinide contraction is for $Rf$ not for $Hf$.Lanthanides are used in optical devices (night vision goggles), petroleum refining, and alloys. Actinides are found primarily in applications where their radioactivity can be used to power devices such as cardiac pacemakers.