Question

Why do Zr and Hf have similar properties?

Answer 1

Zr and Hf belong to the fifth and sixth period respectively of group 4 in the modern periodic table. Even after the addition of a new shell in Hf, the atomic radius of Zr is almost the same as that of Hf because of lanthanoid contraction.

Complete step by step solution:
-Both Zr and Hf belong to group 4 of periodic table and Hf comes after 4f series (lanthanide). Due to poor shielding of f-subshell, the attraction of nucleus on outer shell electrons increases and thus the size of Hf decreases and size of Zr and Hf becomes almost the same. This is the reason why both of them have similar properties.
-To understand it in a better way, we need to know about Lanthanide Contraction. In the same shell, the order of shielding is s>p>d>f. So, s and p subshell electrons have good shielding whereas shielding of d and f subshell electrons is poor. Since shielding is poor, the effective nuclear charge is more in d and f orbitals.
-Effective Nuclear charge (${{Z}_{eff}}$) comes into the picture because there are two types of forces experienced by an atom, one is the attractive force between nucleus and electron and another is the repulsive force between electron and electron.
-Calculation of ${{Z}_{eff}}$ is done by Slater’s Rule and is given as
${{Z}_{eff}}$ = Z-$\sigma$
where Z=Atomic number (No of protons)
$\sigma$=Slater’s constant or Screening constant
-If the value of $\sigma$ is more, then ${{Z}_{eff}}$ will be less and vice versa. In d and f orbitals, screening is less and so ${{Z}_{eff}}$ will be more. ${{Z}_{eff}}$ is inversely proportional to the atomic size. If the nuclear charge will be more, then electrons will be attracted more by the nucleus and so the size of the atom will reduce. So, an increase in ${{Z}_{eff}}$ reduces the atomic size and vice versa.
-Atomic radius of Zr and Hf is 206 pm and 208 pm respectively. Even though a new shell is being added to Hf, the size of both of them is almost the same which is not true for s and p block elements. Introduction of a new orbital always leads to an increment in the size of the atom which is not being observed here.
Thus, Zr and Hf have similar properties though they lie in different periods.

Note: The actinide series follow the same pattern as the lanthanide series but their effect is more than the lanthanide series. This is due to the fact that extra f and g orbitals are being added in the actinides which have poorer shielding effect than that of the lanthanide series. So actinoid contraction is more than the lanthanoid contraction.