3.1 Ionisation Enthalpy

  d and f Block Elements

• Ionisation enthalpies of these elements are very high due to smaller atomic size and high nuclear charge due to poor shielding of $(n-1)d$ electrons.

• These values are between $s$-block and $p$-block elements.

• First ionisation enthalpy values of these elements increases from left to right but the increase is not so sharp.

• First ioniation enthalpy values of third transitional series are higher than the elements of first and second transition series due to very poor shielding of $4f$ electrons in third transition series elements.

• Ionisation enthalpy and thermodynamic stability of transition metal compounds:

• As the sum of ionisation enthalpies of transition metal required to attain a particular oxidation state decreases the stability of the compound of the metal in that oxidation state also increases.

• For example. nickel $(II)$ compounds are more stable than platinum $(IV)$ compounds are more stable than Nickel $(IV)$.

• The ionisation enthalpy required to convert $Ni$ to $N{i^{2 + }}$ is less than ionisation enthalpy required to convert $Pt$ to $P{t^{ + 2}}$.

• Conversion of $Pt$ to $P{t^{ + 4}}$ require less ionisation energy than $Ni$ to $N{i^{ + 4}}$.

• ${K_2}PtC{l_6}$ is well known compound in which $Pt$ is tetravalent where as compounds of $Ni$ are unknown.