Question

What is the correct order of ionisation potential?
(A) $5d > 4d$
(B) $3d < 4d$
(C) $4d$ ~ $5d$
(D) All of the above

Answer 1

Ionisation potential is only the other name for the ionisation energy. We all are very much familiar with the term ionisation energy. So basically ionisation potential is the amount of energy required to remove the most loosely bound electron from the gaseous atom or the molecule.

Complete answer:
Since, ionisation potential is the energy required to remove electrons from the gaseous atom, so after the removal of the electron from the isolated gaseous atom, it becomes a positive ion.
And then the second ionisation energy is always greater than the first ionisation energy except in a few cases. And this is because electrons are easy to be removed from the atom rather than an ion because of greater nuclear charge.
Since, here we are asked about the ionisation energies of the atomic orbitals.
The correct answer is option A. i.e. $5d > 4d$
The reason is: $5d$ series has greater ionisation potential than $4d$ series because of the lanthanide contraction.
$5d$ Series consists of the lanthanides, since according to the lanthanide contraction, the $4f$ has poor shielding effect because of which the nuclear charge per electron increases.
And as a result nucleus binds the electron more tightly and thus electrons are difficult to remove and it needs more ionisation energy to be removed and thus as a result has a higher ionisation potential.
Therefore, from the above discussion we can say that the correct option is A.

Note:
$5d$ series has higher ionisation potential not only than $4d$ series but also has a higher potential than $3d$ series and all because of lanthanoid contraction. The other reason for this is because of the big orbital size of the $5d$ series .