Question

Why $5d$ transition series elements have higher ionization enthalpies than $4d$ transition series elements ?

Answer 1

When we move downward in a particular group the atomic no. is increased, consequently nuclear charge is also increased. Due to this reason $5d$-elements have higher nuclear charge than $4d$ elements, therefore, the outer electrons are attracted strongly by the nucleus is $5d$ elements. On the other hand, the shielding effect of $4f$ electrons is very poor due to lanthanide contraction in $5d$ elements, which further increases from $5d$ elements than $3s$ and $4d$ electrons, where no $4f$ electrons and lanthanide contraction is present.