Question

Why are the ionization energy of 5D elements greater than 3D element?

Answer 1

In 5D series after lanthanum there is unexpected decrease in size due to lanthanide contraction .As a result the effective pull of nuclear charge on valance electron is more in case of 5d elements then in 3d elements and hence ionization energy of the 5d element are greater than 3d elements.

Answer 2

1st ionisation energies of 5d elements are higher than those of 3d or 4d elements this is due to the greater effective nuclear charge acting on outer valence electrons because of weak shielding by 4f electrons!!!!!