This
phenomenon is associated with the intervention of the 4f orbitals which
must be filled before the 5d series of elements begin The filling of 4f
before 5d orbital results in a regular decrease in atomic radii called
Lanthanoid contraction which essentially compensates for the expected
4 3 2 Variation in
Atomic and
Ionic Sizes
of
Transition
Metals
�aH
�/kJ mol
–1
Rationalised 2023-24
94
Chemistry
increase in atomic size with increasing atomic number