Question: what is the major difference between lanthanides and actinides?
Answer: Lanthanides involve the filling of 4f orbitals while actinides involve the filling of 5f orbitals in their atoms. Size of the atom/ion decreases across the period. In the group, size of a lanthanide element is smaller than the actinide of its own group. Atomic and ionic size Size of atom/ion decreases along the period.