The elements in which electrons are progressively filled in 4f-orbital are called
(i) Actinoids
(ii) Transition elements
(iii) Lanthanoids
(iv) Halogens
The answer is the option (iii) Lanthanoids
Explanation: The sixth period (n = 6) contains 32 elements, and after that, the successive electrons enter the 6s, 4f, 5d and 6p orbitals and in that order, filling up of ff orbital begins with cerium (Z=58) and ends at Lutetium(Z=71). This complete 4f inner transition series is called the lanthanoid series.