State the general electronic configuration of lanthanoids. Why there are irregularities in the electronic configuration of lanthanoids?
Solution : The general electronic configuration of lanthanoid elements is `[Xe] 4f^(1-14) 5d^(0-1) 5s^(2)` however, in a tripositive state the general configuration is `[Xe] 4f^(n) (n =1 " to " 14)` <br> <img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/KPK_AIO_CHE_XII_P1_C08_E01_039_S01.png" width="80%"> <br> Reason for the irregularities: The irregularities in electronic configurations are because of very less energy difference between 4f adn 5d orbitals. Hence, the last electron may enter either in 4f and 5d orbitals <br> In lanthanoids, f-orbitals must be filled successively with the increase in atomic number. However, in lanthanum (La), cerium (Ce), gadolinium (Gd) and lutetium, one electron is present in 5d orbital. These irregularities are because of relative stabilities of `f^(0), f^(7) and f^(14)` configuration in lanthanum, gadolinium and lutetium respectively. However, cerium is accepted presently as exception. <br> As lanthanum and lutetium has one 5d electrons, these elements are sometime referred as group 3 elements
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