Isoelectronic species have same number of electrons in them. (a) Pr‌3+ ‌ number of electron =59−3=56 Nd3+ number of electron=60−3=57 ∴Pr3+ and Nd′′ are not isoelectronic. (b) Tb3+ : Total number of electron =65−3=62 Dy 2t : Total number of electron =66−2=64 ∴Tb3+ and Dy2 are not isoelectronic. (c) Eu2+ number of electron =63−2=61 Gd3+ number of electron =64−3=61 ∴Eu2+ and Gd3+ are isoelectronic. (d) Pr3+ number of electron =59−3=56 Ce4+ number of electron =58−4=54 ∴Pr3+ and Cc4+ are not isoelectronic.