Q.Name the radioactive element in lanthanoids series.

Promethium is the one lanthanide with no stable isotopes; all of its isotopes are radioactive.

Among the 14 lanthanoid elements (Ce to Lu), Promethium (Pm, atomic number 61) is unique in that it has no stable isotopes — every isotope of promethium is radioactive, with the longest-lived (Pm-145) having a half-life of only about 17.7 years. Because of this, promethium is not found natural …

Lanthanoids characteristically show the +3 oxidation state, arising from loss of the two 6s electrons and one 5d/4f electron.

The lanthanoids (Ce to Lu, following La) have the general electronic configuration [Xe] 4f^(1-14) 5d^(0-1) 6s^2. They predominantly lose the two 6s electrons and one 4f (or 5d) electron to attain the stable +3 oxidation state, which is common to all of them. A few lanthanoids also show +2 or +4 states when it leads to an especially stable f^0, f^7 or f^14 configuration (e.g. Ce^4+ …