Q.Write the name of radioactive element in lanthanide series.

The lanthanoids (La to Lu, Z 57-71) have the general configuration [Xe] 4f0-14 5d0-2 6s2, with the number of 4f electrons varying gradually from 0 to 14 across the series while 6s stays fixed at 2. Because 4f and 5d are extremely close in energy, several elements depart from the simple, naively-expected filling order for the same extra-stability reason used throughout d-block chemistry (e.g. chromium/copper): lanthanum takes [Xe]6s2 5d1 rather than 4f1 (4f sits slightly higher for La specifically); gadolinium takes an extra 5d1 electron ([Xe]4f7 5d1 6s2) specifically to leave its 4f subshell exactly HALF-FILLED (4f7) rather than the expected 4f8; and lutetium likewise takes an extra 5d1 el …