CBSE Class 12-science Answered

Actinides are elements with atomic numbers from 90 to 103 following element Actinium. They include naturally occurring elements of thorium, protactinium and uranium and eleven transuranic i.e., artificially produced by nuclear reactions. Nevertheless, all actinides are radioactive.

The atomic size/ ionic radii of tri positive actinides ions decrease steadily from Th to Lw due to increasing nuclear charge and electrons entering inner (n-2) f orbital. This gradual decrease in the size with an increasing atomic number is called actinide contraction.