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.
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
- The gradual decrease in the ionic radii with increase in atomic number of actinoids. In the actinoid series, the nuclear charge and number of 5felectrons increases by one unit.
- The size contraction in actinoids is greater as compared to that in lanthanoids. These 5f orbitals have a poorer shielding effect. The similar concentration in lactinides is called lanthanide contraction.