Q.What are actinides? Give three examples.

The actinoids (Th to Lr, following Ac) are all radioactive, mostly with short half-lives; only Th and U occur naturally in significant amounts, with trace Pu found in uranium ores, while Np and heavier elements are made synthetically by nuclear transformation. Like the lanthanoids, they are placed as a detached row at the bottom of the table.

Electronic configuration is less clean-cut than the lanthanoids', because 5f/6d/7s lie even closer in energy than 4f/5d/6s. General formula: [Rn]5f⁰⁻¹⁴6d⁰⁻²7s². Specific irregularities: Ac is [Rn]5f⁰6d¹7s²; Th is [Rn]5f⁰6d²7s² (no 5f electrons at all, unlike Ce which starts 4f filling immediately); Pa/U/Np each retain one 6d electron while adding 5f electrons; from Pu onward the 6d electron drops out entirely until it reappears at the very last element, Lr ([Rn]5f¹⁴6d¹7s²), paralleling Lu. …