<P> All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth . These are used in nuclear reactors and nuclear weapons . Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern smoke detectors . </P> <P> In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively . This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide - formatted periodic table (32 columns) shows the lanthanide and actinide series in their proper columns, as parts of the table's sixth and seventh rows (periods). </P> <P> Transactinide elements (also, transactinides, or super-heavy elements) are the chemical elements with atomic numbers greater than those of the actinides, the heaviest of which is lawrencium (103). All transactinides of period 7 have been discovered, up to oganesson (element 118). </P> <P> Transactinide elements are also transuranic elements, that is, have an atomic number greater than that of uranium (92), an actinide . The further distinction of having an atomic number greater than the actinides is significant in several ways: </P>

The incomplete period in the modern periodic table