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Two of these elements can cause a substance to become metamict [meh-tah- MIKT], meaning the   substance loses its crystal structure. Monazite [MAH-nuh-zite] and allanite are two such substances.  Before it became easier to use lithium, one of these elements was used to power several pacemakers.   The majority of these elements were first synthesized during the 1940s and 50s, with much of the  work done by Glenn Seaborg. Two of these elements were the primary fillings in the atomic bombs  used during World War II. Name these elements, most of which are created by adding electrons to  the 5 f  electron shell, found below the lanthanides on the periodic table.
Two of these elements can cause a substance to become metamict [meh-tah- MIKT], meaning the   substance loses its crystal structure.
actinides [prompt on radioactive elements; do not accept “transuranic” or “transuranium”  or “transplutonic” or “transplutonium”]  1