Roentgenium ( Rg ) belongs to this group of elements based on its theoretical electronic configuration , but it is a short-lived transactinide with a half-life of 26 seconds that has been observed only in laboratory conditions .
Roentgenium ( Rg ) belongs to this group of elements based on its electronic configuration , but it is a short-lived transactinide with a 22.8 seconds half-life that has only been observed in laboratory conditions .