In 1871, Dmitri Mendeleev predicted the existence of an element between thorium and uranium. The actinide series was unknown at the time. Therefore, uranium was positioned below tungsten in group VI, and thorium below zirconium in group IV, leaving the space below tantalum in group V empty; until the acceptance of the actinide concept in the late 1940s, periodic tables were published with this structure. For a long time chemists searched for eka-tantalum as an element with similar chemical properties to tantalum, making a discovery of protactinium nearly impossible. Tantalum's heavier analogue was later found to be the transuranic element dubnium – which, however, does not react like tantalum, but like protactinium.