In the year 1900 the English scientist and inventor Sir William Crookes pointed out that a new radioactive substance was present in some ores of uranium and he called this substance uranium-X. And in the case of protactinium there is an even further complication. Does it mean somebody realizing that a mineral contains a new element, or does it mean the first time an element is actually isolated? Depending on what choice is made the discovery of protactinium can be assigned to different scientists. Of course it depends on what one really means by the discovery of an element. Finally, as Mendeleev also correctly predicted protactinium occurs with uranium, and more specifically in the ore called pitchblende.īut whereas uranium and thorium were isolated in 17 respectively, it was not until the twentieth century before protactinium, was first discovered. Nevertheless protactinium also shows a strong horizontal analogy with thorium and uranium in also displaying the +4 oxidation state, something that Mendeleev does not seem to have anticipated. Even though protactinium has a lower atomic weight than thorium it should be placed after thorium because of its greater atomic number.īut it appears that Mendeleev's brief predictions were broadly fulfilled since the element does indeed show an analogy with tantalum in forming Pa 2O 5 as its highest and most stable oxide. All four known pair reversals, including the one concerning protactinium and thorium were suddenly resolved. When atomic number was discovered it turned out to be a better ordering principle than atomic weight for the elements in the periodic table. This situation occurs when two elements need to be reversed contrary to their atomic weights in order to order them. Even though this seems reasonably accurate, Mendeleev was somewhat unlucky regarding this atomic weight since he was not to know that protactinium is a member of only four 'pair reversals' in the entire periodic table. The modern atomic weight for the predicted element, or protactinium as it is now known, is close to 231. This element should form a highest oxide X 2O 5, like niobium and tantalum to which it should be analogous.' I quote: 'Between thorium and uranium we can further expect an element with an atomic weight of about 235. In 1871 the discoverer of the periodic table, Dimitri Mendeleev, made the following prediction among several others. To tell us more about the mysteries of protactinium here's Eric Scerri This week we've got an element whose origin and location in the periodic table seems to be causing some complications.

Iridium ore