In 1934, Fermi and his team irradiated uranium (Z=92) with neutrons in order to synthesize element 93. They (and others) did not recognize that they had performed a nuclear fission of uranium.

The common explanation for this mistake:

According to the periodic table at that time (see below, shaded elements are missing, no actinide), the transuranium elements (Z>92) are expected to be located in the subgroups IV, V and VI. Hence the chemical experiments to prove the formation of the new element 93 aimed at the chemical similarity to Mn, Ma(Tc) and Re. With the 'discovery' of the actinides this turned out to be wrong. enter image description here

My questions:

The lanthanides were known (except for Pm). Their number and position in the periodic table was correctly described by the electric charge of the nucleus (Moseley, 1913) as well as their electronic configuration (Pauli's Aufbau principle, 1920s).

What is the arrangement of Th, Pa and U based on?

Why did they not expect a second row in the "f-block"?


1 Answer 1


The initial placement of $Th$, $Pa$ and $U$ on the tail of $Ac$ but as transition metals ($d$-block) was wrong but not without merit. Coincidence has it that the main valence of $Th$ is $+4$, as do the elements of Group $4$. Similarly the valence of $Pa$ is $+5$, as does Group $5$. Even $U$'s main valence is $+6$, as expected of a Group $6$ element. Chemically these elements do behave more as transition metals than as Actinides.

Later the elements $Am$, $Cm$ and beyond started showing $+3$ as an important valence and it became clear they belonged to a second $f$-block. The pieces of the puzzle were then reshuffled to the modern and correct version of the Periodic Table.


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