After the discovery of uranium — recognised in 1789 by Klaproth — and thorium (Berzelius, 1828), a considerable period elapsed before actinium (1899) and protactinium (1913) were recognised. For many years they were regarded as a new transition-metal series and placed in groups IIIA–VIA of the Periodic Table. Following the discovery of the transuranium elements, increased chemical and spectroscopic study, particularly of the later actinides with their increasing emphasis on the +3 state, revealed them to be a ‘heavier version’ of the lanthanide series, in which the 5f orbitals were being filled.
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