Following the discoveries of transuranic elements neptunium and plutonium in 1940 and preliminary investigations of their chemistry, their placement as a fourth transition metal series was challenged. These new elements exhibited various properties that suggested a close chemical similarity to uranium rather than their supposed transition metal homologs.[1] Subsequent experiments targeting the then-unknown elements americium and curium raised further questions. Seaborg et al. failed to identify these elements under the premise that they were transition metals, but they were successfully separated and discovered in 1944, following the assumption that they would be chemically similar to the lanthanides.[2] Further experiments corroborated the hypothesis of an actinide (then referred to as "thorides" or "uranides")[3] series.