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Look at the bottom of any periodic table and you will find two loose rows, the lanthanoids and the actinoids, set apart from the main table. The second row is there because Glenn Seaborg argued for it at a time when senior colleagues thought he was wrong. This post tells that story, works through the chemistry that made the case, and ends with what you can learn from how he worked.
Who was Glenn Seaborg?
His Nobel Prize facts page gives his dates: born 19 April 1912 in Ishpeming, Michigan, died 25 February 1999 in Lafayette, California. He shared the 1951 Nobel Prize in Chemistry with Edwin McMillan "for their discoveries in the chemistry of the transuranium elements". The page notes that in 1940 Seaborg created element 94, plutonium.
His Nobel biography says he earned his PhD at Berkeley in 1937, worked as G. N. Lewis's laboratory assistant, and headed the plutonium work of the Manhattan Project at the University of Chicago Metallurgical Laboratory from 1942 to 1946. He was co-discoverer of plutonium and every further transuranium element up to element 102, and the "author of the actinide concept of heavy element electronic structure". A Berkeley Lab profile, written in his own voice, adds that element 106 was later named seaborgium after him.
Why did the heaviest elements need a new row?
Before 1944, chemists expected the elements beyond plutonium to behave much like plutonium itself, which can be oxidised to a +6 state. If that were right, elements 95 and 96 could be separated by oxidising them to high oxidation states.
His Nobel lecture describes what happened next. Attempts to make and identify elements 95 and 96 on that premise "bore no fruit". Progress came only in the middle of the summer of 1944, "upon the first recognition that these elements were part of an actinide transition series". Once the group expected a stable +3 state and rare-earth-like chemistry, curium-242 was identified that summer and element 95 late in 1944.
The Berkeley Lab profile puts it in his words: his theory "required a major realignment of the periodic table", and "Senior colleagues counseled that advocating such a radical concept would ruin my scientific reputation."
Worked example: making curium and reading its configuration
The lecture gives the reaction that made curium: ²³⁹Pu(α, n)²⁴²Cm. Plutonium-239 is hit by an alpha particle and gives off a neutron.
Write it out: ²³⁹₉₄Pu + ⁴₂He → ²⁴²₉₆Cm + ¹₀n.
Check mass numbers: 239 + 4 = 243 on the left; 242 + 1 = 243 on the right.
Check atomic numbers: 94 + 2 = 96 on the left; 96 + 0 = 96 on the right.
Now the electrons. The NCERT Class 12 chapter on d- and f-block elements gives curium (Z = 96) as [Rn] 5f⁷ 6d¹ 7s². Removing three electrons for Cm³⁺ takes the 7s² and 6d¹ electrons, leaving [Rn] 5f⁷: a half-filled 5f subshell.
That is exactly Seaborg's argument. His lecture says the most important criterion for the classification was "the probable presence of seven 5f electrons (analogous to the stable gadolinium structure) in tripositive curium". Gadolinium, the matching lanthanide, has a half-filled 4f subshell in Gd³⁺.
How did the names americium and curium follow?
The lecture explains that americium was named after the Americas by analogy with its rare-earth partner europium, named after Europe, and curium after Pierre and Marie Curie, matching gadolinium, named after Gadolin. The elements were so like the rare earths that separating them took the better part of a year. Meanwhile one group member called them "pandemonium" and "delirium".
Common mistakes students make with actinoids
Counting fifteen. NCERT defines the actinoids as the fourteen elements from Th to Lr. Actinium itself is listed alongside them.
Removing 5f electrons first. When forming M³⁺, remove 7s and then 6d electrons before touching 5f, as in Cm³⁺ above.
Assuming they behave exactly like lanthanoids. NCERT notes that actinoids show a wider range of oxidation states, with the maximum rising from +4 in Th to +5, +6 and +7 in the next elements. Seaborg's lecture makes the same point about the early members.
For the full d- and f-block chapter, with magnetic moments and colour, see our post on d- and f-block configurations. If you want to see how the table itself was first built, read our story of Mendeleev's periodic table.
If you keep losing track of which subshell fills when, the Fermi Concept Library has every concept in physics, chemistry, biology and mathematics with its own lesson and its place in the map, so you can fix that one idea before moving on.
What can a student learn from Seaborg's way of working?
When experiments built on the accepted picture kept failing, Seaborg changed the picture, made a prediction from the new one, and tested it at once. The prediction worked within weeks. When a method keeps failing you, do not just try harder with the same method; ask which assumption it rests on.
Today, write the configurations of Am³⁺ and Cm³⁺ from the NCERT table and explain why Cm³⁺ is special.
