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January 18

Lanthanum and Actinium

These 2 elements are d-block or f-block? Simple google search says f-block, but other sources, I have, say d-block. Both present their claim with confidence. Exclusive Editor _{Notify Me!} 15:47, 18 January 2024 (UTC)[reply]

This issue is discussed at Periodic table#Group 3 and Group 3 element. La and Ac were thought to be d-block back when electron configurations were first being measured: in 1927, Friedrich Hund thought that the usual trivalency of rare earths meant the configurations of La through Lu were 4f^0–145d¹6s², so that La would be d-block, Ce-Lu would be f-block, and then Hf-Hg would continue the d-block. Since then it became known that this doesn't happen (in fact, almost all 4f elements lack a 5d electron as gaseous atoms), and that the relationship between electron configuration and chemistry is not so simple: transition elements are better thought of as having different configurations depending on their environment. Since at least 1965 (doi:10.1119/1.1972042) it has been suggested that La is really an f-element, as it has chemically accessible f-orbitals that are implicated in bonding, and later authors who focused on this issue have tended to agree. More generally, the elements that can use f-orbitals for bonding are La-Yb and Ac-No, so those are the most natural choice for the f-elements, and Lu and Lr are best treated as the first members of the subsequent d-series. With that said, many textbooks still persist in the old version with La as a d-element – probably as a result of a long string of copying back to Sargent–Welch.

(The same issue of course applies to actinium, but it was naturally studied less due to its radioactivity. That it is an f-element was confirmed by calculations last year: doi:10.1002/jcc.26929.)

P.S. I collected a lot of links to papers about this at User:Double sharp/Group 3 sources. Double sharp (talk) 16:39, 18 January 2024 (UTC)[reply]

@ExclusiveEditor: forgot to ping you. Double sharp (talk) 17:16, 21 January 2024 (UTC)[reply]