Actinoids show so many oxidation states but lanthanoids mostly +3, is that only because 5f differs from 4f?

I'm making my lanthanoid vs actinoid comparison table (obviously with colours!!) and I got stuck on the oxidation state row.

NCERT says lanthanoids mostly show +3, with +2 and +4 only in a few cases, while actinoids show a much wider range, like +3 to +7 for some. The book says it's because 5f, 6d and 7s are close in energy. But then the same idea should apply to 4f, 5d and 6s for lanthanoids too, right? What's the real difference?

I also have these rows and I'm not sure about them:

Actinoid contraction vs lanthanoid contraction: is the actinoid one bigger?
Radioactivity: are all actinoids radioactive, and are they all man-made?
Chemical reactivity: do actinoids react with acids and water the way lanthanoids do?

Also which elements are the standard examples of +2 and +4 in the lanthanoids? Like I know Ce shows +4 and Eu shows +2 but why those two? I want a one-line reason in the table.

If somebody has the cleanest version of this table please share, I will copy it in my notes with credit!!

7 answers & replies

[+]Rahul DubeyDropperSample· 1mo ago

The difference is how buried the f orbitals are. 4f is deep inside, shielded by the filled 5s and 5p, so those electrons barely take part in bonding. 5f is more spread out and less shielded, so 5f electrons are much more available for bonding. That's why actinoids can use 5f, 6d and 7s electrons and reach a wide range of states.

Numbers to remember: lanthanoids, +3 is common, +2 in Eu, +4 in Ce. Actinoids, +3 is common, Th mostly +4, Pa +5, U goes +3 to +6, Np and Pu go up to +7.

[+][–]Tanvi Deshmukh12thSample· 1mo agoAuthor
the buried vs spread out explanation is exactly what I needed for the table, adding it with a little diagram!

[+]Farhan ShaikhDropperSample· 1mo ago

I think all actinoids are radioactive and man-made, that's why not much is known about them compared to the lanthanoids. NCERT does say they are radioactive.

[+]Ishita GoyalDropperSample· 1mo ago

All radioactive yes, but not all man-made. Thorium, protactinium and uranium occur naturally. It's the elements after uranium (neptunium onwards, the transuranium elements) which are made artificially in nuclear reactions. Many of the later ones are produced only in tiny amounts and are short-lived, which is why they are hard to study.

[+][–]Tanvi Deshmukh12thSample· 1mo agoAuthor
Thank you!! Fixing my note, I had written "all man-made". Th, Pa, U natural, transuranium elements artificial!!

[+]Aman YadavDropperSample· 1mo ago

Reactivity. Actinoids are highly reactive metals, finely divided ones are pyrophoric. With boiling water they give a mixture of oxide and hydride. HCl attacks all of them. With HNO3 most are only slightly attacked because a protective oxide layer forms. Alkalis do nothing. Lanthanoids are also reactive, but actinoids more so.

[+]Neel Chatterjee12thSample· 1mo ago

Why Ce and Eu: it's about reaching a stable configuration. $Ce^{4+}$ is $4f^0$ , an empty f shell, a noble-gas-like arrangement. $Eu^{2+}$ is $4f^7$ , half-filled. Similarly $Tb^{4+}$ is $4f^7$ and $Yb^{2+}$ is $4f^{14}$ . Because of that $Ce^{4+}$ is a good oxidising agent (it wants to go back to +3) and $Eu^{2+}$ is a reducing agent (it wants to go to +3). The +3 state being the stable one overall is why they act that way.

Actinoid contraction is larger than the lanthanoid one, since 5f electrons shield poorly compared to 4f.

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