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According to Villars and Daams [Journal of Alloys and Compounds, 197 (1993) 177] the atomic volume of U is $2.073\times10^{-2}\,\mathrm{nm^3}$, whereas that of Th is much larger, $3.295\times 10^{-2}\,\mathrm{nm^3}$.

The external electronic configurations of these atoms are:

Th: $[\text{Rn]} 6d^2 7s^2$

U: $[\text{Rn}] 5f^3 6d^1 7s^2$

What's the origin of this large difference?

Why, then, are their ionic sizes (for example the sizes of Th$^{4+}$ and U$^{4+}$) quite similar?

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    $\begingroup$ The title of the question implies there is a different in atomic sizes of Uranium, but the body of the question asks about U vs Th, which are different elements. $\endgroup$
    – hft
    Commented Apr 12, 2022 at 18:07
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    $\begingroup$ Given how the entire concept of atomic sizes is somewhat nebulous, I'd question the consistency of the data before embarking on a journey to explain it. $\endgroup$ Commented Apr 13, 2022 at 9:01

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This is due to the Actinide contraction, analogous to the Lanthanide contraction:

The effect results from poor shielding of nuclear charge (nuclear attractive force on electrons) by 5f electrons; the 6s electrons are drawn towards the nucleus, thus resulting in a smaller atomic radius.

For the Actinide contraction, instead read: "[...] by 4f electrons; the 7s electrons are drawn towards the nucleus, [...]".

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  • $\begingroup$ sorry, but I cannot see how this explain the huge difference between the atomic size of U and Th. Th and U stand below Ce and Nd in the periodic table, but the atomic volume of Ce and Nd are quite similar, whereas that of U is about 60% that of Th. According to Cotton* "The 5f orbitals are not shielded by the filled 6s and 6p subshells as the 4f orbitals of the lanthanides are (by the corresponding 5s and 5p subshells)", but in elemental Th no 5f electrons are present, whereas in U there are 3 5f electrons. On this basis, I would expect larger volume for U. *[Lanthanide and Actinide Chemistry] $\endgroup$
    – gryphys
    Commented Apr 12, 2022 at 19:02
  • $\begingroup$ Firstly, the 'huge difference' looks a lot bigger than when you look at atomic radii rather than volumes (volume requires cubing). Wiki gives empirical data of radius of 180 pm and 175 pm for Th and U resp. (en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page) Secondly, I don't know how 'Cotton' (who?) concludes what he concludes. $\endgroup$
    – Gert
    Commented Apr 12, 2022 at 19:20
  • $\begingroup$ If you look at the Lanthanides, then the contraction of radii isn't spectacular either. Really what you find is no expansion, which in itself is unexpected and explained by the deeper $f$ orbital. $\endgroup$
    – Gert
    Commented Apr 12, 2022 at 19:25
  • $\begingroup$ It seems to me that data reported by Wiki are rather old [Slater (1964)]. I refer to data of Villars and Daams (1993) where, as I wrote the difference in the atomic volumes (not radii) between U and Th are very large. Even if Th and U crystallize with different structures, they have the same coordination number (12). In U the U-U bond distance is 0.275 nm, whereas in Th the Th-Th bond distance is much larger 0.360 nm. Why this difference? This huge difference is not observed for Ce and Pr (where 0.333 and 0.363 bond distances are measured). $\endgroup$
    – gryphys
    Commented Apr 12, 2022 at 19:58
  • $\begingroup$ PS: "... 'Cotton' (who?)". It should be this: birmingham.ac.uk/staff/profiles/chemistry/cotton-simon.aspx $\endgroup$
    – gryphys
    Commented Apr 12, 2022 at 20:01

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