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This was asked in a school exam of mine, GCSE curriculum, exactly as it was worded on the title. I found this question a bit vague, and answered it francium, however of course francium is just an estimation. I couldn't be sure what the answer to the question would be, so can anyone give me some insight?

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    $\begingroup$ The question is more than a bit vague. Privately I consider Cs the right answer, but they most likely wanted you to say Fr, even though Fr is not really a thing. $\endgroup$ – Ivan Neretin May 30 at 7:04
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    $\begingroup$ According to de.wikipedia.org/wiki/Alkalimetalle#Trends Fr is even sligthly less dense than Cs. Also sodium is definitely denser than potassium. The upward trend just isn't perfectly clean. All in all, this is quite a stupid question to ask in an exam. Do they want you to learn these numbers by heart? $\endgroup$ – Karl May 30 at 7:28
  • $\begingroup$ Otoh, all period 7 elements are considerably denser than their counterparts in period 6, and those than theirs in 5. Only periods 2-4 are not continguous. I think I knew this at the end of high school. ;-) $\endgroup$ – Karl May 30 at 9:57
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It's hard to tell what is the answer you were supposed to provide, but yes, generally speaking, you are correct. According to CRC handbook of chemistry and physics [1, p. 4-14] (emphasis mine):

Francium, the heaviest known member of the alkali metal series, occurs as a result of an alpha disintegration of actinium. […]

Because all known isotopes of francium are highly unstable, knowledge of the chemical properties of this element comes from radiochemical techniques. No weighable quantity of the element has been prepared or isolated.

As the result, the majority of physical properties of francium, including density, were obtained by extrapolation and not by direct measurements. According to Bonchev and Kamenska [2], $ρ_\mathrm{calcd.}(\ce{Fr}) \approx \pu{2.9 g cm-3}$, which is in a good agreement with the trend for the measured densities across the group. For comparison, the following data (NTP conditions) is collected from the table PHYSICAL CONSTANTS OF INORGANIC COMPOUNDS [1, p. 4-43]:

$$ \begin{array}{cl} \hline \text{Element} & ρ/\pu{g cm-3} \\ \hline \ce{Li} & 0.534 \\ \ce{Na} & 0.97 \\ \ce{K} & 0.89 \\ \ce{Rb} & 1.53 \\ \ce{Cs} & 1.873 \\ \hline \end{array} $$

References

  1. Haynes, W. M.; Lide, D. R.; Bruno, T. J. CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data.; CRC Press, 2017; Vol. 97. ISBN 978-1-4987-5429-3.
  2. Bonchev, D.; Kamenska, V. Predicting the Properties of the 113-120 Transactinide Elements. J. Phys. Chem. 1981, 85 (9), 1177–1186. https://doi.org/10.1021/j150609a021.
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    $\begingroup$ A linear extrapolation though Li-Cs runs almost exactly though the value of 2.9 at 223. Bit weird, that. $\endgroup$ – Karl May 30 at 9:37

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