What's the densest alkali metal?

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?




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](https://en.wikipedia.org/wiki/Francium), including density, were obtained by extrapolation and not by direct measurements.
According to Bonchev and Kamenska [[2](https://doi.org/10.1021/j150609a021)], $ρ\_\mathrm{calcd.}(\ce{Fr}) \approx \pu{2.9 g cm-3}$, 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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