Hassium is predicted to be the densest element at 41 g/cm ^3, but how do you predict something like that? Wikipedia says there have only been 100 hassium atoms made to date and it's half life is less than ten seconds, so I have no idea how a scientist came up with that figure... any insight?
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1$\begingroup$ Perhaps they made educated guesses about the internuclear distance, based on extrapolated data from other transition metals. This could be used to estimate the volume occupied by a given number of the atoms. Calculating the mass for that number of atoms would be trivial. Mass over volume, you have density. $\endgroup$ – electronpusher Dec 16 '16 at 3:18
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3$\begingroup$ One could do a calculation, say with density functional theory. One could have an educated guess of what the most likely crystal structure is, with another educated guess on lattice parameter. One could use a dart board. $\endgroup$ – Jon Custer Dec 16 '16 at 3:18
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1$\begingroup$ You don't need even one real atom to predict anything. The reliability of prediction is another question. Then again, given the extreme instability of the element, it is a prediction that probably will never be verified, so... $\endgroup$ – Ivan Neretin Dec 16 '16 at 6:19
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$\begingroup$ You do a theoretical calculation based on a model that has been verified by testing, i.e. that correctly predicts the density of other, "similar" elements, which are less elusive. $\endgroup$ – Karl Dec 16 '16 at 20:06