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While studying about Mendeleev's periodic table in my textbook( class 11 NCERT chemistry), I have studied that Mendeleev has predicted some elements like Eka-boron(scandium), Eka-Aluminium(Gallium),Eka-Silicon(Germanium)etc. He also predicted their physical properties which were most accurate. One such element about germanium is given in the below link.

How could he predict such accurate values of such physical properties?enter image description here

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    $\begingroup$ Why, that's simple, and you can do it yourself: he just averaged the properties of the neighboring elements which were already known. $\endgroup$ – Ivan Neretin Jun 22 '17 at 17:18
  • $\begingroup$ Mendeleev's predictions were not accurate, but rather *not too far off". As expected. When you find something surprising in natural sciences, that usually means your previous thinking was stupid, and you should have known. ;-) $\endgroup$ – Karl Jun 22 '17 at 20:56
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    $\begingroup$ This is an interesting question. I hope to see someone explain how Mendeleeev got the boiling points. $\endgroup$ – Pritt says Reinstate Monica Jun 23 '17 at 3:58
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Well, guess what, even I study the exact same textbook (class 11 NCERT, from India). I had asked my teacher the exact same doubt, and the answer was:

Due to its proximity with neighbouring substances, like Arsenic (which was already discovered by then - see here), Mendeleev was able to accurately calculate the average values of density and melting point of Germanium, or eka-Silicon.

The formula of its Oxide and Chloride was predicted with respect to the formula of the Oxides and Chlorides formed by other elements of the group IVA (as per old IUPAC numbering), such as $\ce{CO2}$, $\ce{SiO2}$, $\ce{CCl4}$ and $\ce{SiCl4}$. Their densities were calculated using the knowledge of Chlorine's and Oxygen's atomic weight, and the predicted density and/or atomic mass of the new element.

Although I don't know how the boiling point of a substance can be predicted, my best guess for it is that it was approximated using the boiling point of other similar compounds near it in the Periodic table. For more information on how Boiling point can be possibly approximated, here is a link.

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