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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. $\endgroup$ Commented Jun 22, 2017 at 17:18
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    $\begingroup$ 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
    Commented Jun 22, 2017 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. $\endgroup$ Commented Jun 23, 2017 at 3:58
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    $\begingroup$ Mendeleev did little more than a bit of educated guessing. He was the only one able to do that, not because he was so very educated (he probably was), but because his periodic table gave him the basis to guess from. ;) $\endgroup$
    – Karl
    Commented Feb 17, 2022 at 23:16

2 Answers 2

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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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Germanium is under silicon $\ce{Si}$ $(Z = 14, A = 28.1)$ and above tin $\ce{Sn}$ $(Z = 50, A = 119.7)$. The average of these values may define Ekasilicium. They are : $Z = \frac{14 + 50}{2} = 32, A = \frac{28.1 + 119.7}{2} = 73.9$.

This is not "too far" from Germanium's value : $Z = 32, A = 72.63$.

Same calculations may be done for density, boiling point of chloride (which is rather low), melting point of oxide (which is rather high), etc. The order of magnitude is always correct.

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  • $\begingroup$ There could be done in some cases even two-dimensional interpolation from values of the element grid 3x3, like Al-Si-P/Ga-Ge-As/In-Sn-Sb $\endgroup$
    – Poutnik
    Commented Mar 28, 2022 at 16:57

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