Copper, silver, and gold are often found in combination. That makes sense to me, because they are members of the same chemical family. Likewise, I would expect nickel palladium, and platinum to be found together.

But lead and zinc are often found together, even though they are not of the same chemical family. At least, these metals seem to appear "jointly" more often than lead and tin, even though these are members of the same family.

Why would that be, or am I missing something?


1 Answer 1


Lead and Zinc have in common two electrons "s" in their outer shell. That is why they may sometimes have a similar behavior. You would object that Lead has also two electrons "p" in its outer shell, which are missing in Zinc. It is true. But these p electrons are "modified" and can be "forgotten" by an unusual phenomena in chemistry, namely Einstein's relativity.

When the number of protons in the nucleus is as high as about $80$, and it is $82$ for $\ce{Pb}$, the nucleus charge is so high that the electron has to move at a relativistic speed to stay rotating around the nucleus, according to Bohr's model. Of course Bohr's model is wrong and the electron is not rotating and has no speed. But the calculations made with it are still valid, as was shown by Pekka Pyykko, Relativity and the Periodic Table, Accounts of Chemical Research, Vol. 12, No. 8 (1979) p. 276 - 281.

At relativistic speeds, the dimensions of all moving objects decrease. And Pekka Pyykko has shown that this effect is proportional to the sum of the two quantum numbers n and l. Lead has the following electronic configuration : $\ce{[Xe] 4f^{14} 5d^{10} 6s^2 6p^2}$. The sum n+l is equal to $8$ for $\ce{4f, 5d, 6p}$. It is $7$ for $\ce{6s}$. So the electrons $\ce{4f, 5d, 6p}$ are so contracted that they are mixed with inner electrons and "disappear" from the periphery. Only the two electrons $\ce{6s}$ , with n+l = 7, remain in the outer shell of the atom. That is why Lead $\ce{Pb}$ often behaves like an atom of the $2$nd column : The most usual oxidation number of Pb is $2$ (like in $\ce{Pb^{2+}}$), and not $4$ (like the $4$th column), which exists but is not frequent. Also $\ce{PbSO_4}$ is insoluble in water, like $\ce{BaSO_4}$. So $\ce{Ba}$ is made of $\ce{[Xe] 6s^2}$, and Pb may be considered as a sort of [pseudo-$\ce{Xe] 6s^2}$...

To go back to $\ce{Zn}$ and $\ce{Pb}$ configuration, relativity explains why $\ce{Pb}$ looks like a noble gas (Xenon) plus $2$ electrons "s" in its outer shell. By comparaison, Zinc is $\ce{[Ar] 3d^{10} 4s^2}$. Like $\ce{Pb}$, it also looks like a noble gas (Argon) plus $2$ electrons "s" in its outer shell, if the filled shell $\ce{4d^{10}}$ is "forgotten". Mother Earth is probably not able to recognize the existence of this $\ce{4d}$ shell and has often mixed $\ce{Zn}$ and $\ce{Pb}$ minerals.

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    $\begingroup$ I think this answer is focused on the wrong details. The most common minerals of lead and zinc are galena, PbS and sphalerite, ZnS which are often found together. Trying to understand why those minerals are found together would require a complex discussion of geologic processes. $\endgroup$
    – MaxW
    Commented May 18, 2020 at 22:29
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    $\begingroup$ @MaxW: I think that the above is a very good answer within the realm of Chemistry. You're right that it overlooks non-chemistry issues such as geology. But that is basically an issue for another site, say Earth Sciences. $\endgroup$
    – Tom Au
    Commented May 19, 2020 at 0:07
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    $\begingroup$ @MaxW I was under the impression that lead and silver ores are often found together, or, at least, where there was lead, there was probably also silver. Any truth to that? $\endgroup$
    – Ed V
    Commented May 19, 2020 at 0:40
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    $\begingroup$ @EdV - Yes sulfide minerals are often found together. The mineral $\ce{Ag2S}$ is acanthite. Galena itself can also contain significant quantities of silver. $\endgroup$
    – MaxW
    Commented May 19, 2020 at 0:55

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