I came across the following molecular formula for white lead: $2\mathrm{Pb}\mathrm{CO3}·\mathrm{Pb(OH)_2}$. What does the center dot indicate here? Is this a non-covalent interaction of some kind? https://en.wikipedia.org/wiki/White_lead
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$\begingroup$ it helps - can i conclude then that white lead is a double salt based on the link you showed plus the chemical structure shown on wikipedia? $\endgroup$– imrobertCommented Mar 15 at 16:38
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$\begingroup$ It is rather a basic salt, as it is both a salt and a hydroxide. $\endgroup$– PoutnikCommented Apr 8 at 13:39
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1 Answer
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It could be as well written as $\ce{Pb3(OH)2(CO3)2}$.
The formula $\ce{2 PbCO3 . Pb(OH)2}$ is used as better visualization of composition being an intermediate between lead carbonate and lead hydroxide in ratio 2:1.
The dot has similar meaning as in formulas of salt hydrates like $\ce{CuSO4 . 5 H2O}$. Two substances incorporated into the single one in particular ratio, at level of molecular entities (general term involving molecules, atoms, ions, ionic pairs and any other kinds of the smallest building blocks), i.e. not just a mixture.
Another example are minerals malachite and azurite, containing cupric carbonate and cupric hydroxide in ratio 1:1 or 2:1, respectively.
These substances are members of a larger group called basic salts, where anions of acids are complemented with hydroxide or oxide entities, like basic chlorides of magnesium or zinc, or substances in chlorated lime.
It would be a mixture, if a chemist or a nature process took $\ce{PbCO3(s)}$ and $\ce{Pb(OH)2(s)}$, pulvirized them to some grain size, mixed them together and compacted. In these combined compounds, similarly as for salt hydrated, the component ions are regularly alternated within the single crystal structure.
- For $\ce{PbCO3(s)}$, the crystal structure contains 1 $\ce{CO3^2-}$ for 1 $\ce{Pb^2+}$.
- For $\ce{Pb(OH)2(s)}$, the crystal structure contains 2 $\ce{OH-}$ for 1 $\ce{Pb^2+}$.
- For $\ce{2 PbCO3 . Pb(OH)2(s)}$, the crystal structure contains
2 $\ce{OH-}$ and 2 $\ce{CO3^2-}$ for 3 $\ce{Pb^2+}$.
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$\begingroup$ When you say it's not just a mixture, does that mean the lead carbonate and lead hydroxides are chemically bonded, perhaps covalently? $\endgroup$– imrobertCommented Apr 8 at 13:25
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