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Question:
The atomic weights of lead and gold are close, but their densities are almost 2-1. I thought these values would sort of track each other.. Why do they not do so?



Replies:
Well, going *down* the periodic table they do quite well. Consider lead's column: carbon (2.26 g/cc), silicon (2.33), germanium (5.32), tin (7.30) and lead (11.4). But going across the periodic table, i.e. from gold to lead, they do not. As you go across the table the electronic structure of the atoms changes strongly, and the chemical properties change strongly too. You can go from metals to nonmetals, for example, as in aluminum (3rd row, column 3A) to phosphorus (3rd row, column 5A). That is why the periodic table is organized the way it is. Going across the table you visit different families of elements. Going down you visit elements *within* a family and the properties are similar and change simply. Now in the case of gold the relevant chemical property is the form of crystal gold forms in the solid state. Gold atoms form tighter bonds to their neighbors than do lead atoms, so the melting point is higher (1000 deg C vs. 300) and the distance between neighboring atoms is less (4.08 angstroms vs. 4.95). So the gold atoms are more closely packed, hence the gold is denser.

christopher grayce


One minor correction there. The distance between gold atoms is only 2.88 Angstroms, and between neighboring lead atoms is 3.50. The other variable, besides distance between neighbors, that is important in determining the density is the NUMBER of near neighbors - both gold and lead (and nearly all the metals) have 12 near neighbors, so this does not change much here. However, carbon for example takes the form of either graphite or diamond, and the bond lengths in graphite are shorter than in diamond, but the density is considerably lower first because each atom in graphite has only 3 near neighbors rather than 4 in diamond, and second because in graphite the carbon is packed in layers which are then quite far apart. These materials questions are always very interesting...

Arthur Smith



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