You and me both Chris. I thought the further up the periodic table you went the heavier the elements got, since lead is after gold i would have though it had been heavier. Unless it a bonding thing? Perhaps gold atoms are closer than lead ones meaning there's more in a certain area, which means that although they are lighter overall the difference balances out in gold's favour.* *That's a guess (not based on principles but just me making it up) So if i'm wrong... well i agree. Basically right. The density depends on the lattice structure of the solid which is determined amongst other things by the outer electron shell of the atoms concerned, together with an increasing density as you increase the atomic weight due to the presence of a heavier protected atomic nucleus nucleus. Based on these 2 main criteria, the relatively less dense elements in the list such as aluminium, tin, antimony and lead have s and p orbital electrons in the outer shell whereas the relatively denser ones such as copper, nickel, gold and silver have s and d orbital electrons in the outer shell. Hence you can get misconceived preconceptions of which is metal is or should be "heavier". Add platinum at 21090 kg/m3 to the previous list. It has a less complete outer shell than gold and so offers greater possibilities for bonding. More bonds = tighter lattice. Not a full explanation but hopefully clear enough.
