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<p>[QUOTE=&quot;physics-fan3.14, post: 2235503, member: 19165&quot;]Well, let me break it down and tell you why you&#039;re wrong: a 90% gold coin is mixed by weight, not number of atoms. The atomic weight of gold is 197 (by the way, the gold atom is noticeably larger than copper - but gold is denser, meaning that on the macro scale the same mass will take up less space). That means for a weight of 197 grams of gold, there will be Avogadro&#039;s number of molecules (6.022 x 10^23). The atomic weight of copper is 63 (for a weight of 63 grams, there will be 6.022 x 10^23 molecules).</p><p><br /></p><p>So then, we can ratio that down to 90 grams of gold (which would be 2.75x10^23 molecules) for every 10 grams of copper (which would be 0.955 x 10^23 molecules). The ratio thus becomes about 2.88 gold molecules for every copper molecule. (and thus, just as [USER=7957]@rlm's cents[/USER] said, every 4th atom will be copper)</p><p><br /></p><p>As for the number of molecules in the surface layer itself (here, I will make a number of assumptions, but you&#039;ll get an appreciation for the order of magnitude, which is the important part): The average radius of a gold atom is 144 pm (picometers), the copper atom is 128 pm. We&#039;ll just use an approximation of 140 pm as an average radius, for ease. If the radius of an atom is 140 pm, that means its effective area (its 2-dimensional projection) will be .0615 nm2 (square nanometers). The surface area of a $2.5 gold is 254 mm2 (square millimeters). Thus, I would expect roughly 4.13x10^15 molecules on the surface plane of the coin. Assuming a perfectly homogenous alloy (which might not be the best assumption, but it makes it easier), if 1/4 of those are copper molecules, then I would say that there is plenty of chance for those copper molecules to react and tone. </p><p><br /></p><p>(As I have discussed elsewhere, the layer of the film which causes toning must be 1/2 the wavelength of the light of the complementary color that we see - blue being created by a film about 290 nm thick. It isn&#039;t just the very surface layer. If you look at the width of the atom, we&#039;re talking about 1000 atoms deep for the toning - which means there are even more copper atoms available for toning). </p><p><br /></p><p>Science.[/QUOTE]</p><p><br /></p>

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