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Tribunus Plebis PerpetuusProcurator MonetaeCaesarPosts: 11631Omnes vulnerant, ultima necat. Why is gold yellow? « on: November 26, 2005, 07:16:55 pm »



Today I found an article in 'Spektrum der Wissenschaft, 12/05' (the



The colour of metals naturally is a feature of their surface and their surface consists of atomic lattices. Responsible for the colour are special features of their atoms. I think all of us know the atom model of Nils Bohr.There are electrons circulating on orbits around the atomic nucleus like planets around the sun. Meanwhile we know that the reality is much more complicated. According to the Heisenberg uncertainty principle it is impossible to state place and impuls of an electron arbitrary precisely. Therefore there are no real pathes of electrons, but only states of probability where an electron could be found. The physicists are speaking of orbitals. They distinguish between s-electrons with orbicular orbitals, p-electrons with orbitals having the shape of a bar-bell, and with increasing azimuthal quantum numbers even more complicated orbitals for d- and f-electrons. Important is that with increasing azimuthal quantum number the probability to come in the proximity of the atomic nucleus is decreasing. So a s-electron more often



1928 Dirac and Schrödinger could show that electrons in the proximity of the nucleus could achieve velocities near to the velocity of light. On mercury with the nuclear charge of 80 an electron in the proximity of the nucleus could reach 58% of the velocity of light. That means nothing else than the conditions of the special theory of relativity are valid! The mass of the electron increases about 23%! Thus the



Until 1970 physicists were thinking that electrons only rarely come into the proximity of the nucleus. Meanwhile they know that this is not true. They do! The direct relativistic effects caused by that are most distinct on the s-electrons, but only slight on the d- and f-electrons.Here we have the inverse effect. Because the s-electrons approach more closely to the nucleus they indirect relativistic effects are increasing with increasing nuclear charge. This makes an impact on the size of an atom. Gold has 32 electrons and about double of the mass of silver. Nevertheless it is a



The most remarkable feature of gold is its intense yellow colour as compared with the grey-white of silver. Why is that so? On metals the orbitals broaden to bands which extend over the whole crystal. The most exterior electrons are on the so-called valence band. On gold these are the d-electrons whose energy by indirect relativistic effects - as described - will be increased. Thereby the whole valence band will be raised. Above it - as usually on metals - is located the conduction band. This is formed on gold by s-orbitals. Here the energy will be decreased by direct relativistic effects and the band will be lowered. Thus the distance between these two bands will be decreased. On silver it is



Nice, isn't it?



Best regards

Hi!Today I found an article in 'Spektrum der Wissenschaft, 12/05' (the German edition of 'Scientific American') about the subject, why gold is yellow (and silver white). Because we as coin collectors are daily confronted with these metals, I think this theme is interesting enough to share with you. Because the explanation reaches deep into Quantum Mechanics and the Special Theory of Relativity this will be a bit superficial for a theoretical physicist or chemist. But it will be difficult enough.The colour of metals naturally is a feature of their surface and their surface consists of atomic lattices. Responsible for the colour are special features of their atoms. I think all of us know the atom model of Nils Bohr.There are electrons circulating on orbits around the atomic nucleus like planets around the sun. Meanwhile we know that the reality is much more complicated. According to the Heisenberg uncertainty principle it is impossible to state place and impuls of an electron arbitrary precisely. Therefore there are no real pathes of electrons, but only states of probability where an electron could be found. The physicists are speaking of orbitals. They distinguish between s-electrons with orbicular orbitals, p-electrons with orbitals having the shape of a bar-bell, and with increasing azimuthal quantum numbers even more complicated orbitals for d- and f-electrons. Important is that with increasing azimuthal quantum number the probability to come in the proximity of the atomic nucleus is decreasing. So a s-electron more often comes around near the nucleus than a p-electron, and a p-electron more often than a d-electron and so on.1928 Dirac and Schrödinger could show that electrons in the proximity of the nucleus could achieve velocities near to the velocity of light. On mercury with the nuclear charge of 80 an electron in the proximity of the nucleus could reach 58% of the velocity of light. That means nothing else than the conditions of the special theory of relativity are valid! The mass of the electron increases about 23%! Thus the average distance to the nucleus shrinks, the electrostatic attraction between them raises and the total energy of the system decreases. For this cognition Dirac and Schrödinger get the Nobel prize 1933.Until 1970 physicists were thinking that electrons only rarely come into the proximity of the nucleus. Meanwhile they know that this is not true. They do! Thecaused by that are most distinct on the s-electrons, but only slight on the d- and f-electrons.Here we have the inverse effect. Because the s-electrons approach more closely to the nucleus they shield stronger from the positive charge of the nucleus, and the d- and f-electrons move more outwards. Whereas the p-electrons are heavier to push out, the d-electrons are easier to eliminate. Theseare increasing with increasing nuclear charge. This makes an impact on the size of an atom. Gold has 32 electrons and about double of the mass of silver. Nevertheless it is a bit smaller than a silver atom. The reason for that is the strong contraction of its outer s-orbitals.The most remarkable feature of gold is its intense yellow colour as compared with the grey-white of silver. Why is that so? On metals the orbitals broaden to bands which extend over the whole crystal. The most exterior electrons are on the so-called valence band. On gold these are the d-electrons whose energy by indirect relativistic effects - as described - will be increased. Thereby the whole valence band will be raised. Above it - as usually on metals - is located the conduction band. This is formed on gold by s-orbitals. Here the energy will be decreased by direct relativistic effects and the band will be lowered. Thus the distance between these two bands will be decreased. On silver it is still so big that only ultraviolet photons could lift an electron from the valence band to the conduction band. On gold these two bands are so narrow that even photons with less energy from the blue range of the spectrum can do this. Hence gold absorbs blue light and appears to our eyes in the complementary colour, the intense yellow we all love so much!Nice, isn't it?Best regards Logged https://www.forumancientcoins.com/gallery/index.php?cat=10672