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I have studied that sulphide salts of s block are colourless while p and d block salts are coloured.

But colour intensity of a salt is directly proportional to polarisation in salt.

And s block cations being smaller offer more polarisation. According to this logic, s block salts should be coloured.

Is my knowledge incomplete? Or am I using wrong logic?

priyal
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  • Polarisation and colour are not related. Well, at least not directly. – Ivan Neretin May 13 '20 at 08:39
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    sodium sulfide is yellow. – Nilay Ghosh May 13 '20 at 09:07
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    Pure sodium sulfide is colorless. I do have a bottle of $\ce{Na2S·9H2O}$ in my collection. Technical sodium sulfide is yellow because of the presence of sulfur as impurities. And this impurity of often produced by atmospheric oxidation releasing $\ce{Na2O}$ and free sulfur. Then free sulfur reacts with $\ce{Na2S}$ to produce disulfide $\ce{Na2S2}$ which contains the ion $\ce{S^{2-}}$ which is yellow, the same yellow color as in pyrite $\ce{FeS2}$ – – Maurice May 13 '20 at 09:25
  • "But colour intensity of a salt is directly proportional to polarisation in salt." This is a false premise. – Mithoron May 13 '20 at 19:40
  • Color of a salt is mainly due electronic transitions, MLCT, LMCT in transition metal complexes. Also convergence of HOMO-LUMO gap leads to easier excitations, small HOMO-LUMO gap make π electrons mobile, since it is easy for the electron to jump to a higher energy level that is close in energy. "Higher polarisation" in salts definitely lead to decrease in HOMO-LUMO gap. But polarization in group 1 salts is not so well due to large size of cations. – Zenix May 13 '20 at 19:47
  • See the answer of this question: https://chemistry.stackexchange.com/questions/138309/the-color-of-the-most-sulfides-of-p-and-d-block-elements – Nilay Ghosh Nov 23 '20 at 06:27

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