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I have noticed that the colours that the F-centres impart in Alkali metal halides are the same that the corresponding alkali metals impart to their flames.

$\ce{NaCl}$ when heated in an atmosphere of $\ce{Na}$ becomes yellow.

$\ce{LiCl} \rightarrow$ pink

$\ce{KCl} \rightarrow$ violet/lilac

Why ?

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    $\begingroup$ However, ZnO gives yellow color due to F-center formation while its flame coloration is blue-green to pale green. $\endgroup$ – adianadiadi Feb 29 '16 at 6:50
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Why are the colours the same as the metal. Basically, the anion doesn't affect the colour, but the cation does. And since the cation is just the metal missing an electron or several, of course, they exhibit similar behaviour!

Colours we see are determined by the transitions made by electrons "falling" from a high energy state to a lower energy state, and in the process releasing one or some photons with a frequency in what we call the visible spectrum.

When an F-centre is formed in the described conditions, we have effectively extended the lattice with additional cations the same as those already present. But rather than additional anions that match as well, free electrons take the place in the lattice that would be occupied by one.

Now the anion already has its orbitals filled - there is nowhere for an associated electron to move into when excited, nor somewhere for other electrons to enter when relaxed, and thus there is no way for them to release energy as photons that we sense as light. So the anion doesn't affect the colour.

However the cation has empty orbitals when neutral, let alone when positively charged (i.e. has an electron(s) removed). This means any excitation or relaxation will involve these orbitals.

The result is that the electron movements in the F-centre lattice are the same as those that would occur in the ionisation of the metal which the cation is derived from, and hence the colours are also the same.

Why those specific colours for the respective compounds? That would be a completely different question, and probably found in a quantum mechanics discussion rather than a chemistry area.

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  • $\begingroup$ There are some exceptions, as always, due to the way certain orbitals can interact - like those in zinc oxide, for example. As mentioned though, again, this is closer to QM than chemistry. $\endgroup$ – Nij Feb 29 '16 at 12:14
  • $\begingroup$ The colours in case of an F-centre are formed due to excitation (followed by deexcitation) of unpaired electrons which occupy the anion vacancy sites in a crystal and not due to the electrons in the cations.. $\endgroup$ – Abhirikshma Mar 8 '16 at 7:29
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I thought F-center absorptions tracked the lattice parameter fairly closely. This means that the color is a function of the size of the vacancy the electron fills (determined by the specific anion that's missing). As a counter example RbCl and KBr have nearly identical colors due to f-center, but a flame test can clearly distinguish Rb and K. Likewise the color due to f-center defects in KF is much different than in KBr.

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