Explain the metal-excess defect that arises from anion vacancies (F-centres), using $\text{NaCl}$ heated in sodium vapour as an example. Why does this defect give the crystal a colour?

Explain the metal-excess defect that arises from anion vacancies (F-centres), using NaCl heated in sodium vapo

Example · Example 15

\text{NaCl}

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When $\text{NaCl}$ crystals are heated in the presence of sodium vapour, extra sodium atoms diffuse into the crystal and deposit on lattice sites. Each incoming sodium atom loses its single valence electron to become $\text{Na}^+$ , which occupies a normal cation site in the lattice — but because there is now one extra $\text{Na}^+$ without a corresponding $\text{Cl}^-$ to balance it (this is a non-stoichiometric metal-excess situation), the freed electron does not simply vanish; instead, it becomes trapped in a nearby vacant site that would normally be occupied by a $\text{Cl}^-$ ion (an anion vacancy). This trapped electron, sitting in the electrostatic potential well left by the missing anion, is called an F-centre (from German Farbzentre, "colour centre"). Because the trapped electron occupies discrete, quantised energy levels, it can absorb photons of visible light and jump to a higher-energy level; the wavelengths absorbed depend on the local crystal environment, and the remaining, transmitted/re …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2022Set ANNUAL1 markMCQ
$Q. NaCl does not show Frenkel defect because$
(a) cations and anions have almost equal size
(b) there is large differences in size of cations and anions
(c) cations and anions have low coordination number
(d) anions cannot be accommodated in voids
›Reveal solutionSolution

Frenkel defect needs a large cation–anion size mismatch so the smaller ion can slip into an interstitial void; in NaCl the two ions are close enough in size that this does not happen, so option (a) is correct.

Why Frenkel defect needs a size mismatch

A Frenkel defect arises when a smaller ion (usually the cation) leaves its regular lattice site and occupies an interstitial position, creating a vacancy at its original site and an interstitial ion elsewhere — the overall stoichiometry and electrical neutrality of the crystal are preserved. This defect is favoured only when there is a large difference in the sizes of the cation and anion, because a small cation can then be accommodated in the small interstitial voids of the anion lattice without much strain. Classic examples are $AgBr$ , $AgCl$ , $AgI$ and $ZnS$ , where $Ag^+$ / $Zn^{2+}$ are much smaller than $Br^-$ / $Cl^-$ / $S^{2-}$ .

Why NaCl is different

…

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CBSE 2021Set OC1 mark
$Q. What are 'point defects' in crystalline substances?$
›Reveal solutionSolution
A point defect is a localised irregularity at a single lattice site in an otherwise perfectly ordered crystal, unlike line or planar defects which extend over many lattice points.

Point defects in crystals

An ideal crystalline solid has its constituent particles (atoms, ions, or molecules) arranged in a perfectly ordered, repeating three-dimensional pattern. In real crystals, however, deviations from this perfect order occur at isolated points in the lattice, called point defects. They arise mainly due to thermal vibrations (some particles gain enough energy to leave their regular sites) during crystallisation.

Common types of point defects include:

Vacancy defect — a lattice site that is unoccupied (an atom/ion is missing from its position); this lowers the density of the solid.

Interstitial defect — an extra particle occupies a normally empty interstitial (in-between) site, increasing the density.

Impurity defect — a foreign atom/ion occupies a regular lattice site or an interstitial site (this is the basis of doping). …
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Q.Explain the metal-excess defect that arises from anion vacancies (F-centres), using $\text{NaCl}$ heated in sodium vapour as an example. Why does this defect give the crystal a colour?

Concept understanding — Point Defects in Solids

Q.Explain the metal-excess defect that arises from anion vacancies (F-centres), using NaCl\text{NaCl}NaCl heated in sodium vapour as an example. Why does this defect give the crystal a colour?

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Q.Explain the metal-excess defect that arises from anion vacancies (F-centres), using $\text{NaCl}$ heated in sodium vapour as an example. Why does this defect give the crystal a colour?

Unlock everything free for 14 days