The flame colors of metal ions are due to
A.Metal excess defect
B.Metal deficiency defect
C.Schottky defect
D.Frenkel defect

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The flame test is an analytical procedure used to detect the presence of certain elements; primary metals ions, based on each element's characteristic emission spectrum. But in this question we have to tell the defect that causes the flame colors of ions. In general, the missing and lacking of atoms or ions in an ideal or imaginary crystal structure or lattice and the misalignment of unit cells in real crystals are called defects or solid defects.

Complete step by step answer:
- The flame colors of metal ions are due to metal excess defect.
-The free electrons can be excited to higher energy levels giving absorption spectra and as a consequence their compounds are coloured.
-The metal excess defect is caused due to anionic vacancies and by the presence of extra cations in the interstitial sites. Interstitial sites are the position between the regular positions in an array of atoms or ions that can be occupied by other atoms or ions.
-When alkali metal halides are heated in an atmosphere of vapour of the alkali metal, anion vacancies are created.
- The crystals with metal excess defects are generally coloured due to the presence of free electrons in them. The crystals with this defect conduct electricity due to the presence of free electrons and are semiconductors.

Hence, option A is correct.

Additional information:
Non –stoichiometric defect is of two types:
1.Metal Excess Defect
2.Metal Deficiency defect
-The non stoichiometric defects disturb the stoichiometry of the compounds. These defects are either due to the presence of excess metal ions or deficiency of metal ions.
-In metal deficiency defect, a cation is missing from its lattice site. To maintain electrical neutrality, one of the nearest metal ions acquires an extra positive charge.
-This type of defect occurs in compounds where the metal exhibits variable valency e.g. -Transition metal complexes like Nio, FeO, FeS etc.
-Further, stoichiometric defects are those defects which do not disturb the stoichiometry (the ratio of numbers of positive and negative ions).
The compounds in which the numbers of positive and negative ions are exactly in the ratios indicated by their chemical formula are called stoichiometric compounds.
They are of 4 types:
1.Vacancy defect
2.Interstitial defect
3.Frenkel defect
4.Schottky defect

The color of the flame in general also depends on the temperature. During the process of crystallization, the defects in solids occur due to the fast or moderate rate of formation of crystals.