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Chemistry Notes For Class 12 - Non-Stoichiometric Defects  Class 12 Chemistry Non-Stoichiometric Defects are of two types: Metal Deficiency Defects when negative ions are in excess. Metal Excess Defects when positive ions are in excess. Metal Deficiency Defects: – These defects can occur in two possible ways as follows: In this way, one of the positive ions may be missing from its lattice point. The extra negative charge is balanced may be balanced by some nearby metal ion acquiring two charges instead of one. The metal must be in a position to acquire variable valency so, this defect is generally found in compounds of transition metals like FeO, FeS, NiO, etc. In another way, an extra negative ion may find an interstitial position. The extra negative charge may be balanced by one of the adjacent metal ion acquiring an extra charge. In this case, also, the metal ion should also be in a position to acquire variable valency. As negative ions are usually very large,