Question

What are non-stoichiometric compounds?

Answer 1

Any solid compound has some imperfections in it. Any deviation from a perfect crystal leads to certain defects. These defects are of 3 types: stoichiometric defects, non-stoichiometric defects and impurity defects.

Complete answer:
We all know that all compounds are made up of ions, there are many compounds in which the ratio of positive and negative ions that are present in the crystal are different from the chemical formula of the compound . Such compounds are called non-stoichiometric compounds.
Example: FeO can have ratios like $\;F{e_{0.93}}O$ to $F{e_{0.95}}O$,
Similarly vanadium exists as$V{O_{0.6}}$to $V{O_{1.3}}$
There are 2 types of non stoichiometric defects:
Metal excess defect :
Due to anionic vacancies: in a compound if a negative ion is missing from the compound, leaving a hole it is occupied by an unpaired electron (called F-Centre).
Due to cationic vacancies: there are certain extra ions in a metal, these metal ions accommodate at the interstitial sites and electrons are trapped in neighboring interstitial sites.
Metal deficiency defect: in these compounds the amount of metal is less than the expected stoichiometric proportion. For example: FeO is found in the form of $\;F{e_{0.93}}O$ because in this compound $\;F{e^{2 + }}$ cations are missing and it is filled in by $F{e^{3 + }}$ ions. Even in the $C{u_2}O$ crystal some cuprous ions are replaced by cupric ions. That is $C{u^ + }$ is replaced by ${\text{ }}C{u^{ + + }}$.

Note:
In these non-stoichiometric compounds the balance of charges is maintained either by extra electrons ( negative charges) or extra positive charges. We can distinguish stoichiometric and non-stoichiometric compounds by keeping in mind that stoichiometric compounds do not impart colour whereas non-stoichiometric compounds impart color on reactions.