Question

Analysis shows that ${{FeO}}$ has a non-stoichiometric composition with formula ${{F}}{{{e}}_{0.95}}{{O}}$. Give reason.

Answer 1

Hint: A crystal having the same unit cell and lattice points are referred to as an ideal crystal. But this does not occur in practice. There will be defects which are the deviations from the arrangement of its constituents. When ionic solids are considered, any defect in stoichiometry denotes that there is a metal excess or deficient for what is needed.

Complete step by step solution:
Non-stoichiometric defects are influenced by the number of excess or deficient positive or negative ions respectively. Metal deficiency defects have less number of positive ions than the number of negative ions. This has occurred due to the vacancies in cations or excess anions in interstitial sites.
Cation vacancy
In this type, some positive ions may be missing from their sites. Thus there will be excess negative charge. It is balanced by the metal ion having $ + 2$ charge.
This defect usually occurred in the metal ions showing varied oxidation state. For example, ${{FeS,NiO,FeO}}$ etc.
In ${{FeO}}$, the composition generally ranges from ${{F}}{{{e}}_{0.93}} - {{F}}{{{e}}_{0.96}}{{O}}$. Some ${{F}}{{{e}}^{2 + }}$ are missing and this is balanced by some ${{F}}{{{e}}^{3 + }}$ ions. For each vacancy, two ${{F}}{{{e}}^{2 + }}$ ions are replaced by ${{F}}{{{e}}^{3 + }}$ ions. The remaining two electrons are needed by the excess oxygen. Here, the charge is made up by converting ferrous to ferric ions, i.e. $3$ ${{F}}{{{e}}^{2 + }}$ is replaced by $2$${{F}}{{{e}}^{3 + }}$.
So we can say that the compound ${{FeO}}$ has a different composition as ${{F}}{{{e}}_{0.95}}{{O}}$ is because of its metal deficiency defect.

Note:
In this compound, there occurs an exchange of electrons. So it can be said that the substances are conductors. Crystals with these defects are generally semiconductors since the electrons are moved from one ion to another.