Which of the following compounds is nonstoichiometric ?

Options

A. `FeO`
B. `Cu_(2)O`
C. `Cu_(2)S`
D. All of these

Answer

4

Explanation

Various lattice defects occur during crystllization. Crystal planes may be misaligned or sites in the crystal lattice may reamain vacant.
It is also possible to have an unequal number of cation and anion vacancies in an ionic crystal. For example, iron (II) oxide.`FeO` usually crystallizes with some iron (II) sites left unoccupied. Enough of the remaining `Fe` atoms have `+3` charges to gives an electrically balanced crystal.As a result there are more oxygen atoms then `Fe` atoms in the crystal . Moreover, the exact composition of the crystal can vary, so the formula `FeO` is only approximate, Such a compound whose composition varies slightly from its idealized formula is said to be nonstoichiometric.
Other Example of nonstoichiometric compounds are `Cu_(2)O` and `Cu_(2)S`. Each usually has less `Cu` than expected from the formula.The newly discovered ceramic materials having superconducting properties at relatively high temperatures are nonstoichiometric compound. An example is yttrium barium copper oxide ,`YBa_(2)CU_(3)O_(7-x)` where `x` is approximately `0.1`. It is a compound with `O` atom vacancies.