Question

From the following compounds, non-stoichiometric compounds is/are: (This question can have multiple correct answers)
(A) \[C{u_2}O\]
(B) \[C{u_2}S\]
(C) $FeO$
(D) \[H{g_2}B{a_2}YCaC{u_2}{O_7}\]

Answer 1

Hint: It is important to understand what non-stoichiometric compounds are, and what makes a compound non-stoichiometric. They may appear from their structures that they are stoichiometric, but they may not necessarily be one. Try to find a common link that exists among all non-stoichiometric compounds.

Complete answer: Compounds whose elemental composition can’t be shown in the form of ratios of natural numbers are called Non-stoichiometric compounds. They are mostly inorganic in nature and invariably metallic solids. These are generally observed as defects in the crystal lattice, such as atoms missing in a lattice or packing of too many atoms in the lattice structure. Many metal oxides and sulfides show non-stoichiometric defects. Let’s discuss the options given in the question. Yttrium barium copper oxide, which has the general formula ${Y_x}B{a_2}C{u_3}{O_7}$ , is a non-stoichiometric superconductor. As option 4 has a similar non-stoichiometric structure, it is one of the answers.
While preparing cuprous oxide, \[C{u_2}O\] , the ratio of copper to oxygen is slightly less than 2:1, making it a non-stoichiometric crystal. To maintain electrical neutrality, there is a replacement of one $C{u^{2 + }}$ ion for every \[2C{u^ + }\] ions, making a hole. Hence \[C{u_2}O\] also serves as a p-type semiconductor. Similar to \[C{u_2}O\] , \[C{u_2}S\] is also non-stoichiometric as the ratio of Cu to S is slightly less than 2:1. In fact the ratio lies between 1.96 - 2; making it a non-stoichiometric compound. Same thing is applicable for $FeO$ as well.
Hence all given compounds are non-stoichiometric compounds.
Additional information: Non-stoichiometric compounds resulting out of these defects exhibit some special chemical and electrical properties. Hence they find their application in ceramics, superconductive materials, and in battery system designs. Modern definition treats non-stoichiometric compounds as homogeneous and not as mixtures. Non-Stoichiometric compounds are of two types, viz. compounds with metal excess defects and compounds with metal deficiency defects.

Note:Non-stoichiometric compounds are generally metal oxides and sulfides. Their structure doesn’t tell you about their non-stoichiometry, but the ratio of the number of atoms in a unit cell doesn’t give a natural number. That is the reason all the four options are non-stoichiometric compounds.