Read the following passage and answer the questions.
A student measures the resistivity of three unlabelled solids, X, Y and Z, at room temperature. Her readings are about \(1.7\times10^{-8}\ \Omega\,\mathrm{m}\) for X, \(2\times10^{3}\ \Omega\,\mathrm{m}\) for Y and \(10^{14}\ \Omega\,\mathrm{m}\) for Z. She plans to warm each sample a little and take the readings again. To explain her results, she will compare the samples with the energy-band pictures of solids. Such a picture shows a valence band and a conduction band. In some solids the two bands are separated by an energy gap, and the size of this gap differs from one solid to another. She also notes four elements of group IV of the periodic table: carbon (as diamond), silicon, germanium and tin. Each of them has four valence electrons, yet they behave very differently in electric circuits.
Consider the following statements about the classification of solids:
(A) The resistivity of metals is of the order of \(10^{-2}\) to \(10^{-8}\ \Omega\,\mathrm{m}\).
(B) Anthracene is an example of an organic semiconductor.
(C) GaAs and CdSe are elemental semiconductors.
(D) Polypyrrole is an inorganic semiconductor.
Which of the above statements are incorrect?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (C) and (D) only
Solids are grouped by their resistivity as metals, semiconductors and insulators. Semiconductors are further grouped as elemental (Si and Ge) or compound, and a compound one can be inorganic or organic.
(A) The resistivity of metals is of the order of \( 10^{-2} \) to \( 10^{-8}\ \Omega\,\mathrm{m} \). — Correct statement. Metals have very low resistivity, in this range.
(B) Anthracene is an example of an organic semiconductor. — Correct statement. Anthracene is listed among the organic semiconductors.
(C) GaAs and CdSe are elemental semiconductors. — Incorrect statement. Each of them is made of two elements, so they are inorganic compound semiconductors. Only Si and Ge are elemental.
(D) Polypyrrole is an inorganic semiconductor. — Incorrect statement. Polypyrrole is an organic polymer semiconductor, like polythiophene.
NCERT (2026-27):
Classification of Metals, Conductors and Semiconductors – “(i) Metals: They possess very low resistivity (or high conductivity). ρ ~ 10–2 – 10–8 Ω m”
Classification of Metals, Conductors and Semiconductors – “• Organic: anthracene, doped pthalocyanines, etc.”
Classification of Metals, Conductors and Semiconductors – “(i) Elemental semiconductors: Si and Ge (ii) Compound semiconductors: Examples are: • Inorganic: CdS, GaAs, CdSe, InP, etc.”
Classification of Metals, Conductors and Semiconductors – “• Organic polymers: polypyrrole, polyaniline, polythiophene, etc.”