Room-temperature conductivity versus superconductivity at low temperature Elements that become superconducting only at extremely low temperatures are typically:
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Agood conductors at room temperature (e.g., copper, silver)
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Bferromagnetic at room temperature
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Crelatively poor conductors at room temperature
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Dinsulators at room temperature
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Esemiconductors at room temperature
Answer
Correct Answer: relatively poor conductors at room temperature
Explanation
Introduction / Context:Superconductivity is characterized by zero DC resistivity and flux expulsion below a critical temperature Tc. Curiously, the best normal-state conductors at room temperature (Cu, Ag, Au) typically do not exhibit superconductivity at ambient pressure, whereas many poorer conductors do at cryogenic temperatures.
Given Data / Assumptions:
- We consider elemental superconductors under standard pressure (no high-pressure phases).
- “Poor conductor at room temperature” means higher resistivity compared with noble metals.
- Examples include Pb, Sn, Hg, Al (moderate conductivity but not the very best), which become superconducting at low Tc.
Concept / Approach:Electron–phonon coupling strength and electronic structure govern Tc in conventional (BCS) superconductors. Strong coupling and favorable density of states can occur in materials that are not the very best normal conductors. Meanwhile, the noble metals have electronic structures that disfavor the Cooper pairing required for superconductivity.
Step-by-Step Solution:Observe empirical trend: elements with very low room-temperature resistivity (Cu, Ag, Au) lack superconductivity at ambient pressure.Note many elements of moderate/poorer conductivity (Pb ≈ 7.2 K, Sn ≈ 3.7 K, Hg ≈ 4.2 K, Al ≈ 1.2 K) become superconducting at low T.Therefore, the best general statement is: they are relatively poor conductors at room temperature.
Verification / Alternative check:Handbooks listing elemental Tc values corroborate the pattern: noble metals are not superconductors; many other metals are, but only at cryogenic temperatures.
Why Other Options Are Wrong:Good conductors at room temperature: Typically not superconducting at 1 atm. Ferromagnetic: Long-range ferromagnetism is generally antagonistic to conventional superconductivity. Insulators/semiconductors: Elements that are insulators at room temperature are not the usual low-T elemental superconductors.
Common Pitfalls:Assuming “good conductor” at 300 K implies superconductivity at low T; the mechanisms are distinct.
Final Answer:relatively poor conductors at room temperature