UPSC Prelims 2007 Q104: Science & Technology – Electrical Conductivity of Non - metals
Which one of the following non - metals is NOT a poor conductor of electricity?
- (a) Sulphur
- (b) Selenium
- (c) Bromide
- (d) Phosphorus
Answer: (b) Selenium
Explanation
Which non-metal is NOT a poor conductor? — Selenium
| Element | Electrical behaviour |
|---|---|
| Selenium | Semiconductor and photoconductor — conductivity rises sharply in light |
| Sulphur | Insulator |
| Bromine | Poor conductor (liquid non-metal) |
| Phosphorus | Poor conductor |
Selenium's uses (because it conducts):
- Photocopiers (xerography — early drums used selenium)
- Light meters and photocells
- Old rectifiers
Other non-metal conductors to remember: Graphite (carbon) — conducts well; Silicon — semiconductor (metalloid).
(The option "Bromide" refers to bromine.)
Answer: (b) Selenium.
Why each option is right or wrong
- (a) Incorrect. Sulphur is a classic insulator.
- (b) CORRECT. Selenium is a semiconductor/photoconductor, so it is not a poor conductor.
- (c) Incorrect. Bromine is a poor conductor.
- (d) Incorrect. Phosphorus is a poor conductor.
Key point
Selenium = photoconductive semiconductor (photocopiers, light meters). Graphite = the non-metal that conducts well.
Remember
Selenium is a photoconductor — not a poor conductor.
Common mistakes
- Assuming all non-metals are insulators — graphite and selenium are exceptions.
- Getting confused by the option 'Bromide' — it means bromine, still a poor conductor.
- Forgetting selenium's role in xerography (photocopying).
Exam tip
Non-metal exceptions: Graphite conducts; Selenium conducts in light; Iodine is lustrous.
Sources
- NCERT Class VIII — Science, Ch 4 (Materials: Metals and Non-Metals)
- NCERT Class X — Science, Ch 3 (Metals and Non-metals)