CDS (I) 2021 General Knowledge · Question 101 of 120

Correct — D, Iodine. Non-metals are normally dull, and iodine is the standard exception every textbook names: solid iodine is a semi-lustrous, greyish-violet solid with a distinctly metallic sheen, even though it is a non-metal — the fourth halogen and the heaviest of the stable ones. Its other properties are firmly non-metallic. It is brittle rather than malleable, it does not conduct electricity, and it is famous for going almost straight from solid to violet vapour on warming, which is the classroom demonstration of sublimation. Iodine matters as the counter-example: physical properties describe a general trend for metals and non-metals, not a law, and this element is the exception the trend is taught with.

• (a)Carbon — The most tempting wrong answer, because two forms of carbon do shine — diamond sparkles and graphite is grey and glossy. The exam's named lustrous non-metal is iodine, and carbon in its ordinary forms of coal and charcoal is dull black.
• (b)Silicon — Silicon is a metalloid, not a non-metal, so it fails the stem's own wording whatever its sheen. Its shiny grey crystals are exactly why it sits on the dividing line of the periodic table.
• (c)Germanium — A metalloid too, and one used as a semiconductor for the same reason. Like silicon it is greyish-white and lustrous, which is precisely the ambiguity that puts it outside the non-metal column.

Metals are typically lustrous, malleable, ductile, sonorous and good conductors of heat and electricity; non-metals are typically dull, brittle in the solid state and poor conductors. Both lists are trends with named exceptions, and a small block of elements — the metalloids, including silicon and germanium — sits between the two and shares properties with each.

The item is built to reward a candidate who knows both the exception list and the metalloid boundary. Silicon and germanium can be struck out on classification alone: the stem asks for an element that is a non-metal, and neither is one. That leaves carbon and iodine, and carbon is the trap, since diamond and graphite both shine. The exception the syllabus teaches by name is iodine, and it is the one that combines a metallic sheen with everything else about it being non-metallic. Keep the sibling exceptions with it — graphite conducts electricity though non-metals do not, mercury is a liquid metal, and bromine is a liquid non-metal.

• Solid iodine is a semi-lustrous, non-metallic solid with a metallic grey appearance.
• Iodine melts at about 114 °C to a deep violet liquid and boils at about 184 °C to a violet gas.
• It is the fourth halogen, group 17, and the heaviest of the stable halogens.
• Silicon and germanium are metalloids, not non-metals, and both are used as semiconductors.
• Other named exceptions to the metal and non-metal trends are graphite, which conducts, and mercury, which is a liquid metal.

• Choosing carbon on the strength of diamond or graphite, when the exception the syllabus names is iodine.
• Treating silicon and germanium as non-metals rather than metalloids.
• Assuming a lustrous element must conduct electricity — iodine does not.

As an exception item: which non-metal is lustrous, which non-metal conducts, which metal is liquid at room temperature.

• practice — not a real PYQ

  Which one of the following non-metals is a good conductor of electricity?

  • (a)Sulphur
  • (b)Graphite
  • (c)Phosphorus
  • (d)Iodine

  Answer(b) Graphite — the one form of carbon in which each atom bonds to only three others, leaving one electron free to move and carry current.

• practice — not a real PYQ

  Which one of the following pairs is correctly matched as element and classification?

  • (a)Silicon — non-metal
  • (b)Germanium — metal
  • (c)Iodine — metalloid
  • (d)Bromine — non-metal

  Answer(d) Bromine — non-metal — bromine is a non-metal and the only one that is liquid at room temperature; silicon and germanium are metalloids and iodine is a non-metal.