NDA (I) 2018 General Ability Test · Question 87 of 150

Correct — C, Fullerene. Fullerenes are molecules of pure carbon with a definite formula and a definite shape — the first one identified, C₆₀, has sixty carbon atoms arranged in a closed cage of hexagons and pentagons that looks like a football, which is why it was named after the architect Buckminster Fuller and his geodesic domes. Because a fullerene is a single well-defined molecule containing carbon and nothing else, it is the purest form in the list. The other three are all amorphous, ill-defined solids obtained from wood, coal or oil, and each of them carries hydrogen, oxygen, sulphur or mineral ash left over from the material it came from.

• (a)Charcoal — Charcoal is what remains when wood is heated out of contact with air. It is porous and highly useful as an adsorbent, but it retains hydrogen, oxygen and mineral ash from the wood, so it is far from pure carbon.
• (b)Coke — Coke is the residue of heating coal without air. Its carbon content is high, which makes it a good fuel and reducing agent, but it still contains ash and sulphur inherited from the coal.
• (d)Carbon black — Carbon black is soot collected from burning hydrocarbons in a restricted supply of air. It is a very fine and fairly clean powder, which makes it the closest rival among the wrong options, but it is amorphous, of no fixed molecular formula, and carries adsorbed hydrocarbon residues.

Forms of carbon divide into two families. The crystalline allotropes have atoms arranged in a regular, definable pattern — diamond, graphite, and the more recently discovered fullerenes, graphene and nanotubes. The amorphous or impure forms have no such order and are named for their source rather than their structure — charcoal from wood, coke from coal, lampblack and carbon black from burning hydrocarbons, bone black from bones. Fullerenes were discovered in 1985 by Kroto, Curl and Smalley, whose work on C₆₀ won the Nobel Prize in Chemistry in 1996 and opened up the field of carbon nanomaterials.

The word 'purest' is doing all the work in this stem, and the way to read it is as 'best defined chemically', not 'blackest' or 'most carbon by weight'. Once the list is split into crystalline allotropes and impure amorphous forms, only one option sits on the crystalline side and the answer follows without any comparison of percentages. It is worth noting where diamond stands, because candidates often expect it in this slot: diamond is also essentially pure carbon, and had it been on the list a paper could reasonably have keyed it, but here the only option with a defined molecular identity is the fullerene. As of this 2018 paper fullerenes were the newest form of carbon a school textbook covered; graphene, isolated in 2004, has since become just as standard, and NDA has itself listed graphene among the allotropes in a later paper.

• Fullerenes are molecules made entirely of carbon; the first identified was C₆₀, a closed cage of hexagons and pentagons.
• C₆₀ is named buckminsterfullerene after the architect whose geodesic domes it resembles.
• Fullerenes were discovered in 1985, and the discovery won the 1996 Nobel Prize in Chemistry.
• Charcoal, coke and carbon black are amorphous forms carrying impurities from wood, coal and oil respectively.
• Diamond, graphite, fullerene, graphene and carbon nanotubes are the crystalline allotropes of carbon.

• Reading 'purest' as 'highest carbon percentage' rather than 'chemically best defined'.
• Assuming carbon black must be the answer because it looks like pure soot.
• Placing fullerene among the amorphous forms because it is a powder in practice; it is a crystalline molecular solid.

NDA asks candidates to identify the purest form, to spot the item that is not an allotrope, or to name the cage-shaped molecule among several forms of carbon.

• practice — not a real PYQ

  Buckminsterfullerene, C₆₀, has its carbon atoms arranged in

  • (a)a flat sheet of hexagons
  • (b)a closed cage of hexagons and pentagons
  • (c)a tetrahedral three-dimensional network
  • (d)long parallel chains

  Answer(b) a closed cage of hexagons and pentagons — the football shape that gave it its name.

• practice — not a real PYQ

  Which one of the following is NOT a crystalline allotrope of carbon?

  • (a)Graphite
  • (b)Diamond
  • (c)Charcoal
  • (d)Fullerene

  Answer(c) Charcoal — an amorphous, impure form obtained by heating wood without air.