Molecules of which of the following has cage like structure ? 1. Diamond 2. Graphite 3. Fullerenes Select the correct answer using the code given below :
- (a)1, 2 and 3
- (b)2 and 3 only
- (c)2 only
- (d)3 only
Correct — D, 3 only. Fullerenes are the one carbon allotrope built as a closed cage. The first to be identified, C-60, has sixty carbon atoms sitting at the corners of a hollow shell made of twelve five-membered rings and twenty six-membered rings, which gives it the shape of a football. NCERT describes it in exactly those terms and adds that the molecule was named after the American architect Buckminster Fuller because the shell looks like his geodesic dome. Diamond and graphite have no cage anywhere in them — diamond is a rigid three-dimensional network in which every carbon is bonded to four others, and graphite is a stack of flat sheets of six-membered rings in which every carbon is bonded to three others. Only the third item therefore qualifies.
- (a)1, 2 and 3 — This would make diamond and graphite cages as well. Diamond is an endless tetrahedral network of carbon atoms and graphite is a set of flat layers — neither closes on itself, and neither exists as a separate molecule of a fixed size the way a fullerene does.
- (b)2 and 3 only — Graphite is layered, not caged. Each sheet is a plane of hexagonal rings, and the sheets are held to one another by weak forces, which is what lets them slide and makes graphite a lubricant. A flat sheet is the opposite of a closed cage.
- (c)2 only — This picks graphite alone and leaves out the one allotrope that really is a cage. Graphite's structure is planar and open, while the fullerene shell encloses a hollow space.
Carbon shows allotropy — the same element crystallising in different structural forms with very different properties. Diamond binds each atom to four neighbours in a rigid three-dimensional lattice, which makes it the hardest known substance and an electrical insulator. Graphite binds each atom to three neighbours in flat hexagonal sheets, leaving one electron per atom free, which makes it soft, slippery and a good conductor. Fullerenes are discrete cage molecules such as C-60, C-70 and their relatives.
The word 'molecules' in the stem is a clue in itself. Diamond and graphite are giant covalent structures with no molecular formula of fixed size, whereas a fullerene really is a molecule you can write as C-60 and weigh. That is why fullerenes dissolve in some organic solvents while diamond and graphite do not. Examiners also like a related pair of near-misses — graphene is a single sheet peeled from graphite, and carbon nanotubes are rolled sheets rather than closed cages, so neither is a fullerene.
- NCERT Class 10 Science records that the first fullerene identified was C-60, whose carbon atoms are arranged in the shape of a football, and that it was named after the geodesic domes of the architect Buckminster Fuller.
- The C-60 cage is built from twelve five-membered rings and twenty six-membered rings.
- In diamond each carbon atom is bonded to four others in a three-dimensional network, making it the hardest natural substance and an insulator.
- In graphite each carbon atom is bonded to three others in flat hexagonal sheets, and the one electron left free per atom makes it conduct electricity.
- Graphene is a single layer of graphite and carbon nanotubes are rolled sheets, so neither is a closed cage like a fullerene.
Only fullerenes form closed cages, so the answer is item 3 alone.
- Assuming that because all three are carbon they must share a structure — the properties differ precisely because the structures differ.
- Calling graphene or a nanotube a fullerene; a fullerene is a closed cage.
- Believing diamond conducts electricity because it is a crystal; it has no free electrons and does not conduct.
NDA asks for the odd allotrope out, or for the element that Buckminsterfullerene belongs to, or for the false statement in a list about diamond and graphite.
Buckminster fullerene, which looks like a football, is an allotropic form of
- (a) Phosphorus
- (b) Sulfur
- (c) Carbon
- (d) Tin
Answer(c) Carbon
Names the same cage molecule and its football shape, which is the exact property this 2017 question is testing.
Which one of the following materials is not an allotrope of carbon?
- (a) Diamond
- (b) Graphene
- (c) Fly ash
- (d) Fullerene
Answer(c) Fly ash
Runs over the same family of carbon allotropes, including the fullerene, and is a reminder that graphene belongs to the list while a combustion residue does not.
- practice — not a real PYQ
Buckminsterfullerene, the first fullerene to be identified, has the formula
- (a)C-12
- (b)C-20
- (c)C-60
- (d)C-100
Answer(c) C-60 — sixty carbon atoms forming a closed football-shaped cage of twelve pentagons and twenty hexagons.
- practice — not a real PYQ
Graphite conducts electricity while diamond does not, because in graphite
- (a)each carbon atom is bonded to four others
- (b)one electron of each carbon atom is free to move
- (c)the carbon atoms are arranged in cages
- (d)the carbon atoms carry a permanent charge
Answer(b) one electron of each carbon atom is free to move — each carbon in graphite bonds to only three neighbours, leaving one delocalised electron per atom.