CDS (II) 2021 General Knowledge · Question 8 of 120

Correct — C, 2 only. Graphite is the one allotrope on the list that conducts electricity well. In graphite each carbon atom is bonded to only three neighbours in the same plane, forming hexagonal sheets stacked one above another; that leaves one electron per atom outside the sigma framework, free to move across the whole sheet. Delocalised electrons are what electrical conduction requires, and the textbook notes the point explicitly — graphite is a very good conductor of electricity, unlike other non-metals. Diamond fails for the opposite reason. Each carbon there is bonded to four others in a rigid three-dimensional network, every valence electron is locked into a bond, and nothing is left over to carry charge, so diamond is an electrical insulator even though it is the best known conductor of heat. Fullerene fails too. C-60 is a closed cage of sixty carbon atoms shaped like a football, and its electrons are confined to individual molecules rather than shared across a continuous lattice; solid C-60 behaves as a semiconductor with a substantial band gap, not as a good conductor. Only statement 2 survives.

• (a)1 only — Names the one allotrope that certainly does not conduct. Diamond's four bonds per atom leave no free electrons, which is why it is used as an insulating heat spreader rather than as a conductor.
• (b)1 and 2 only — Correct about graphite and wrong about diamond. The two differ in conductivity precisely because they differ in the number of bonds each carbon atom forms.
• (d)1 and 3 only — Picks the two non-conductors and omits the conductor. Fullerene is a molecular cage, not an extended sheet, so it has no delocalised network for charge to travel through.

Allotropes are different structural forms of the same element, and carbon's are the classic case: diamond, graphite and the fullerenes are chemically identical yet physically very different. Diamond bonds each carbon to four neighbours in a rigid three-dimensional lattice. Graphite bonds each carbon to three neighbours in a flat hexagonal array, with the sheets stacked in layers. Fullerenes are closed cages, the first identified being C-60 with its atoms arranged in the shape of a football. Structure, not composition, is what sets hardness, slipperiness and conductivity apart.

The item is settled entirely by counting bonds per carbon atom. Four bonds means every electron is committed, giving hardness and insulation; three bonds means one electron per atom is spare, giving soft slippery layers and conduction along them. Fullerene is the option that separates a memoriser from someone who has understood the rule, because it does have some spare electrons but they belong to a molecule rather than to an unbroken lattice, so there is no path for a current to flow along. It is worth knowing that graphite's conductivity is strongly directional — high within a layer, poor across the stack — and that this same layered structure is what makes it a lubricant and the 'lead' of a pencil. Conductivity of heat runs the other way between the two: diamond is one of the best thermal conductors known.

• In diamond each carbon atom is bonded to four others in a rigid three-dimensional structure; in graphite each is bonded to three others in the same plane, giving a hexagonal array in stacked layers.
• Graphite is a very good conductor of electricity, unlike other non-metals; the delocalised electron on each carbon atom is what carries the current.
• Diamond is the hardest substance known and is an electrical insulator, though it conducts heat extremely well.
• The first fullerene identified was C-60, whose carbon atoms are arranged in the shape of a football; it is named after the architect Buckminster Fuller.
• Solid C-60 is a semiconductor with a band gap of roughly 1.5 to 1.9 electron volts, so it is not a good electrical conductor.

• Assuming that allotropes of the same element must share physical properties; only their chemical properties are the same.
• Calling diamond a conductor because it conducts heat so well — heat and charge are carried by different mechanisms here.
• Treating fullerene as a graphite-like conductor because both involve hexagonal rings.

As a diamond-versus-graphite comparison, as a statements item on conductivity or hardness, or as a match-list pairing each allotrope with its structure or use.

• practice — not a real PYQ

  Diamond does not conduct electricity because

  • (a)it has a very high melting point
  • (b)all four valence electrons of each carbon atom are used in bonding
  • (c)it is the hardest known substance
  • (d)its carbon atoms are arranged in hexagonal layers

  Answer(b) all four valence electrons of each carbon atom are used in bonding — no delocalised electrons are left to carry a current.

• practice — not a real PYQ

  The allotrope of carbon in which the atoms are arranged in the shape of a football is

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

  Answer(c) Fullerene — C-60, the first fullerene identified, is a closed cage of sixty carbon atoms resembling a football.