CDS (II) 2023 General Knowledge · Question 40 of 120

Correct — B, Diamond is hard but graphite is smooth and slippery. Diamond and graphite are allotropes — the same element, carbon, arranged in two different ways — and the difference in arrangement is what produces the difference in feel. In diamond each carbon atom is bonded to four others, giving a rigid three-dimensional network in which every bond has to be broken before the crystal will yield; that is why diamond is the hardest substance known. In graphite each carbon atom is bonded to only three others in the same plane, producing hexagonal sheets that are stacked one above the other and held together only weakly, so the layers slide over one another under the slightest force. NCERT states the property difference almost word for word: diamond is the hardest substance known while graphite is smooth and slippery. That sliding is what makes graphite useful as a dry lubricant and as the writing material in a pencil, where layers rub off onto the paper.

• (a)Diamond and graphite have similar physical and chemical properties. — Half of this is true and the half that is stated first is false. Being allotropes of one element, the two have the same chemical properties — both are carbon and both burn to carbon dioxide. Their physical properties are very different indeed: hardness, appearance, density and electrical behaviour all differ.
• (c)Diamond and graphite are both non-conductors of electricity. — Graphite conducts. Because each carbon in graphite bonds to only three neighbours, one electron per atom is left free to move along the layers, and NCERT notes that graphite is a very good conductor of electricity unlike other non-metals. Diamond, with all four electrons locked into bonds, is an electrical insulator.
• (d)Both diamond and graphite have similar structures. — The structures are the one thing that is definitely not shared. Diamond is a rigid three-dimensional tetrahedral network; graphite is a stack of flat hexagonal layers. Every other difference between them follows from this.

Allotropy is the existence of an element in two or more forms in the same physical state, differing in the arrangement of its atoms. Carbon is the classic case: diamond, graphite and the fullerenes are all pure carbon. Because the atoms are the same, the chemical properties are the same — all three burn in oxygen to give carbon dioxide. Because the arrangement differs, the physical properties diverge sharply, and hardness, electrical conductivity, density and appearance can all be traced back to how many neighbours each carbon atom is bonded to and how those units are held together.

The reliable route is to argue from bonding to property rather than to recall a list. Four bonds per atom in three dimensions means nothing can move, so diamond is hard and, with no spare electrons, an insulator. Three bonds per atom in a plane means one electron per atom is loose and whole sheets can slip, so graphite conducts and is slippery. That single line of reasoning disposes of options (b), (c) and (d) at once. Two refinements are worth carrying. Graphite's conduction is along the layers rather than equally in every direction. And diamond, although a poor conductor of electricity, is an outstanding conductor of heat — a distinction examiners occasionally exploit.

• NCERT: 'Diamond is the hardest substance known while graphite is smooth and slippery.'
• NCERT: 'Graphite is also a very good conductor of electricity unlike other non-metals.'
• NCERT on structure: 'In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional structure. In graphite, each carbon atom is bonded to three other carbon atoms in the same plane giving a hexagonal array … placed in layers one above the other.'
• Diamond and graphite have 'very different physical properties even though their chemical properties are the same' — the definition of allotropes at work.
• Fullerenes are a third allotrope of carbon, built as closed cage-like molecules rather than as networks or sheets.

• Assuming that because both are non-metals neither can conduct; graphite is the standard exception among non-metals.
• Saying the two have different chemical properties. As allotropes of one element their chemistry is the same; it is the physical properties that differ.
• Confusing electrical with thermal conduction. Diamond blocks current but carries heat exceptionally well.

As a single true-or-false statement about the pair, as a which-allotrope-conducts item, or as a structure-to-property match.

• practice — not a real PYQ

  Graphite is able to conduct electricity mainly because

  • (a)it contains metallic impurities
  • (b)each carbon atom is bonded to three others, leaving one electron free to move
  • (c)its layers are held together by strong covalent bonds
  • (d)it has a three-dimensional tetrahedral network

  Answer(b) each carbon atom is bonded to three others, leaving one electron free to move — the delocalised electrons travel along the layers, which is why graphite conducts while diamond does not.

• practice — not a real PYQ

  Diamond, graphite and fullerene are best described as which one of the following?

  • (a)Isotopes of carbon
  • (b)Isomers of carbon
  • (c)Allotropes of carbon
  • (d)Compounds of carbon

  Answer(c) Allotropes of carbon — they are the same element in different atomic arrangements, which is why they share chemical properties but differ physically.