NDA (II) 2018 General Ability Test · Question 81 of 146

Correct — D, the claim that graphite's layers are held together by carbon-carbon single bonds. They are not. Within a layer every carbon is sp² hybridised and bonded to three neighbours by strong covalent bonds, with the fourth electron delocalised over the sheet; between one layer and the next there are no covalent bonds at all, only weak van der Waals attraction across a gap of about 340 picometres. That is why graphite flakes slide over one another and leave a mark on paper, and why it works as a dry lubricant. Statement (d) also contradicts statement (b) in the same question, so one of the two has to be the false one, and (b) is the one that matches the accepted structure.

• (a)All carbons in diamond are linked by carbon-carbon single bond. — This is correct, so it cannot be the answer to a 'not correct' question. Each carbon in diamond is sp³ hybridised and joined to four others by single covalent bonds in a rigid three-dimensional tetrahedral network — the reason diamond is the hardest natural substance and does not conduct electricity.
• (b)Graphite is layered structure in which layers are held together by weak van der Waals forces. — This is the true statement about interlayer bonding, and it is exactly what makes option (d) false. Weak forces between sheets give graphite its softness, its slipperiness and its use in pencils and as a lubricant.
• (c)Graphite layers are formed by hexagonal rings of carbon atoms. — Also correct. Each layer is a flat honeycomb of fused hexagons, every carbon sharing three sigma bonds within the plane. The leftover p electrons form a delocalised cloud above and below the sheet, which is why graphite conducts electricity along the layers.

Diamond and graphite are both pure carbon, and the entire difference between them is how the bonds are arranged. In diamond every carbon uses all four valence electrons in sp³ single bonds to four neighbours, producing one giant rigid lattice with no free electrons — hard, non-conducting, high melting. In graphite every carbon uses three electrons in sp² bonds inside a flat hexagonal sheet and leaves the fourth delocalised; the sheets stack loosely, held only by van der Waals forces. So graphite is soft and slippery along the stacking direction and a good conductor along the sheets, even though it is made of the same element as diamond.

Questions of this shape put three true statements and one false one in front of you, and the quickest way in is to look for the internal contradiction. Statements (b) and (d) make opposite claims about the same bond — weak van der Waals forces versus carbon-carbon single bonds — so the false one has to be among them, and no time need be spent on diamond at all. Choosing between the two then rests on a property you already know: if the layers were covalently bonded to each other, graphite would be a rigid three-dimensional solid like diamond, and a pencil would not write.

• In diamond each carbon is sp³ hybridised and covalently bonded to four others in a tetrahedral network.
• In graphite each carbon is sp² hybridised and covalently bonded to three others within a flat hexagonal sheet.
• Graphite's sheets are held to one another only by weak van der Waals forces, about 340 pm apart, which is why they slide.
• The fourth, delocalised electron on each graphite carbon makes graphite an electrical conductor, while diamond is an insulator.

• Assuming that two forms of the same element must have similar properties. Diamond and graphite differ in hardness and conductivity as sharply as any two solids can.
• Reading a 'not correct' stem as 'correct' and marking the first true statement you meet.
• Thinking graphite is soft because its bonds are weak. The bonds inside a sheet are among the strongest known; it is the space between sheets that is weakly held.

NDA repeatedly sets a four-statement 'which is not correct' item on the diamond–graphite contrast, usually hinging on hybridisation or on interlayer bonding.

• practice — not a real PYQ

  Graphite conducts electricity because

  • (a)each carbon atom is bonded to four others
  • (b)one electron per carbon atom is delocalised over the layer
  • (c)its layers are held by covalent bonds
  • (d)it contains free hydrogen atoms

  Answer(b) one electron per carbon atom is delocalised over the layer — those mobile electrons carry current along the sheets.

• practice — not a real PYQ

  The hybridisation of each carbon atom in diamond and in graphite is respectively

  • (a)sp² and sp³
  • (b)sp³ and sp²
  • (c)sp³ and sp³
  • (d)sp and sp²

  Answer(b) sp³ and sp² — diamond's tetrahedral network against graphite's flat hexagonal sheets.