Covalent bonding and dative bondsEdexcel International A Level Chemistry: Subtopic test

1. 1
   Nitrogen atoms have atomic number 7. In nitrogen gas, N₂, two atoms are joined by a triple bond. X-ray diffraction can be used to produce an electron density map of a molecule such as N₂.

   (a)

   How many electrons are shared between the two atoms in N₂?

   [1 mark]

   • A2
   • B4
   • C3
   • D6

   (b)

   What does the electron density map of a covalent molecule such as N₂ show?

   [1 mark]

   • AThe electron density is zero between the two nuclei.
   • BA region of high electron density between the two nuclei.
   • CThe electron density is centred on one nucleus only.
   • DThe electron density is spread evenly throughout the molecule.

   (c)

   Explain, in terms of electrostatic forces, how the shared electrons hold the two nitrogen nuclei together.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ammonia gas reacts with hydrogen chloride gas to form solid ammonium chloride, which contains the ammonium ion, NH₄⁺. In the ammonium ion all four N–H bonds are identical in length and strength. A hydrogen ion, H⁺, has no electrons.

   (a)

   Which statement describes the dative covalent bond in the ammonium ion?

   [1 mark]

   • ABoth electrons in the shared pair come from the hydrogen ion.
   • BEach atom supplies one electron to the shared pair.
   • CBoth electrons in the shared pair come from the lone pair on the nitrogen atom.
   • DIt is an ionic attraction between nitrogen and the hydrogen ion.

   (b)

   Which species can donate a pair of electrons to form a dative covalent bond with H⁺?

   [1 mark]

   • AH₂O
   • BCH₄
   • CNH₄⁺
   • DMg²⁺

   (c)

   Explain why all four N–H bonds in the ammonium ion are identical even though one of them is a dative covalent bond.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The vapour of aluminium chloride just above its sublimation temperature has a relative formula mass of 267 (Ar: Al = 27.0, Cl = 35.5). In an AlCl₃ molecule the aluminium atom has only six electrons in its outer shell.

   (a)

   Deduce the formula of the species in the vapour and explain how it forms from AlCl₃.

   [3 marks]

   (b)

   Describe the bonding around each aluminium atom in Al₂Cl₆.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is preparing a revision poster on covalent bonding. She has collected this evidence: diamond, a giant structure of carbon atoms, is very hard and melts at over 3500 °C; iodine, made of simple molecules, melts at 114 °C; and electron density maps are available for molecules such as hydrogen and hydrogen chloride. The species she will draw are ethene, C₂H₄, hydrogen cyanide, HCN, and the ammonium ion, NH₄⁺.

   (a)

   Explain how the evidence she has collected supports the idea that a covalent bond is a strong attraction between two nuclei and a shared pair of electrons.

   [6 marks]

   (b)

   Describe the covalent bonding in ethene, hydrogen cyanide and the ammonium ion, stating the number of electron pairs shared in each bond and the origin of the electrons in any dative bond.

   [6 marks]

   Total for question 4: 12 marks

End of questions