Covalent and dative covalent bondingAQA A-Level Chemistry: Subtopic test

1. 1
   Nitrogen gas from the air is used to make ammonia. Ammonia is then converted into fertilisers that contain the ammonium ion, NH₄⁺.

   (a)

   How many pairs of electrons are shared between the two atoms in a molecule of nitrogen, N₂?

   [1 mark]

   • A2
   • B3
   • C4
   • D6

   (b)

   Which statement describes a dative covalent (co-ordinate) bond?

   [1 mark]

   • AA shared pair of electrons in which both electrons come from one atom
   • BA shared pair of electrons in which each atom supplies one electron
   • CAn attraction between oppositely charged ions
   • DA lone pair of electrons that is not shared with another atom

   (c)

   Explain how an ammonia molecule forms an ammonium ion with a hydrogen ion, H⁺.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Carbon monoxide, CO, is a toxic gas formed by the incomplete combustion of carbon-containing fuels. A carbon atom has four outer-shell electrons and an oxygen atom has six. In carbon monoxide the carbon and oxygen atoms are joined by a triple bond, and each atom has one lone pair of electrons.

   (a)

   How many of the six electrons in the triple bond of carbon monoxide come from the oxygen atom?

   [1 mark]

   • A2
   • B3
   • C4
   • D6

   (b)

   Which statement about the bonds in carbon monoxide is correct?

   [1 mark]

   • AAll three bonds are formed with one electron from each atom
   • BTwo bonds are covalent and one is a dative covalent bond
   • CThe triple bond is an ionic bond
   • DOne bond is covalent and two are dative covalent bonds

   (c)

   Explain why one of the bonds in carbon monoxide is a dative covalent bond, and state how this bond is represented in a displayed formula.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Boron trifluoride, BF₃, is a gas in which the boron atom has only six electrons in its outer shell. It reacts with ammonia, NH₃, to form a solid compound with the formula H₃N→BF₃.

   (a)

   Explain how the bond between the nitrogen atom and the boron atom forms.

   [3 marks]

   (b)

   Deduce the number of bonding pairs and lone pairs of electrons around the nitrogen atom in NH₃ and in H₃N→BF₃, and the number of bonding pairs around the boron atom in BF₃ and in H₃N→BF₃.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student studies the bonding in ethene, C₂H₄, and in the oxonium ion, H₃O⁺, which forms when a water molecule accepts a hydrogen ion, H⁺, from an acid.

   (a)

   Describe the covalent bonding in a molecule of ethene, including the number of pairs of electrons shared and how the bonds are represented in a displayed formula.

   [6 marks]

   (b)

   Explain how the oxonium ion is formed from a water molecule and an H⁺ ion, and describe the bonding in the ion, including how the dative covalent bond is represented.

   [6 marks]

   Total for question 4: 12 marks

End of questions