Faulty A-level bonding explanations

I tried these examples with my classes this year. They are all mistakes I have seen students make in exams. Some of them are obviously wrong and others are more subtly wrong.

So can you work out what is wrong with the following explanations?

  1. Calcium has a higher melting point than Barium because there are stronger intermolecular forces between its atoms
  2. PH3 has covalent bonding because phosphorus has a small electronegativity
  3. Methane has a low boiling point because it has weak van der waals between its atoms
  4. Ammonia (NH3) has a trigonal planar shape because it has 3 bond pairs trying to get as far as way as possible from each other
  5. A metallic bond in an element is the electrostatic force of attraction between its nucleus and its delocalised electrons
  6. CH4 has a tetrahedral shape as the four hydrogen atoms try to repel as far away from each other as possible
  7. Water has a higher boiling point than H­2S because it can form hydrogen bonds between its O and H atoms
  8. NaF has a higher melting point than NaBr as fluorine is a smaller atom and so has stronger bonding between its atoms
  9. CO2 is a gas because it has weak covalent bonds
  10. CCl4 is a non-polar molecule because there is no electronegativity difference between C and Cl
  11. NaCl can dissolve in water because it can hydrogen bond with water
  12. Sodium chloride can conduct electricity when molten because it has free electrons.

3 thoughts on “Faulty A-level bonding explanations

    1. chemrevise Post author

      1. Calcium has a higher melting point than Barium because there are stronger intermolecular forces between its atoms.
      Calcium and Barium have metallic bonding. Calcium has stronger metallic bonding because it is a smaller atom and so forms a stronger electrostatic force of attraction between the positive ions and delocalised electrons
      2. PH3 has covalent bonding because phosphorus has a small electronegativity
      There is a small electronegativity difference between phosphorus and hydrogen
      3. Methane has a low boiling point because it has weak van der waals between its atoms
      The weak van der waals forces are between molecules of methane
      4. Ammonia (NH3) has a trigonal planar shape because it has 3 bond pairs trying to get as far as way as possible from each other
      Ammonia has 3 bond pairs and one lone pair on the Nitrogen. This makes the shape trigonal pyramidal.
      5. A metallic bond in an element is the electrostatic force of attraction between its nucleus and its delocalised electrons
      A metallic bond is an electrostatic force of attraction between the positive ions and delocalised electron.
      6. CH4 has a tetrahedral shape as the four hydrogen atoms try to repel as far away from each other as possible
      It is the bonding electron pairs trying to repel as far away from each other as possible (not atoms)
      7. Water has a higher boiling point than H2S because it can form hydrogen bonds between its O and H atoms
      This vague answer does not make explicit that hydrogen bonding is between different molecules. It is better to say the hydrogen bond form between the oxygen in one molecule and the hydrogen in another molecule.
      8. NaF has a higher melting point than NaBr as fluorine is a smaller atom and so has stronger bonding between its atoms
      This is about ionic bonding. It should be the fluoride ion is a smaller ion and has stronger ionic bonding between its sodium and fluoride ions.
      9. CO2 is a gas because it has weak covalent bonds
      It should be CO2 is a gas because it has weak van der waals forces between its molecules.
      10. CCl4 is a non-polar molecule because there is no electronegativity difference between C and Cl
      There is an electronegativity difference beween C and Cl. The molecules is symmetrical and all the polar bonds cancel out.
      11. NaCl can dissolve in water because it can hydrogen bond with water
      Sodium and chloride ions can form electrostatic attractions to polar water molecules but these are not hydrogen bonds
      12. Sodium chloride can conduct electricity when molten because it has free electrons.
      It can conduct electricity because when molten it contains ions that are free to move

      Reply

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