Monthly Archives: December 2013

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.