across period 3

• Na to Mg to Al

  • The melting points increase from Na to Mg to Al because the strength of the metallic bonding increases, so more energy is needed to break the metallic bonds
  • Na has the least number of delocalised electrons, Mg has more delocalised electrons and Al has the most delocalised electrons.
• Silicon

  • Silicon has the highest melting point. It has a giant macromolecular structure very similar to diamond held together by strong covalent bonds. These require a lot of energy to break hence Si has a very high melting point
• P, S, Cl and Ar

  • Phosphorus, Sulfur and Chlorine all exist as simple covalent molecules
  • their melting and boiling points don’t depend on the strength of the covalent bonds they form, but the strength of the induced dipole dipole intermolecular forces. These forces are weak, so the melting points for these four will be much lower than silicon as less energy is needed to break the forces