Edexcel International A Level Chemistry

Revision Notes

1.5.2 Periodicity - Thermal Trends

Test Yourself

Trends in Melting & Boiling Point

Melting point

  • Period 2 and 3 elements follow the same pattern in relation to their melting points

Melting Points of the Elements Across Period 3 Table

The Periodic Table - Table 3_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

The Periodic Table - Melting Point Graph, downloadable AS & A Level Chemistry revision notes

Ions of Period 3 elements with increasing positive charge (metals) and increasing of number of outer electrons across the period

  • A general increase in melting point for the Period 3 elements up to silicon is observed
  • Silicon has the highest melting point
  • After the Si element the melting points of the elements decreases significantly
  • The above trends can be explained by looking at the bonding and structure of the elements

Bonding & Structure of the Elements Table

The Periodic Table - Table 4_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

  • The table shows that Na, Mg and Al are metallic elements which form positive ions arranged in a giant lattice in which the ions are held together by a 'sea' of delocalised electrons  

The Periodic Table - Metallic Lattice, downloadable AS & A Level Chemistry revision notes

Metal cations form a giant lattice held together by electrons that can freely move around

Exam Tip

  • Remember: At room temperature and pressure, metals (except for mercury) are solid
  • This means that the lattice structure should:
    • Have a regular arrangement of positive ions (rows and columns)
    • Have the ions tightly packed / close together
  • The lattice structure should not:
    • Have large gaps between the positive ions
      • This could lose marks in an exam as examiners may not be satisfied that a solid is being shown
    • Have a random arrangement of particles
      • Examiners would consider a randomly arranged, close packed structure to be a liquid and penalise answers accordingly
  • The delocalised electrons do not have to be specifically shown

  • The electrons in the ‘sea’ of delocalised electrons are those from the valence shell of the atoms
  • Na will donate one electron into the ‘sea’ of delocalised electrons, Mg will donate two and Al three electrons
  • As a result of this, the metallic bonding in Al is stronger than in Na
  • This is because the electrostatic forces between a 3+ ion and the larger number of negatively charged delocalised electrons is much larger compared to a 1+ ion and the smaller number of delocalised electrons in Na
  • Because of this, the melting points increase going from Na to Al
  • Si has the highest melting point due to its giant molecular structure in which each Si atom is held to its neighbouring Si atoms by strong covalent bonds
  • P, S, Cl and Ar are non-metallic elements and exist as simple molecules (P4, S8, Cl2 and Ar as a single atom)
  • The covalent bonds within the molecules are strong, however, between the molecules, there are only weak instantaneous dipole-induced dipole forces
  • It doesn’t take much energy to break these intermolecular forces
  • Therefore, the melting points decrease going from P to Ar (note that the melting point of S is higher than that of P as sulphur exists as larger S8 molecules compared to the smaller P4 molecule)

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