CIE AS Chemistry (9701) exams from 2022

Revision Notes

2.1.7 Period 3 Chlorides & Oxides

Bonding in Period 3 Chlorides & Oxides

Period 3 chlorides

  • The bonding and structure of the Period 3 elements are summarised in the table below:

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 around them
  • 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 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)
  • The presence of a ‘sea’ of delocalised electrons also determines whether the element is a good conductor or not
  • Going across the period the electrical conductivity of the elements decreases due to lack of delocalised electrons


  • The electronegativities of the Period 3 elements therefore determines the chemical bonding and structure of their chlorides and oxides

The Periodic Table - Chemical Periodicity Table 2_Bonding in Period 3 Chlorides & Oxides, downloadable AS & A Level Chemistry revision notes

  • Going across Period 3, their chlorides and oxidised become more covalent and their structure shifts from a giant ionic to a simple molecular structure
  • Their reactions with water become more vigorous as a result of this as it becomes easier to hydrolyse the chlorides and oxides

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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