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
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