CIE AS Chemistry (9701) exams from 2022

Revision Notes

2.1.2 Period 3 Elements: Structure & Bonding

Period 3: Structure & Bonding

Melting point

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 outer electrons across the period

  • 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 around them


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

  • 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)

Electrical conductivity

  • The electrical conductivity decreases going across the Period 3 elements

Electrical conductivity decreases Period 3 elements table

The Periodic Table - Table 5_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

  • Going from Na to Al, there is an increase in the number of valence electrons that are donated to the ‘sea’ of delocalised electrons
  • Because of this, in Al there are more electrons available to move around through the structure when it conducts electricity, making Al a better electrical conductor than Na


  • Due to the giant molecular structure of Si, there are no delocalised electrons that can freely move around within the structure
  • Si is therefore not a good electrical conductor and is classified as a semimetal (metalloid)
  • The lack of delocalised electrons is also why P and S cannot conduct electricity

Exam Tip

Intermolecular forces are forces between molecules

Intramolecular forces are forces within a molecule

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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