AQA A Level Chemistry

Revision Notes

2.1.4 Trends of Period 3 Elements: Melting Point

Trend: Melting Point

Melting points of the elements across period 3

  • A pattern is a little harder to see from the data, but you can see that it rises and falls:

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

Melting points of the period 3 elements

  • The trends in melting point can be explained by looking at the bonding and structure of the elements

Bonding & Structure of the Period 3 Elements

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 move around freely

  • 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 neighboring Si atoms by four 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 across Period 3 Elements

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

The SI unit of electrical conductivity is siemens per metre named after Ernest von Siemens.

Previously, the unit used to be known as the mho, where not only is 1 mho the reciprocal of 1 ohm ( the unit of resistance), it is also ohm spelled backwards!

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