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
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 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
Metal cations form a giant lattice held together by electrons that can freely move around
Examiner 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
- Have large gaps between the positive ions
- 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)
