Explaining Physical Properties of the Period 3 Elements
Melting point
Melting points of the elements across Period 3 table
| Period 3 element | Na | Mg | Al | Si | P | S | Cl | Ar |
| Melting point (K) |
371 | 923 | 932 | 1638 | 317 | 392 | 172 | 84 |
Graph of melting points across Period 3
There is a general increase in melting point from Na to Si, followed by a sharp drop to the lower melting points of P to Ar
- The above trends can be explained by looking at the bonding and structure of the elements
Bonding and structure of the Period 3 elements table
| Period 3 element | Na | Mg | Al | Si | P | S | Cl | Ar |
| Bonding | Metallic | Metallic | Metallic | Covalent | Covalent | Covalent | Covalent | - |
| Structure | Giant metallic | Giant metallic | Giant metallic | Giant molecular | Simple molecular | Simple molecular | Simple molecular | Simple molecular |
- 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 structure of metals
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
- Due to 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
Trends in electrical conductivity across Period 3 table
| Period 3 element | Na | Mg | Al | Si | P | S | Cl | Ar |
| Electrical conductivity (S m-1) |
0.218 | 0.224 | 0.382 | 2 x 10-10 | 10-17 | 10-23 | - | - |
- 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
