Melting points across Period 3
Unlike atomic radius and first ionisation energy, melting point does not change in one steady direction across Period 3. It rises from sodium to silicon, falls sharply at phosphorus, rises from phosphorus to sulfur, and then falls through chlorine to argon.
| Element | Melting point / K |
|---|---|
| Na | 371 |
| Mg | 923 |
| Al | 932 |
| Si | 1683 |
| P | 317 |
| S | 392 |
| Cl | 172 |
| Ar | 84 |
The changes can be understood by considering the structure of each element and the bonding or forces that must be overcome when it melts.
| Elements | Structure | Main bonding or forces involved |
|---|---|---|
| Na, Mg, Al | Giant metallic | Metallic bonding |
| Si | Giant covalent | Strong covalent bonds |
| , , | Simple molecular | Instantaneous dipole–induced dipole attractions between molecules |
| Ar | Monatomic | Instantaneous dipole–induced dipole attractions between atoms |
Exam Tip: Link the element's structure to the particular attraction that must be overcome on melting, then use the relative strength of that attraction to justify its melting point.