Trends across Period 3


  • Atomic radius decreases across Period 3


  • This is because across the period, the atoms have more protons and the electrons in the outer shell are pulled closer to the positive nucleus.


  • Melting and boiling point fluctuates across Period 3


This is because of the bonding between molecules and structures the atoms form.


Element Na Mg Al Si P S Cl Ar
Bonding Metallic Metallic Metallic Macromolecular Simple Covalent Simple Covalent Simple Covalent None
Melting Point (K) 371 922 933 1683 317 392 172 84


  • Sodium, magnesium and aluminium all form giant metallic crystals. The melting point increases from Na to Al because each atom loses more electrons to become metal ions in the metallic structure (Na+, Mg2+, Al3+), as well as because the atomic radius decreases. These two factors combined give the metals a higher charge density, holding the ions together more strongly.
  • Silicon is a giant covalent (macromolecular) tetrahedral structure with strong covalent bonds between atoms that require lots of energy to break.
  • Phosphorus, sulphur and chlorine are all simple covalent (molecular) structures. Because of the nature of their electron shells, these atoms form P4, S8 and Cl2 molecules respectively. Bigger molecules have stronger Van der Waals forces between them, explaining the increase in melting point between P and S and decrease between S and Cl.
  • Argon has a very low melting point because it is a monoatomic atom and does not form bonds.



  • Ionisation energy increases across period 3.
  • This is because of the increasing attraction from the positive nucleus across the period.
  • There is a decrease between magnesium and aluminium. This is because aluminium’s outer electron is being removed from a higher energy principal energy level (3p instead of 3s) so is further from the positive nucleus. The 3p orbital is also shielded by the 3s electrons from the positive nucleus, making it harder to remove.
  • There is a decrease between phosphorus and sulphur due to sulphur’s outer electron being removed from an orbital containing 2 electrons, instead of 1 (like phosphorus). This means there is mutual repulsion between electrons, making it easier to remove and lowering the ionization energy.