9701 · 9.1
Periodicity of physical properties of the elements in Period 3
As you move from left to right across Period 3, from sodium to argon, the properties of elements change in a predictable pattern. This is because the number of protons increases, pulling electrons in tighter and changing how atoms bond with each other.
Need to know
What you need to know
- **Atomic Radius:** Decreases from Na to Ar.
- **Reason:** Increasing nuclear charge attracts the same outer shell ($n=3$) more strongly, with relatively constant shielding.
- **Ionic Radius:** For cations (Na⁺, Mg²⁺, Al³⁺, Si⁴⁺), the radius decreases significantly as the nuclear charge increases for an isoelectronic series. For anions (P³⁻, S²⁻, Cl⁻), the radius also decreases for the same reason. Note that anions are much larger than cations in the same period, as they have an extra shell of electrons compared to the cations.
Explanation
The Period 3 Property Parade
- Atomic radius decreases across Period 3 as increasing nuclear charge pulls the outer electron shell closer.
- First ionisation energy generally rises due to stronger nuclear attraction, but dips at Group 13 (Al) and 16 (S) due to sub-shell changes and electron-pair repulsion.
- Melting point peaks at Group 14 (Si) because of its strong giant covalent structure, contrasting with the metallic and simple molecular elements.
- Electrical conductivity is high for the metals (Na, Mg, Al), then drops significantly for the semi-metal (Si) and non-metals (P, S, Cl, Ar) as mobile electrons become unavailable.