Periodicity of Physical Properties of the Elements in Period 3

Full Notes

The period 3 elements are sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl) and argon (Ar).

They are in period 3 because atoms of each element have three occupied energy levels (shells) however their physical properties change across the period — this is called periodicity.

Atomic Radius Trend

Atomic radius decreases from Na to Ar.

Why:

• Number of protons increases, so nuclear charge increases.
• Electron shielding stays the same or similar (same number of inner shells).
• Outer electrons are pulled closer to the nucleus due to stronger attraction to increased positive charge of nucleus.
• This makes atoms smaller as you move across the period.

Ionic Radius Trend

Metals (Na⁺, Mg²⁺, Al³⁺) lose electrons and form smaller ions than their atoms.

• The ionic radius decreases from Na⁺ to Al³⁺ as the nuclear charge increases whilst the number of electrons stays the same (the ions all have the same electron configuration), meaning greater attraction from nucleus pulls outer-shell of ion more tightly, decreasing ionic radius.

Non-metals (P³⁻, S²⁻, Cl⁻) gain electrons and form larger ions than their atoms.

• There's a noticeable jump in ionic radius between Al³⁺ and P³⁻, due to the switch from cations to anions.

Note that Silicon (Si) and Argon (Ar) don’t readily form ions.

Melting Point Trend

Melting points vary due to different bonding and structures.

Explanation of melting point trends:

• Metals (Na, Mg, Al)
  • High melting points due to strong metallic bonding.
  • Metallic bonding increases in strength from Na to Al due to more delocalised electrons and greater positive charge of metal ions.
• Silicon (Si)
  • Highest melting point due to its giant covalent structure.
  • Strong covalent bonds require a lot of energy to break.
• Non-metals (P₄, S₈, Cl₂, Ar)
  • Low melting points due to weak intermolecular forces.
  • S₈ has a higher melting point than P₄ and Cl₂ because it is a larger molecule, meaning stronger van der Waals forces.

Matt’s exam tip

Don’t forget that sulfur has a slightly higher melting point than phosphorus. This is because sulfur exists as molecules of S₈, whereas phosphorus is most commonly found as P₄ molecules. S₈ molecules are larger than P₄ meaning stronger van der Waals forces and a higher melting point.

Electrical Conductivity Trend

Metals conduct well — conductivity increases from Na to Al due to more delocalised electrons in the metallic lattice.

Si is a metalloid — it can conduct slightly due to its semi-metallic nature.

Non-metals don't conduct — they form molecules or atoms with no free electrons.

Summary

• Period 3 elements are Na, Mg, Al, Si, P, S, Cl, Ar.
• Atomic radius decreases across the period due to increasing nuclear charge with similar shielding.
• Ionic radius decreases across metals, then jumps for non-metals due to change from cations to anions.
• Melting points: high for metals, highest for Si, low for non-metals (but S₈ > P₄ > Cl₂ > Ar).
• Electrical conductivity: increases across metals, Si conducts slightly, non-metals don’t conduct.