Physical properties of Period 3 elements — radius, melting point, conductivity

Period 3 runs Na Mg Al Si P S Cl Ar. Going left to right, the nuclear charge increases by one each step while electrons fill the same principal shell (n = 3). That single fact drives almost every physical trend in the period.

The elements start as metals (Na, Mg, Al) with delocalised electrons, pass through a metalloid (Si) with giant covalent bonding, and end as non-metals (P, S, Cl, Ar) with simple molecular or atomic structures.

Atomic radius decreases across the period. The increasing nuclear charge pulls the outer electrons closer, so the atoms get smaller. Argon is not assigned a covalent radius in the Data Booklet because it does not form covalent bonds.

Ionic radius follows a different pattern:

• Cations (Na⁺, Mg²⁺, Al³⁺) are smaller than their parent atoms — they have lost their outer shell, and the increased effective nuclear charge pulls the remaining electrons tighter.
• Anions (P³⁻, S²⁻, Cl⁻) are larger than their parent atoms — they have gained electrons into the same shell, increasing electron–electron repulsion.

The Data Booklet values (in pm) give the picture:

• Na 186, Mg 160, Al 143, Si 117, P 110, S 104, Cl 99 (atomic, covalent radii)
• Na⁺ 102, Mg²⁺ 72, Al³⁺ 53 (cationic radii)
• P³⁻ 212, S²⁻ 184, Cl⁻ 181 (anionic radii)