The Periodic Table: Chemical Periodicity

• Atomic radius decreases across Period 3; ionic radius falls from Na⁺ to Si⁴⁺, jumps up for P³⁻, then falls to Cl⁻.
• Melting point rises Na → Si, then drops to P, S, Cl, Ar; conductivity is high for Na, Mg, Al, low for Si and nil after.
• Oxides change from basic (Na₂O, MgO) through amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₂, SO₃).
• Highest oxidation number in oxides and chlorides equals the number of outer-shell electrons.
• Ionic chlorides (NaCl, MgCl₂) dissolve to near-neutral solutions; covalent ones (AlCl₃, SiCl₄, PCl₅) hydrolyse to give acidic solutions.
• Periodic trends let you predict properties of other elements and identify unknowns.

Na to Ar

Explaining the trends

• Atomic radius decreases: nuclear charge increases, electrons go into the same shell with similar shielding, so they are pulled closer.
• Ionic radius: Na⁺ to Si⁴⁺ are isoelectronic (2,8), and increasing nuclear charge shrinks them. P³⁻ to Cl⁻ have an extra shell (2,8,8) so are much larger, and shrink as nuclear charge increases.
• Melting point Na → Al rises: metallic bonding gets stronger because the ions have higher charge and smaller radius and there are more delocalised electrons per atom (1, 2, 3).
• Si has the highest melting point: giant covalent lattice, many strong covalent bonds must be broken.
• P, S, Cl, Ar are simple molecular (or atomic); only weak id-id forces are overcome. S₈ (more electrons) > P₄ > Cl₂ > Ar.
• Conductivity: Na, Mg, Al have delocalised electrons (more per atom from Na to Al); Si is a semiconductor; P to Ar have no mobile electrons.

With oxygen, chlorine and water

With water (Na and Mg only)

• Sodium: reacts vigorously with cold water, fizzing and moving on the surface: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). Solution pH 13–14.
• Magnesium

Set by the outer electrons