AS Chemistry: Periodicity Across Period 3 — Revision Notes

Condensed for the final weeks. For the full explanation, use the Periodicity Across Period 3 study guide.

Physical trends across Period 3 (Na → Ar)

Property	Trend	Reason
Atomic radius	Decreases	Nuclear charge increases; shielding roughly constant
Ionic radius	Jumps up sharply from Al³⁺ to P³⁻, then decreases again to Cl⁻	Cations (Na⁺, Mg²⁺, Al³⁺) have one fewer electron shell than anions (P³⁻, S²⁻, Cl⁻); within each group, radius falls as nuclear charge rises for the same electron count
First ionisation energy	Increases (with dips at Al and S)	Greater nuclear charge, smaller radius
Electronegativity	Increases	Same reasons
Melting point	Rises to Si, then falls sharply	See below

Melting point — the shape everyone must explain

Na  Mg  Al        metallic, increasing:  more delocalised electrons
                  per ion and smaller ions -> stronger bonding

Si                GIANT COVALENT -- HIGHEST melting point
                  many strong covalent bonds must be broken

P4  S8  Cl2       simple molecular -- LOW melting points
Ar  (monatomic)   only weak van der Waals (dispersion) forces
                  between molecules/atoms
                  S8 > P4 because S8 is a larger molecule; Ar is a
                  single atom, not a molecule, but is grouped here
                  because it also has only weak dispersion forces
                  and a low melting point

The peak at silicon is the key feature; the drop after it is because the structure changes, not because bonds get weaker.

Oxidation number across the period

The oxidation number of each element in its oxide/chloride rises steadily across the period: Na (+1), Mg (+2), Al (+3), Si (+4), P (+5), S (+6 in SO₃) — because each element uses progressively more of its outer-shell electrons in bonding as the number of valence electrons increases.

The oxides with water

Oxide	Structure	With water	Resulting pH
Na₂O	Ionic	Reacts → NaOH	13–14
MgO	Ionic	Slightly soluble → Mg(OH)₂	9–10
Al₂O₃	Ionic/covalent	Insoluble — amphoteric	7
SiO₂	Giant covalent	Insoluble	7
P₄O₁₀	Simple molecular	Reacts → H₃PO₄	1–2
SO₂ / SO₃	Simple molecular	Reacts → H₂SO₃ / H₂SO₄	1–3

The pattern: metal oxides are basic, non-metal oxides are acidic, and Al₂O₃ is amphoteric (reacts with both acids and bases) sitting on the boundary:

Al2O3 + 6HCl  ->  2AlCl3 + 3H2O        (reacts with acid)
Al2O3 + 2NaOH  ->  2NaAlO2 + H2O       (reacts with base)

MgO gives only a weakly alkaline pH because Mg(OH)₂ is sparingly soluble.

The chlorides with water

Chloride	With water	pH
NaCl	Dissolves, no reaction	7
MgCl₂	Slight hydrolysis	6.5
AlCl₃	Hydrolyses	3
SiCl₄	Violent hydrolysis, misty HCl fumes	2
PCl₅	Violent hydrolysis, misty HCl fumes	2

Ionic chlorides simply dissolve; covalent chlorides hydrolyse, releasing HCl. The transition from dissolving to hydrolysing tracks the change from ionic to covalent bonding across the period.

Why the split happens where it does: electronegativity increases across the period while chlorine’s stays fixed, so the electronegativity difference shrinks — Na–Cl and Mg–Cl bonds stay ionic enough that the chloride simply dissolves as ions, while Si–Cl and P–Cl bonds become covalent enough that water attacks the bond directly, hydrolysing it. MgCl₂ (pH ≈ 6) is only weakly acidic because the small, doubly-charged Mg²⁺ ion polarises coordinated water enough to release a little H⁺; AlCl₃ (pH ≈ 3) is more acidic still because Al³⁺ is smaller and more highly charged, polarising coordinated water even more strongly.

Worked example — predicting properties. An unknown Period-3-like element X forms a strongly acidic oxide, and its chloride hydrolyses violently, “fuming” in moist air. Where does X sit in its period? A violently hydrolysing chloride indicates a covalent chloride (by analogy with Si and P), and a strongly acidic oxide points to a non-metal — both place X on the right-hand side of the period.

Exam traps

• Saying melting point “decreases after Si because bonds weaken” — the structure changes to simple molecular.
• Forgetting Al₂O₃ is amphoteric.
• Confusing dissolving (NaCl) with hydrolysis (SiCl₄).
• Giving MgO a pH of 13 — it is only sparingly soluble, so 9–10.
• Omitting the misty HCl fumes when describing covalent chloride hydrolysis.
• Forgetting Al₂O₃ needs a specific equation with each of an acid and a base to demonstrate amphoteric behaviour, not just the word “amphoteric”.
• Explaining the ionic-to-covalent chloride split without mentioning the shrinking electronegativity difference to chlorine.

Self-test

1. Why does atomic radius decrease across Period 3?
2. Explain why silicon has the highest melting point in the period.
3. Which Period 3 oxide is amphoteric, and what does that mean?
4. What is observed when silicon tetrachloride is added to water?
5. Why is the pH of magnesium oxide in water only about 9–10?
6. Write equations showing Al₂O₃ reacting with both an acid and a base.
7. Why is AlCl₃’s solution more acidic than MgCl₂’s?
8. An unknown Period-3-like element X has a strongly acidic oxide and a chloride that hydrolyses violently. Where in the period does X sit, and why?

Answers: 1. Nuclear charge increases while electrons enter the same shell with roughly constant shielding, so the outer electrons are pulled in more strongly. 2. It has a giant covalent structure; melting requires breaking many strong covalent bonds throughout the lattice. 3. Al₂O₃ — it reacts with both acids and bases. 4. Violent hydrolysis producing misty fumes of hydrogen chloride and an acidic solution of about pH 2. 5. Magnesium hydroxide is only sparingly soluble, so few hydroxide ions enter solution. 6. Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O. 7. Al³⁺ is smaller and more highly charged than Mg²⁺, so it polarises coordinated water molecules more strongly, releasing more H⁺. 8. The right-hand side — a strongly acidic oxide indicates a non-metal, and a violently hydrolysing (covalent) chloride is typical of elements towards the right of a period, by analogy with Si and P.