Describe and explain the periodic variation across Period 3 in atomic radius, ionic radius, melting point and electrical conductivity, and the trends in the bonding, structure and acid-base behaviour of the oxides and chlorides

What this dot point is asking

SEAB wants you to describe and explain the periodic trends across Period 3 (Na to Ar) in atomic and ionic radius, melting point and electrical conductivity, and the change in bonding, structure and acid-base behaviour of the oxides and chlorides. The melting-point trend and the acid-base behaviour of oxides are reliable Paper 2 questions.

The answer

Atomic and ionic radius

Atomic radius decreases across Period 3. The nuclear charge increases while electrons are added to the same shell, so the increasing nuclear attraction pulls the outer shell in. Shielding stays roughly constant.

Ionic radius shows a step: the cations ($\text{Na}^+$ to $\text{Al}^{3+}$) are small (lost the outer shell), then the anions ($\text{P}^{3-}$ to $\text{Cl}^-$) are larger (gained electrons, more repulsion). Within the cations, radius decreases with charge; within the anions, radius decreases with nuclear charge.

Melting point

Melting point depends on structure, not a simple trend:

• Na, Mg, Al (metallic): rises with more delocalised electrons and higher cation charge.
• Si (giant covalent): highest melting point, many strong covalent bonds.
• $\text{P}_4$, $\text{S}_8$, $\text{Cl}_2$ (simple molecular): low, set by van der Waals forces; $\text{S}_8 > \text{P}_4 > \text{Cl}_2$ by molecule size.
• Ar (monatomic): lowest.

Electrical conductivity

Conductivity rises Na to Al (more mobile delocalised electrons per atom), drops sharply at Si (semiconductor), and is essentially zero for the molecular non-metals and argon (no mobile charge carriers).

Bonding in the oxides and chlorides

Across the period, bonding shifts from ionic (metal compounds) to covalent (non-metal compounds) as the electronegativity difference falls:

• Oxides: $\text{Na}_2\text{O}$, MgO (ionic) to $\text{SiO}_2$ (giant covalent) to $\text{P}_4\text{O}_{10}$, $\text{SO}_3$ (simple molecular covalent).
• Chlorides: NaCl, $\text{MgCl}_2$ (ionic) to $\text{AlCl}_3$, $\text{SiCl}_4$, $\text{PCl}_5$ (covalent).

Acid-base behaviour of the oxides

The clearest periodic trend is in acid-base character, from basic to amphoteric to acidic:

• Basic (ionic metal oxides): $\text{Na}_2\text{O}$, MgO react with water or acids to give alkaline solutions or salts.
• Amphoteric: $\text{Al}_2\text{O}_3$ reacts with both acids and bases.
• Acidic (covalent non-metal oxides): $\text{SiO}_2$ (weakly), $\text{P}_4\text{O}_{10}$, $\text{SO}_2$, $\text{SO}_3$ react with water or bases to give acids or salts.

Behaviour of the chlorides with water

• Ionic chlorides (NaCl, $\text{MgCl}_2$) dissolve to give near-neutral or slightly acidic solutions.
• Covalent chlorides hydrolyse, releasing HCl fumes and giving acidic solutions: $\text{SiCl}_4 + 2\text{H}_2\text{O} \rightarrow \text{SiO}_2 + 4\text{HCl}$, and $\text{PCl}_5$ reacts vigorously with water.

Examples in context

Example 1. Identifying an unknown oxide. A Paper 2 question gives the pH of the solution formed when an unknown Period 3 oxide is shaken with water. A strongly alkaline result points to $\text{Na}_2\text{O}$; a strongly acidic result points to $\text{P}_4\text{O}_{10}$ or $\text{SO}_3$; an oxide that dissolves in both acid and alkali is $\text{Al}_2\text{O}_3$. The acid-base trend is the key identifying feature.

Example 2. Why silicon is special. Silicon's giant covalent structure gives it the highest melting point in the period and makes it a semiconductor, which is exactly why it underpins the electronics industry. SEAB uses this to test the link between structure (giant covalent) and physical properties (high melting point, intermediate conductivity).

Try this

Q1. Explain why atomic radius decreases across Period 3. [2 marks]

• Cue. Nuclear charge increases while electrons enter the same shell with roughly constant shielding, so the outer electrons are pulled in.

Q2. State the structure and bonding of (a) $\text{SiO}_2$ and (b) $\text{SO}_2$, and predict which has the higher melting point. [3 marks]

• Cue. (a) Giant covalent. (b) Simple molecular. $\text{SiO}_2$ has the much higher melting point (covalent bonds versus van der Waals).

Q3. Write equations for the reaction of $\text{Al}_2\text{O}_3$ with (a) hydrochloric acid and (b) sodium hydroxide, to show it is amphoteric. [2 marks]

• Cue. (a) $\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$. (b) $\text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4$.